1.5 SQL

MySQL fully supports ANSI SQL 92, entry level. A SQL reference for MySQL is thus largely a general SQL reference. Nevertheless, MySQL contains some proprietary enhancements that can help you at the mysql command line. This section thus provides a reference for the SQL query language as it is supported in MySQL.

SQL is a kind of controlled English language consisting of verb phrases. Each of these verb phrases begins with an SQL command followed by other SQL keywords, literals, identifiers, or punctuation.

1.5.1 Case Sensitivity

Case-sensitivity in MySQL depends on a variety of factors, including the token in question and the underlying operating system. Table 1-3 shows the case-sensitivity of different SQL tokens in MySQL.

Table 1-3. The case-sensitivity of MySQL.

Token type	Case-sensitivity
Keywords	Case-insensitive.
Identifiers (databases and tables)	Dependent on the case-sensitivity for the underlying OS. On all UNIX systems except Mac OS X using HFS+, database and table names are case-sensitive. On Mac OS X using HFS+ and Windows, they are case-insensitive.
Table aliases	Case-sensitive
Column aliases	Case-insensitive

1.5.2 Literals

Literals come in the following varieties:

String

String literals may be enclosed either by single or double quotes. If you wish to be ANSI compatible, you should always use single quotes. Within a string literal, you may represent special characters through escape sequences. An escape sequence is a backslash followed by another character to indicate to MySQL that the second character has a meaning other than its normal meaning. Table 1-4 shows the MySQL escape sequences. Quotes can also be escaped by doubling them up: 'This is a ''quote'''. However, you do not need to double up on single quotes when the string is enclosed by double quotes: "This is a 'quote'".

Table 1-4. MySQL escape sequences

Escape sequence	Value
\0	NUL
\`	Single quote
\"	Double quote
\b	Backspace
\n	Newline
\r	Carriage return
\t	Tab
\z	Ctrl-z (workaround for Windows use of Ctrl-z as EOF)
\\	Backslash
\%	Percent sign (only in contexts where a percent sign would be interpreted as a wildcard)
\_	Underscore (only in contexts where an underscore would be interpreted as a wildcard)

Binary

Like string literals, binary literals are enclosed in single or double quotes. You must use escape sequences in binary data to escape NUL (ASCII 0), " (ASCII 34), ' (ASCII 39), and \ (ASCII 92).

Decimal

Numbers appear as a sequence of digits. Negative numbers are preceded by a - sign and a . indicates a decimal point. You may also use scientific notation, as in: -45198.2164e+10.

Hexadecimal

The way in which a hexadecimal is interpreted is dependent on the context. In a numeric context, the hexadecimal literal is treated is a numeric value. In a non-numeric context, it is treated as a binary value. For example, 0x1 + 1 is 2, but 0x4d7953514c by itself is MySQL.

Null

The special keyword NULL signifies a null literal in SQL. In the context of import files, the special escape sequence \N signifies a null value.

1.5.3 Identifiers

You can reference any given object on a MySQL server—assuming you have the proper rights—using one of the following conventions:

Absolute naming

Absolute naming specifies the full path of the object you are referencing. For example, the column BALANCE in the table ACCOUNT in the database BANK would be referenced absolutely as:

BANK.ACCOUNT.BALANCE

Relative naming

Relative naming allows you to specify only part of the object's name, with the rest of the name being assumed based on your current context. For example, if you are currently connected to the BANK database, you can reference the BANK.ACCOUNT.BALANCE column as ACCOUNT.BALANCE. In an SQL query where you have specified that you are selecting from the ACCOUNT table, you may reference the column using only BALANCE. You must provide an extra layer of context whenever relative naming might result in ambiguity. An example of such ambiguity would be a SELECT statement pulling from two tables that both have BALANCE columns.

Aliasing

Aliasing enables you to reference an object using an alternate name that helps avoid both ambiguity and the need to fully qualify a long name.

In general, MySQL allows you to use any character in an identifier. (Older versions of MySQL limited identifiers to valid alphanumeric characters from the default character set, as well as $ and _.) This rule is limited, however, for databases and tables, because these values must be treated as files on the local filesystem. You can therefore use only characters valid for the underlying filesystem's naming conventions in a database or table name. Specifically, you may not use / or . in a database or table name. You can never use NUL (ASCII 0) or ASCII 255 in an identifier.

When an identifier is also an SQL keyword, you must enclose the identifier in backticks:

CREATE TABLE 'select' ( 'table' INT NOT NULL PRIMARY KEY AUTO_INCREMENT);

Since Version 3.23.6, MySQL supports the quoting of identifiers using both backticks and double quotes. For ANSI compatibility, however, you should use double quotes for quoting identifiers. You must, however, be running MySQL in ANSI mode.

1.5.4 Comments

You can introduce comments in your SQL to specify text that should not be interpreted by MySQL. This is particularly useful in batch scripts for creating tables and loading data. MySQL specifically supports three kinds of commenting: C, shell-script, and ANSI SQL commenting.

C commenting treats anything between /* and */ as comments. Using this form of commenting, your comments can span multiple lines. For example:

/*
 * Creates a table for storing customer account information.
*/
DROP TABLE IF EXISTS ACCOUNT;
   
CREATE TABLE ACCOUNT ( ACCOUNT_ID BIGINT NOT NULL
                       PRIMARY KEY AUTO_INCREMENT,
                       BALANCE DECIMAL(9,2) NOT NULL );

Within C comments, MySQL still treats single and double quotes as a start to a string literal. In addition, a semicolon in the comment will cause MySQL to think you are done with the current statement.

Shell-script commenting treats anything from a # character to the end of a line as a comment:

CREATE TABLE ACCOUNT ( ACCOUNT_ID BIGINT NOT NULL 
                       PRIMARY KEY AUTO_INCREMENT,
                       BALANCE DECIMAL(9,2) 
                       NOT NULL ); # Not null ok?

MySQL does not really support ANSI SQL commenting, but it comes close. ANSI SQL commenting is distinguished by adding -- to the end of a line. MySQL supports two dashes and a space ('-- `) followed by the comment. The space is the non-ANSI part:

DROP TABLE IF EXISTS ACCOUNT; -- Drop the table if it already exists

1.5.5 Commands

This section presents the full syntax of all commands accepted by MySQL.

ALTER [IGNORE] TABLE table action_list

The ALTER statement covers a wide range of actions that modify the structure of a table. This statement is used to add, change, or remove columns from an existing table as well as to remove indexes. To perform modifications on the table, MySQL creates a copy of the table and changes it, meanwhile queuing all table altering queries. When the change is done, the old table is removed and the new table put in its place. At this point the queued queries are performed.

As a safety precaution, if any of the queued queries create duplicate keys that should be unique, the ALTER statement is rolled back and cancelled. If the IGNORE keyword is present in the statement, duplicate unique keys are ignored and the ALTER statement proceeds as normal. Be warned that using IGNORE on an active table with unique keys invites table corruption.

Possible actions in action_list include:

ADD [COLUMN] create_clause [FIRST | AFTER column]

ADD [COLUMN] ( create_clause, create_clause,...)

Adds a new column to the table. The create_clause is the SQL that would define the column in a normal table creation (see CREATE TABLE for the syntax and valid options). The column will be created as the first column if the FIRST keyword is specified. Alternately, you can use the AFTER keyword to specify which column it should be added after. If neither FIRST nor AFTER is specified, the column is added at the end of the table's column list. You may add multiple columns at once by enclosing multiple create clauses separated with commas, inside parentheses.

ADD [CONSTRAINT symbol] FOREIGN KEY name ( column, ...)[ reference]

Currently applies only to the InnoDB table type, which supports foreign keys. This syntax adds a foreign key reference to your table.

ADD FULLTEXT [ name] ( column, ...)

Adds a new full text index to the table using the specified columns.

ADD INDEX [ name] ( column, ...)

Adds an index to the altered table, indexing the specified columns. If the name is omitted, MySQL will choose one automatically.

ADD PRIMARY KEY ( column, ...)

Adds a primary key consisting of the specified columns to the table. An error occurs if the table already has a primary key.

ADD UNIQUE[ name] ( column, ...)

Adds a unique index to the altered table; similar to the ADD INDEX statement.

ALTER [COLUMN] column SET DEFAULT value

Assigns a new default value for the specified column. The COLUMN keyword is optional and has no effect.

ALTER [COLUMN] column DROP DEFAULT

Drops the current default value for the specified column. A new default value is assigned to the column based on the CREATE statement used to create the table. The COLUMN keyword is optional and has no effect.

DISABLE KEYS

Tells MySQL to stop updating indexes for MyISAM tables. This clause applies only to non-unique indexes. Because MySQL is more efficient at rebuilding its keys than it is at building them one at a time, you may want to disable keys while performing bulk inserts into a database. You should avoid this trick, however, if you have read operations going against the table while the inserts are running.

ENABLE KEYS

Recreates the indexes no longer being updated because of a prior call to DISABLE KEYS.

CHANGE [COLUMN] column create_clause

MODIFY [COLUMN] create_clause [FIRST | AFTER column]

Alters the definition of a column. This statement is used to change a column from one type to a different type while affecting the data as little as possible. The create clause is the same syntax as in the CREATE TABLE statement. This includes the name of the column. The MODIFY version is the same as CHANGE if the new column has the same name as the old. The COLUMN keyword is optional and has no effect. MySQL will try its best to perform a reasonable conversion. Under no circumstance will MySQL give up and return an error when using this statement; a conversion of some sort will always be performed. With this in mind, you should make a backup of the data before the conversion and immediately check the new values to see if they are reasonable.

DROP [COLUMN] column

Deletes a column from a table. This statement will remove a column and all its data from a table permanently. There is no way to recover data destroyed in this manner other than from backups. All references to this column in indexes will be removed. Any indexes where this was the sole column will be destroyed as well. (The COLUMN keyword is optional and has no effect.)

DROP PRIMARY KEY

Drops the primary key from the table. If no primary key is found in the table, the first unique key is deleted.

DROP INDEX key

Removes an index from a table. This statement will completely erase an index from a table. This statement will not delete or alter any of the table data itself, only the index data. Therefore, an index removed in this manner can be recreated using the ALTER TABLE ... ADD INDEX statement.

RENAME [AS] new_table

RENAME [TO] new_table

Changes the name of the table. This operation does not affect any of the data or indexes within the table, only the table's name. If this statement is performed alone, without any other ALTER TABLE clauses, MySQL will not create a temporary table as with the other clauses, but simply perform a fast Unix-level rename of the table files.

ORDER BY column [ASC | DESC]

Forces the table to be reordered by sorting on the specified column name. The table will no longer be in this order when new rows are inserted. This option is useful for optimizing tables for common sorting queries. You can specify multiple columns.

table_options

Enables a redefinition of the tables options such as the table type.

Multiple ALTER statements may be combined into one using commas, as in the following example:

ALTER TABLE mytable DROP myoldcolumn, ADD mynewcolumn INT

To perform any of the ALTER TABLE actions, you must have SELECT, INSERT, DELETE, UPDATE, CREATE, and DROP privileges for the table in question.

Examples

# Add the field 'address2' to the table 'people' and make 
# it of type 'VARCHAR' with a maximum length of 100.
ALTER TABLE people ADD COLUMN address2 VARCHAR(100)
# Add two new indexes to the 'hr' table, one regular index 
# for the 'salary' field and one unique index for the 'id' 
# field. Also, continue operation if duplicate values are 
# found while creating the 'id_idx' index 
# (very dangerous!).
ALTER TABLE hr ADD INDEX salary_idx ( salary )
ALTER IGNORE TABLE hr ADD UNIQUE id_idx ( id )
# Change the default value of the 'price' field in the 
# 'sprockets' table to $19.95.
ALTER TABLE sprockets ALTER price SET DEFAULT '$19.95'
# Remove the default value of the 'middle_name' field in
# the 'names' table.
ALTER TABLE names ALTER middle_name DROP DEFAULT
# Change the type of the field 'profits' from its previous 
# value (which was perhaps INTEGER) to BIGINT. The first
# instance of 'profits' is the column to change, and the
# second is part of the create clause.
ALTER TABLE finances CHANGE COLUMN profits profits BIGINT
# Remove the 'secret_stuff' field from the table
# 'not_private_anymore'
ALTER TABLE not_private_anymore DROP secret_stuff
# Delete the named index 'id_index' as well as the primary 
# key from the table 'cars'.
ALTER TABLE cars DROP INDEX id_index, DROP PRIMARY KEY
# Rename the table 'rates_current' to 'rates_1997'
ALTER TABLE rates_current RENAME AS rates_1997

ANALYZE TABLE table1, table2, ..., tablen

Acquires a read lock on the table and performs an analysis on it for MyISAM and BDB tables. The analysis examines the key distribution in the table. It returns a result set with the following columns:

Table

The name of the table.

Op

The value analyze.

Msg_type

One of status, error, or warning.

Msg_text

The message resulting from the analysis.

CREATE DATABASE [IF NOT EXISTS] dbname

Creates a new database with the specified name. You must have the proper privileges to create the database. Running this command is the same as running the mysqladmincreate utility.

Example

CREATE DATABASE Bank;

CREATE [AGGREGATE] FUNCTION name 
RETURNS return_type SONAME library

The CREATE FUNCTION statement allows MySQL statements to access precompiled executable functions known as user-defined functions (UDFs). These functions can perform practically any operation, since they are designed and implemented by the user. The return value of the function can be STRING, for character data; REAL, for floating point numbers; or INTEGER, for integer numbers. MySQL will translate the return value of the C function to the indicated type. The library file that contains the function must be a standard shared library that MySQL can dynamically link into the server.

Example

CREATE FUNCTION multiply RETURNS REAL SONAME mymath.so

CREATE [UNIQUE|FULLTEXT] INDEX name ON table (column, ...)

The CREATE INDEX statement is provided for compatibility with other implementations of SQL. In older versions of SQL, this statement does nothing. As of 3.22, this statement is equivalent to the ALTER TABLE ADD INDEX statement. To perform the CREATE INDEX statement, you must have INDEX privileges for the table in question.

The UNIQUE keyword constrains the table to having only one row in which the index columns have a given value. If the index is multicolumn, individual column values may be repeated; the whole index must be unique.

The FULLTEXT keyword enables keyword searching on the indexed column or columns.

Example

CREATE UNIQUE INDEX TransIDX ON Translation ( language, 
locale, code );

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] table
(create_clause, ...) [table_options]
[[IGNORE|REPLACE] select]

The CREATE TABLE statement defines the structure of a table within the database. This statement is how all MySQL tables are created. If the TEMPORARY keyword is used, the table exists only as long as the current client connection exists, or until you explicitly drop the table.

The IF NOT EXISTS clause tells MySQL to create the table only if the table does not already exist. If the table does exist, nothing happens. If the table exists and IF NOT EXISTS and TEMPORARY are not specified, an error will occur. If TEMPORARY is specified and the table exists but IF NOT EXISTS is not specified, the existing table will simply be invisible to this client for the duration of the new temporary table's life.

The CREATE clause can either define the structure of a specific column or define a meta-structure for the column. A CREATE clause that defines a column consists of the name of the new table followed by any number of field definitions. The syntax of a field definition is:

column type [NOT NULL | NULL] [DEFAULT value]
[AUTO_INCREMENT] [PRIMARY KEY] [reference]

The modifiers in this syntax are:

AUTO_INCREMENT

Indicates that the column should be automatically incremented using the current greatest value for that column. Only whole number columns may be auto-incremented.

DEFAULT value

This attribute assigns a default value to a field. If a row is inserted into the table without a value for this field, this value will be inserted. If a default is not defined, a null value is inserted, unless the field is defined as NOT NULL in which case MySQL picks a value based on the type of the field.

NOT NULL

This attribute guarantees that every entry in the column will have some non-null value. Attempting to insert a NULL value into a field defined with NOT NULL will generate an error.

NULL

This attribute specifies that the field is allowed to contain NULL values. This is the default if neither this nor the NOT NULL modifier are specified. Fields that are contained within an index cannot contain the NULL modifier. (The attribute will be ignored, without warning, if it does exist in such a field.)

PRIMARY KEY

This attribute automatically makes the field the primary key (see later) for the table. Only one primary key may exist for a table. Any field that is a primary key must also contain the NOT NULL modifier.

REFERENCES table [(column, . . .)] [MATCH FULL | MATCH PARTIAL ] [ON DELETE option] [ON UPDATE option]

Creates a foreign key reference. Currently applies only to the InnoDB table type.

You may specify meta-structure such as indexes and constraints via the following clauses:

FULLTEXT ( column, ... )

Since MySQL 3.23.23, MySQL has supported full text indexing. The use and results of this search are described in the online MySQL reference manual. To create a full text index, use the FULLTEXT keyword:

CREATE TABLE Item ( itemid INT NOT NULL PRIMARY KEY,
       name VARCHAR(25) NOT NULL,
       description TEXT NOT NULL,
       FULLTEXT ( name, description )
);

INDEX [name] (column, ...)

Creates a regular index of all of the named columns (KEY and INDEX, in this context, are synonyms). Optionally the index may be given a name. If no name is provided, a name is assigned based on the first column given and a trailing number, if necessary, for uniqueness. If a key contains more than one column, leftmost subsets of those columns are also included in the index. Consider the following index definition:

INDEX idx1 ( name, rank, serial );

When this index is created, the following groups of columns will be indexed:

• name, rank, serial

• name, rank

• name

KEY [name] (column, ...)

Synonym for INDEX.

PRIMARY KEY

Creates the primary key of the table. A primary key is a special key that can be defined only once in a table. The primary key is a UNIQUE key with the name PRIMARY. Despite its privileged status, it behaves almost the same as every other unique key, except it does not allow NULL values.

UNIQUE [name] (column, ...)

Creates a special index where every value contained in the index (and therefore in the fields indexed) must be unique. Attempting to insert a value that already exists into a unique index will generate an error. The following would create a unique index of the nicknames field:

UNIQUE (nicknames);

When indexing character fields (CHAR, VARCHAR, and their synonyms only), it is possible to index only a prefix of the entire field. For example, the following will create an index of the numeric field id along with the first 20 characters of the character field address:

INDEX adds ( id, address(20) );

When performing any searches of the field address, only the first 20 characters will be used for comparison, unless more than one match is found that contains the same first 20 characters, in which case a regular search of the data is performed. Therefore, it can be a big performance bonus to index only the number of characters in a text field that you know will make the value unique. This feature is, however, dependent on the underlying table type.

In addition, MySQL supports the following special "types," and the MySQL team is working on adding functionality to support them:

FOREIGN KEY (name (column, [column2, . . . ])

CHECK

As of MySQL 3.23, you can specify table options at the end of a CREATE TABLE statement. These options are:

AUTO_INCREMENT = start

Specifies the first value to be used for an AUTO_INCREMENT column. Works only with MyISAM tables.

AVG_ROW_LENGTH = length

An option for tables containing large amounts of variable-length data. The average row length is an optimization hint to help MySQL manage this data.

CHECKSUM = 0 or 1

When set to 1, this option forces MySQL to maintain a checksum for the table to improve data consistency. This option creates a performance penalty.

COMMENT = comment

Provides a comment for the table. The comment may not exceed 60 characters.

DELAY_KEY_WRITE = 0 or 1

For MyISAM tables only. When set, this option delays key table updates until the table is closed.

MAX_ROWS = rowcount

The maximum number of rows you intend to store in the table.

MIN_ROWS = rowcount

The minimum number of rows you intend to store in the table.

PACK_KEYS = 0 or 1

For MyISAM and ISAM tables only. This option provides a performance booster for read-heavy tables. Set to 1, this option causes smaller keys to be created and thus slows down writes while speeding up reads.

PASSWORD = 'password'

Available only to MySQL customers with special commercial licenses. This option uses the specified password to encrypt the table's .frm file. This option has no effect on the standard version of MySQL.

ROW_FORMAT = DYNAMIC or STATIC

For MyISAM tables only. Defines how the rows should be stored in a table.

TYPE = rowtype

Specifies the table type of the database. If the selected table type is not available, the closest table type available is used. For example, BDB is not available yet for Mac OS X. If you specified TYPE=BDB on a Mac OS X system, MySQL will instead create the table as a MyISAM table (the default table type). Supported table types are described later.

Finally, you can create a table and populate it straight from the results of a SQL query:

CREATE TABLE tblname SELECT query

You must have CREATE privileges on a database to use the CREATE TABLE statement.

Examples

# Create the new empty database 'employees'
CREATE DATABASE employees;
# Create a simple table
CREATE TABLE emp_data ( id INT, name CHAR(50) );
# Create a complex table
CREATE TABLE IF NOT EXISTS emp_review (
 id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
 emp_id INT NOT NULL REFERENCES emp_data ( id ),
 review TEXT NOT NULL,
 INDEX ( emp_id ),
 FULLTEXT ( review )
) AUTO_INCREMENT = 1, TYPE=InnoDB;
# Make the function make_coffee (which returns a string
# value and is stored in the myfuncs.so shared library)
# available to MySQL.
CREATE FUNCTION make_coffee RETURNS string SONAME "myfuncs.so";
# Create a table using the resultss from another query
CREATE TABLE Stadium
SELECT stadiumName, stadiumLocation
FROM City;

DELETE [LOW_PRIORITY | QUICK]
FROM table [WHERE clause] [ORDER BY column, ...]
[LIMIT n]
DELETE [LOW_PRIORITY | QUICK]
table1[.*], table2[.*], ..., tablen[.*]
FROM tablex, tabley, ..., tablez [WHERE clause]
DELETE [LOW_PRIORITY | QUICK]
FROM table1[.*], table2[.*], ..., tablen[.*]
USING references[WHERE clause]

Deletes rows from a table. When used without a WHERE clause, this will erase the entire table and recreate it as an empty table. With a WHERE clause, it will delete the rows that match the condition of the clause. This statement returns the number of rows deleted.

In versions prior to MySQL 4, omitting the WHERE clause will erase this entire table. This is done by using an efficient method that is much faster than deleting each row individually. When using this method, MySQL returns 0 to the user because it has no way of knowing how many rows it deleted. In the current design, this method simply deletes all the files associated with the table except for the file that contains the actual table definition. Therefore, this is a handy method of zeroing out tables with unrecoverably corrupt data files. You will lose the data, but the table structure will still be in place. If you really wish to get a full count of all deleted tables, use a WHERE clause with an expression that always evaluates to true:

DELETE FROM TBL WHERE 1 = 1;

The LOW_PRIORITY modifier causes MySQL to wait until no clients are reading from the table before executing the delete. For MyISAM tables, QUICK causes the table handler to suspend the merging of indexes during the DELETE, to enhance the speed of the DELETE.

The LIMIT clause establishes the maximum number of rows that will be deleted in a single shot.

When deleting from MyISAM tables, MySQL simply deletes references in a linked list to the space formerly occupied by the deleted rows. The space itself is not returned to the operating system. Future inserts will eventually occupy the deleted space. If, however, you need the space immediately, run the OPTIMIZE TABLE statement or use the myisamchk utility.

The second two syntaxes are new multi-table DELETE statements that enable the deletion of rows from multiple tables. The first is new as of MySQL 4.0.0, and the second was introduced in MySQL 4.0.2.

In the first multi-table DELETE syntax, the FROM clause does not name the tables from which the DELETEs occur. Instead, the objects of the DELETE command are the tables from which the deletes should occur. The FROM clause in this syntax works like a FROM clause in a SELECT in that it names all of the tables that appear either as objects of the DELETE or in the WHERE clause.

I recommend the second multi-table DELETE syntax because it avoids confusion with the single table DELETE. In other words, it deletes rows from the tables specified in the FROM clause. The USING clause describes all the referenced tables in the FROM and WHERE clauses. The following two DELETEs do the exact same thing. Specifically, they delete all records from the emp_data and emp_review tables for employees in a specific department.

DELETE emp_data, emp_review
FROM emp_data, emp_review, dept
WHERE dept.id = emp_data.dept_id
AND emp_data.id = emp_review.emp_id
AND dept.id = 32;
DELETE FROM emp_data, emp_review
USING emp_data, emp_review, dept
WHERE dept.id = emp_data.dept_id
AND emp_data.id = emp_review.emp_id
AND dept.id = 32;

You must have DELETE privileges on a database to use the DELETE statement.

Examples

# Erase all of the data (but not the table itself) 
for the table 'olddata'.
DELETE FROM olddata
# Erase all records in the 'sales' table where the 'syear' 
field is '1995'.
DELETE FROM sales WHERE syear=1995

DESCRIBE table [column]
DESC table [column]

Gives information about a table or column. While this statement works as advertised, its functionality is available (along with much more) in the SHOW statement. This statement is included solely for compatibility with Oracle SQL. The optional column name can contain SQL wildcards, in which case information will be displayed for all matching columns.

Example

# Describe the layout of the table 'messy'
DESCRIBE messy
# Show the information about any columns starting 
# with 'my_' in the 'big' table.
# Remember: '_' is a wildcard, too, so it must be 
# escaped to be used literally.
DESC big my\_%

Synonym for DESCRIBE.

DROP DATABASE [IF EXISTS] name

Permanently remove a database from MySQL. Once you execute this statement, none of the tables or data that made up the database are available. All support files for the database are deleted from the filesystem. The number of files deleted will be returned to the user. This statement is equivalent to running the mysqladmindrop utility. As with running mysqladmin, you must be the administrative user for MySQL (usually root or mysql) to perform this statement. You may use the IF EXISTS clause to prevent any error message that would result from an attempt to drop a nonexistent table.

DROP FUNCTION name

Will remove a user-defined function from the running MySQL server process. This does not actually delete the library file containing the function. You may add the function again at any time using the CREATE FUNCTION statement. In the current implementation, DROP FUNCTION simply removes the function from the function table within the MySQL database. This table keeps track of all active functions.

DROP INDEX idx_name ON tbl_name

Provides compatibility with other SQL implementations. In older versions of MySQL, this statement does nothing. As of 3.22, this statement is equivalent to ALTER TABLE ... DROP INDEX. To perform the DROP INDEX statement, you must have SELECT, INSERT, DELETE, UPDATE, CREATE, and DROP privileges for the table in question.

DROP TABLE [IF EXISTS] name [, name2, ...]
[RESTRICT | CASCADE]

Will erase an entire table permanently. In the current implementation, MySQL simply deletes the files associated with the table. As of 3.22, you may specify IF EXISTS to make MySQL not return an error if you attempt to remove a table that does not exist. The RESTRICT and CASCADE keywords do nothing; they exist solely for ANSI compatibility. You must have DELETE privileges on the table to use this statement.

EXPLAIN [table_name | sql_statement]

Used with a table name, this command is an alias for SHOW COLUMNS FROM table_name.

Used with an SQL statement, this command displays verbose information about the order and structure of a SELECT statement. This can be used to see where keys are not being used efficiently. This information is returned as a result set with the following columns:

table

The name of the table referenced by the result set row explaining the query.

type

The type of join that will be performed.

possible_keys

Indicates which indexes MySQL could use to build the join. If this column is empty, there are no relevant indexes and you should probably build some to enhance performance.

key

Indicates which index MySQL decided to use.

key_len

Provides the length of the key MySQL decided to use for the join.

ref

Describes which columns or constants were used with the key to build the join.

rows

Indicates the number of rows MySQL estimates it will need to examine to perform the query.

Extra

Additional information indicating how MySQL will perform the query.

Example

EXPLAIN SELECT customer.name, product.name FROM customer, 
product, purchases 
WHERE purchases.customer=customer.id AND purchases.
product=product.id

FLUSH option[, option...]

Flushes or resets various internal processes depending on the options given. You must have RELOAD privileges to execute this statement. The option can be any of the following:

DES_KEY_FILE

Reloads the DES keys from the file originally specified with the --des-key-file option.

HOSTS

Empties the cache table that stores hostname information for clients. This should be used if a client changes IP addresses, or if there are errors related to connecting to the host.

LOGS

Closes all the standard log files and reopens them. This can be used if a log file has changed its inode number. If no specific extension has been given to the update log, a new update log will be opened with the extension incremented by one.

PRIVILEGES

Reloads all the internal MySQL permissions grant tables. This must be run for any changes to the tables to take effect unless those changes occurred through a GRANT/REVOKE statement.

QUERY CACHE

For better memory use, this command defragments the query cache but it does not delete queries from the cache.

STATUS

Resets the status variables that keep track of the current state of the server.

TABLE table

TABLES table, table2, . .., tablen

Flushes only the specified tables.

TABLES [WITH READ LOCK]

Closes all currently open tables and flushes any cached data to disk. With a read lock, it acquires a read lock that will not be released until UNLOCK TABLES is issued. Read locks are ineffective with InnoDB tables.

GRANT privilege
[ (column, ...) ] [, privilege [( column, ...) ] ...]
ON {table} TO user [IDENTIFIED BY 'password']
[, user [IDENTIFIED BY 'password'] ...]
[REQUIRE [{SSL | X509 |
CIPHER cipher [AND]
[ISSUER issuer [AND]]
[SUBJECT subject]]]
[WITH [GRANT OPTION | MAX_QUERIES_PER_HOUR limit]]

In versions prior to MySQL 3.22.11, the GRANT statement was recognized but did nothing. In current versions, GRANT is functional. This statement enables access rights to a user (or users). Access can be granted per database, table or individual column. The table can be given as a table within the current database; use * to affect all tables within the current database, *.* to affect all tables within all databases or database.* to affect all tables within the given database.

The following privileges are currently supported:

ALL PRIVILEGES/ALL

Assigns all privileges except FILE, PROCESS, RELOAD, and SHUTDOWN.

ALTER

To alter the structure of tables.

CREATE

To create new tables.

DELETE

To delete rows from tables.

DROP

To delete entire tables.

FILE

To create and remove entire databases, as well as manage log files.

INDEX

To create and delete indexes from tables.

INSERT

To insert data into tables.

PROCESS

To kill process threads.

REFERENCES

Not implemented (yet).

RELOAD

To refresh various internal tables (see the FLUSH statement).

SELECT

To read data from tables.

SHUTDOWN

To shut down the database server.

UPDATE

To alter rows within tables.

USAGE

No privileges at all.

The user variable is of the form user@hostname. Either the user or the hostname can contain SQL wildcards. When wildcards are used, either the whole name must be quoted, or just the parts with the wildcards (e.g., joe@"%.com " and "joe@%.com" are both valid)^[1]. A user without a hostname is considered to be the same as user@"%".

^[1] In fact, the rules governing when you need to use quotes are more complex. As a good rule of thumb, whenever you have non-alphanumeric characters, quote them.

If you have a global GRANT privilege, you may specify an optional INDENTIFIED BY modifier. If the user in the statement does not exist, it will be created with the given password. Otherwise, the existing user will have her password changed.

The GRANT privilege is given to a user with the WITH GRANT OPTION modifier. If this is used, the user may grant any privilege she has to another user. You may alternately chose to limit the number of queries made by a particular user ID through the MAX_QUERIES_PER_HOUR option.

Support for secure SSL encryptions, as well as X.509 authentication, has recently been added to MySQL. The REQUIRE clause enables you to require a user to authenticate in one of these manners and identify the credentials to be used. Just specifying REQUIRE SSL tells MySQL that the user can connect to MySQL using only an SSL connection. Similarly, REQUIRE X509 requires the user to authenticate using an X.509 certificate. You can place the following restrictions on the connection:

ISSUER issuer

Demands that the certificate have the issuer specified.

SUBJECT subject

Not only does the user have to have a valid certificate, but it must have a certificate for the specified subject.

CIPHER cipher

Enables MySQL to enforce a minimum encryption strength. The connection must use one of the ciphers specified here.

Examples

# Give full access to joe@carthage for the Account table
GRANT ALL ON bankdb.Account TO joe@carthage;
# Give full access to jane@carthage for the
# Account table and create a user ID for her
GRANT ALL ON bankdb.Account TO jane@carthage IDENTIFIED BY 'mypass';
# Give joe the ability
# to SELECT from any table on the webdb database
GRANT SELECT ON webdb.* TO joe;
# Give joe on the local machine access to everything in webdb but
# require some special security
GRANT ALL on webdb.* TO joe@localhost
IDENTIFIED BY 'mypass'
REQUIRE SUBJECT `C=US, ST=MN, L=Minneapolis, O=My Cert,
CN=Joe Friday/Email=joe@localhost'
AND ISSUER='C=US, ST=MN, L=Minneapolis, O=Imaginet,
CN=Joe Friday/Email=joe@localhost'
AND CIPHER='RSA-DES-3DES-SHA';

INSERT [DELAYED | LOW_PRIORITY ] [IGNORE] 
[INTO] table [ (column, ...) ] 
VALUES ( values [, values... ])

INSERT [DELAYED | LOW_PRIORITY] [IGNORE]
[INTO] table [ (column, ...) ]
SELECT ...

INSERT [DELAYED | LOW_PRIORITY] [IGNORE]
[INTO] table
SET column=value, column=value,...

Inserts data into a table. The first form of this statement simply inserts the given values into the given columns. Columns in the table that are not given values are set to their default values or NULL. The second form takes the results of a SELECT query and inserts them into the table. The third form is simply an alternate version of the first form that more explicitly shows which columns correspond with which values. If the DELAYED modifier is present in the first form, all incoming SELECT statements will be given priority over the insert, which will wait until the other activity has finished before inserting the data. In a similar way, using the LOW_PRIORITY modifier with any form of INSERT causes the insertion to be postponed until all other operations from other clients have been finished.

Starting with MySQL 3.22.5, it is possible to insert more than one row into a table at a time. This is done by adding additional value lists to the statement separated by commas.

You must have INSERT privileges to use this statement.

Examples

# Insert a record into the 'people' table.
INSERT INTO people ( name, rank, serial_number )
VALUES ( 'Bob Smith', 'Captain', 12345 );
# Copy all records from 'data' that are older than a 
certain date into
# 'old_data'. This would usually be followed by deleting 
the old data from
# 'data'.
INSERT INTO old_data ( id, date, field )
SELECT ( id, date, field)
FROM data
WHERE date < 87459300;
# Insert 3 new records into the 'people' table.
INSERT INTO people (name, rank, serial_number )
VALUES ( 'Tim O\'Reilly', 'General', 1), 
 ('Andy Oram', 'Major', 4342),
 ('Randy Yarger', 'Private', 9943);

KILL thread_id

Terminates the specified thread. The thread ID numbers can be found using SHOW PROCESSLIST. Killing threads owned by users other than yourself requires PROCESS privilege. In MySQL 4.x, this privilege is now the SUPER privilege.

Example

# Terminate thread 3
KILL 3

LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL]
INFILE file [REPLACE|IGNORE]
INTO TABLE table [delimiters] [(columns)]

Reads a text file and inserts its data into a database table. This method of inserting data is much quicker than using multiple INSERT statements. Although the statement may be sent from all clients like any other SQL statement, the file referred to in the statement is assumed to be located on the server unless the LOCAL keyword is used. If the filename does not have a fully qualified path, MySQL looks under the directory of the current database for the file.

With no delimiters specified, LOAD DATA INFILE will assume that the file is tab delimited with character fields, special characters escaped with backslashes (\), and lines terminated with newline characters.

In addition to the default behavior, you may specify your own delimiters using the following keywords. Delimiters apply to all tables in the statement.

FIELDS TERMINATED BY 'c'

Specifies the character used to delimit the fields. Standard C language escape codes can be used to designate special characters. This value may contain more than one character. For example, FIELDS TERMINATED BY ',' denotes a comma-delimited file and FIELDS TERMINATED BY '\t' denotes tab delimited. The default value is tab delimited.

FIELDS ENCLOSED BY 'c'

Specifies the character used to enclose character strings. For example, FIELD ENCLOSED BY '"' would mean that a line containing "one, two", "other", "last" would be taken to have three fields:

• one, two

• other

• last

The default behavior is to assume that no quoting is used in the file.

FIELDS ESCAPED BY 'c'

Specifies the character used to indicate that the next character is not special, even though it would usually be a special character. For example, with FIELDS ESCAPED BY '^' a line consisting of First,Second^,Third,Fourth would be parsed as three fields: "First", "Second,Third", and "Fourth". The exceptions to this rule are the null characters. Assuming the FIELDS ESCAPED BY value is a backslash, \0 indicates an ASCII NUL (character number 0) and \N indicates a MySQL NULL value. The default value is the backslash character. Note that MySQL itself considers the backslash character to be special. Therefore, to indicate backslash in that statement, you must backslash the backslash like this: FIELDS ESCAPED BY '\\'.

IGNORE number LINES

Ignores the specified number of lines before it loads.

LINES TERMINATED BY 'c'

Specifies the character that indicates the start of a new record. This value can contain more than one character. For example, with LINES TERMINATED BY '.', a file consisting of a,b,c.d,e,f.g,h,k. would be parsed as three separate records, each containing three fields. The default is the newline character. This means that by default, MySQL assumes each line is a separate record.

By default, if a value read from the file is the same as an existing value in the table for a field that is part of a unique key, an error is given. If the REPLACE keyword is added to the statement, the entire row from the table will be replaced with values from the file. Conversely, the IGNORE keyword causes MySQL to ignore the new value and keep the old one.

The word NULL encountered in the data file is considered to indicate a null value unless the FIELDS ENCLOSED BY character encloses it, or if no FIELDS ENCLOSED BY clause is specified.

Using the same character for more than one delimiter can confuse MySQL. For example, FIELDS TERMINATED BY ',' ENCLOSED BY ',' would produce unpredictable behavior.

If a list of columns is provided, the data is inserted into those particular fields in the table. If no columns are provided, the number of fields in the data must match the number of fields in the table, and they must be in the same order as the fields are defined in the table.

You must have SELECT and INSERT privileges on the table to use this statement.

Example

# Load in the data contained in 'mydata.txt' into the 
table 'mydata'. Assume
# that the file is tab delimited with no quotes 
surrounding the fields.
LOAD DATA INFILE 'mydata.txt' INTO TABLE mydata
# Load in the data contained in 'newdata.txt' Look for two 
comma delimited
# fields and insert their values into the fields 'field1' 
and 'field2' in
# the 'newtable' table.
LOAD DATA INFILE 'newdata.txt'
INTO TABLE newtable
FIELDS TERMINATED BY ','
( field1, field2 )

LOCK TABLES name 
[AS alias] {READ | [READ LOCAL] | [LOW_PRIORITY] WRITE}
[, name2 [AS alias] {READ | [READ LOCAL] | LOW_PRIORITY] 
WRITE, ...]

Locks a table for the use of a specific thread. This command is generally used to emulate transactions. If a thread creates a READ lock, all other threads may read from the table, but only the controlling thread can write to the table. If a thread creates a WRITE lock, no other thread may read from or write to the table.

Example

# Lock tables 'table1' and 'table3' to prevent updates, 
and block all access 
# to 'table2'. Also create the alias 't3' for 'table3' in 
the current thread.
LOCK TABLES table1 READ, table2 WRITE, table3 AS t3 READ

OPTIMIZE TABLE name

Recreates a table, eliminating any wasted space and sorting any unsorted index pages. Also updates any statistics that are not currently up to date. This task is performed by creating the optimized table as a separate, temporary table and using it to replace the current table. This command currently works only for MyISAM and BDB tables. If you want the syntax to work no matter what table type you use, you should run mysqld with --skip-new or --safe-mode on. Under these circumstances, OPTIMIZE TABLE is an alias for ALTER TABLE.

Example

OPTIMIZE TABLE mytable

REPLACE [DELAYED | LOW_PRIORITY]
INTO table [(column, ...)]
VALUES (value, ...)
REPLACE [DELAYED | LOW_PRIORITY]
INTO table [(column, ...)]
SELECT select_clause
REPLACE [DELAYED | LOW_PRIORITY]
INTO table
SET column=value, column=value, ...

Inserts data into a table, replacing any old data that conflicts. This statement is identical to INSERT except that if a value conflicts with an existing unique key, the new value replaces the old one. The first form of this statement simply inserts the given values into the given columns. Columns in the table that are not given values are set to their default values or to NULL. The second form takes the results of a SELECT query and inserts them into the table. The final form inserts specific values using a syntax similar to an UPDATE statement.

Examples

# Insert a record into the 'people' table.
REPLACE INTO people ( name, rank, serial_number )
VALUES ( 'Bob Smith', 'Captain', 12345 )
# Copy all records from `data' that are older than a 
certain date into
# 'old_data'. This would usually be followed by deleting 
the old data from
# 'data'.
REPLACE INTO old_data ( id, date, field )
SELECT ( id, date, field)
FROM data
WHERE date < 87459300

REVOKE privilege [(column, ...)] [, privilege [(column, ...) ...]
ON table FROM user

Removes a privilege from a user. The values of privilege, table, and user are the same as for the GRANT statement. You must have the GRANT privilege to be able to execute this statement.

SELECT [STRAIGHT_JOIN]
[SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
[SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
[HIGH_PRIORITY]
[DISTINCT | | DISTINCTROW | ALL] 
column [[AS] alias][, ...]
[INTO {OUTFILE | DUMPFILE} 'filename' delimiters]
[FROM table [[AS] alias]
[USE INDEX (keys)] [IGNORE INDEX (keys)][, ...]
[constraints]]
[UNION [ALL] select substatement]

Retrieves data from a database. The SELECT statement is the primary method of reading data from database tables.

If the DISTINCT keyword is present, only one row of data will be output for every group of rows that is identical. The ALL keyword is the opposite of DISTINCT and displays all returned data. The default behavior is ALL. DISTINCT and DISTINCTROWS are synonyms.

MySQL provides several extensions to the basic ANSI SQL syntax that help modify how your query runs:

HIGH_PRIORITY

Increases the priority with which the query is run, even to the extent of ignoring tables waiting to be locked for update. You can cause database updates to grind to a halt if you use this option with long-running queries.

STRAIGHT_JOIN

If you specify more than one table, MySQL will automatically join the tables so that you can compare values between them. In cases where MySQL does not perform the join in an efficient manner, you can specify STRAIGHT_JOIN to force MySQL to join the tables in the order you enter them in the query.

SQL_BUFFER_RESULT

Forces MySQL to store the result in a temporary table.

SQL_CALC_FOUND_ROWS

Enables you to find out how many rows the query would return without a LIMIT clause. You can retrieve this value using SELECT FOUND_ROWS( ).

SQL_BIG_RESULT

SQL_SMALL_RESULT

Tells MySQL what size you think the result set will be for use with GROUP BY or DISTINCT. With small results, MySQL will place the results in fast temporary tables instead of using sorting. Big results, however, will be placed in disk-based temporary tables and use sorting.

SQL_CACHE

SQL_NO_CACHE

SQL_NO_CACHE dictates that MySQL should not store the query results in a query cache. SQL_CACHE, on the other hand, indicates that the results should be stored in a query cache if you are using cache on demand (SQL_QUERY_CACHE_TYPE=2).

The selected columns' values can be any one of the following:

Aliases

Any complex column name or function can be simplified by creating an alias for it. The value can be referred to by its alias anywhere else in the SELECT statement (e.g., SELECT DATE_FORMAT(date,"%W, %M %d %Y") as nice_date FROM calendar). You cannot use aliases in WHERE clauses, as their values are not be calculated at that point.

Column names

These can be specified as column, table.column or database.table.column. The longer forms are necessary only to disambiguate columns with the same name, but can be used at any time (e.g., SELECT name FROM people; SELECT mydata.people.name FROM people).

Functions

MySQL supports a wide range of built-in functions such as SELECT COS(angle) FROM triangle (see later). In addition, user defined functions can be added at any time using the CREATE FUNCTION statement.

By default, MySQL sends all output to the client that sent the query. It is possible however, to have the output redirected to a file. In this way you can dump the contents of a table (or selected parts of it) to a formatted file that can either be human readable, or formatted for easy parsing by another database system.

The INTO OUTFILE 'filename' modifier is the means in which output redirection is accomplished. With this, the results of the SELECT query are put into filename. The format of the file is determined by the delimiters arguments, which are the same as the LOAD DATA INFILE statement with the following additions:

• The OPTIONALLY keyword may be added to the FIELDS ENCLOSED BY modifier. This will cause MySQL to treat enclosed data as strings and non-enclosed data as numeric.

• Removing all field delimiters (i.e., FIELDS TERMINATED BY '' ENCLOSED BY '') will cause a fixed-width format to be used. Data will be exported according to the display size of each field. Many spreadsheets and desktop databases can import fixed-width format files. You must have FILE permissions to execute this command.

The default behavior with no delimiters is to export tab delimited data using backslash (\) as the escape character and to write one record per line. You may optionally specify a DUMPFILE instead of an OUTFILE. This syntax will cause a single row to be placed into the file with no field or line separators. It is used for outputting binary fields.

The list of tables to join may be specified in the following ways:

Table1, Table2, Table3, . . .

This is the simplest form. The tables are joined in the manner that MySQL deems most efficient. This method can also be written as Table1 JOIN Table2 JOIN Table3, .... The CROSS keyword can also be used, but it has no effect (e.g., Table1 CROSS JOIN Table2) Only rows that match the conditions for both columns are included in the joined table. For example, SELECT * FROM people, homes WHERE people.id=homes.owner would create a joined table containing the rows in the people table that have id fields that match the owner field in the homes table.

Like values, table names can also be aliased (e.g., SELECT t1.name, t2.address FROM long_table_name t1, longer_table_name t2)

Table1 INNER JOIN Table2 {[ON expr] | [USING (columns)]}

Performs a standard inner join. This method is identical to the method just described, except you specify the USING clause to describe the join columns instead of a WHERE clause.

Table1 STRAIGHT_JOIN Table2

This is identical to the first method, except that the left table is always read before the right table. This should be used if MySQL performs inefficient sorts by joining the tables in the wrong order.

Table1 LEFT [OUTER] JOIN Table2 ON expression

This checks the right table against the clause. For each row that does not match, a row of NULLs is used to join with the left table. Using the previous example, SELECT * FROM people, homes LEFT JOIN people, homes ON people.id=homes.owner, the joined table would contain all the rows that match in both tables, as well as any rows in the people table that do not have matching rows in the homes table; NULL values would be used for the homes fields in these rows. The OUTER keyword is optional and has no effect.

Table1 LEFT [OUTER] JOIN Table2 USING (column[, column2 . . .])

This joins the specified columns only if they exist in both tables (e.g., SELECT * FROM old LEFT OUTER JOIN new USING (id)).

Table1 NATURAL LEFT [OUTER] JOIN Table2

This joins only the columns that exist in both tables. This would be the same as using the previous method and specifying all the columns in both tables (e.g., SELECT rich_people.salary, poor_people.salary FROM rich_people NATURAL LEFT JOIN poor_people).

{oj Table1 LEFT OUTER JOIN Table2 ON clause }

This is identical to Table1 LEFT JOIN Table2 ON clause and is included only for ODBC compatibility.

MySQL also supports right joins using the same syntax as left joins—except for the OJ syntax. For portability, however, it is recommended that you formulate your joins as left joins.

If no constraints are provided, SELECT returns all the data in the selected tables. You may also optionally tell MySQL whether to use or ignore specific indexes on a join using USE INDEX and IGNORE INDEX.

The search constraints can contain any of the following substatements:

WHERE statement

The WHERE statement construct is the most common way of searching for data in SQL. This statement is usually a comparison of some type but can also include any of the following functions, except for the aggregate functions. Named values, such as column names and aliases, and literal numbers and strings can be used in the statement.

FOR UPDATE

Creates a write lock on the rows returned by the query. This constraint is useful if you intend to immediately modify the query data and update the database.

LOCK IN SHARE MODE

Creates a shared mode lock on the read so that the query returns no data that is part of an uncommitted transaction.

GROUP BY column[ , column2,...]

This gathers all the rows that contain data with some value from a certain column. This allows aggregate functions to be performed on the columns (e.g., SELECT name,MAX(age) FROM people GROUP BY name). The column value may be an unsigned integer representing a column number or a formula, instead of an actual column name.

HAVING clause

This is the same as a WHERE clause except it is performed upon the data that has already been retrieved from the database. The HAVING statement is a good place to perform aggregate functions on relatively small sets of data that have been retrieved from large tables. This way, the function does not have to act upon the whole table, only the data that has already been selected (e.g., SELECT name,MAX(age) FROM people GROUP BY name HAVING MAX(age)>80).

ORDER BY column [ ASC| DESC][ , column2 [ ASC| DESC] ,...]

Sorts the returned data using the given column(s). If DESC is present, the data is sorted in descending order, otherwise ascending order is used (e.g., SELECT name, age FROM people ORDER BY age DESC). Ascending order can also be explicitly stated with the ASC keyword. As with GROUP BY, the column value may be an unsigned integer or a formula (though not an aggregate), instead of the column name.

LIMIT [ start,] rows

Returns only the specified number of rows. If the start value is supplied, that many rows are skipped before the data is returned. The first row is number (e.g., SELECT url FROM links LIMIT 5,10 returns URLs numbered 5 through 14).

PROCEDURE name ([ arg_list])

In early versions of MySQL, this does not do anything. It was provided to make importing data from other SQL servers easier. Starting with MySQL 3.22, this substatement lets you specify a procedure that modifies the query result before returning it to the client.

SELECT supports functions. MySQL defines several built-in functions that can operate on the data in the table, returning the computed value(s) to the user. With some functions, the value returned depends on whether the user wants to receive a numerical or string value. This is regarded as the "context" of the function. When selecting values to be displayed to the user, only text context is used, but when selecting data to be inserted into a field, or to be used as the argument of another function, the context depends upon what the receiver is expecting. For instance, selecting data to be inserted into a numerical field will place the function into a numerical context.

MySQL 4.0 introduced support for unions. A UNION clause enables the results from two SELECT statements to be joined as a single result set. The two queries should have columns that match in type and number. Matching in type allows for columns to have types that are convertible.

Examples

# Find all names in the 'people' table where the 'state' 
Sfield is 'MI'.
SELECT name FROM people WHERE state='MI'
# Display all of the data in the `mytable' table.
SELECT * FROM mytable

SET OPTION SQL_OPTION=value

Defines an option for the current session. Values set by this statement are not in effect anywhere but the current connection, and they disappear at the end of the connection. The following options are currently supported:

AUTOCOMMIT=0 or 1

When set to the default value of 1, each statement sent to the database is automatically committed unless preceded by BEGIN. Otherwise, you need to send a COMMIT or ROLLBACK to end a transaction.

CHARACTER SET charsetname or DEFAULT

Changes the character set used by MySQL. Specifying DEFAULT will return to the original character set.

LAST_INSERT_ID =number

Determines the value returned from the LAST_INSERT_ID( ) function.

PASSWORD=PASSWORD('password')

Sets the password for the current user.

PASSWORD FOR user = PASSWORD('password')

Sets the password for the specified user.

SQL_AUTO_IS_NULL= 0 or 1

When set to the default value of 1, you can find the last inserted row in a table with WHERE auto_increment_column IS NULL.

SQL_BIG_SELECTS=0 or 1

Determines the behavior when a large SELECT query is encountered. If set to 1, MySQL will abort the query with an error, if the query would probably take too long to compute. MySQL decides that a query will take too long if it will have to examine more rows than the value of the max_join_size server variable. The default value of the variable is 0, which allows all queries.

SQL_BIG_TABLES=0 or 1

Determines the behavior of temporary tables (usually generated when dealing with large data sets). If this value is 1, temporary tables are stored on disk, which is slower than primary memory but can prevent errors on systems with low memory. The default value is 0, which stores temporary tables in RAM.

SQL_BUFFER_RESULT=0 or 1

A value of 1 is the same as specifying SQL_BUFFER_RESULT for every SELECT statement. It forces MySQL to place results into a temporary table.

SQL_LOG_OFF=0 or 1

When set to 1, turns off standard logging for the current session. This does not stop logging to the ISAM log or the update log. You must have PROCESS LIST (SUPER as of MySQL 4.0.2) privileges to use this option. The default is 0, which enables standard logging.

SQL_LOG_UPDATE=0 or 1

Enables a client to turn off its update log only if the client has PROCESS (SUPER as of MySQL 4.0.2) privileges.

SQL_LOW_PRIORITY_UPDATES=0 or 1

Tells MySQL to wait until no pending SELECT or LOCK TABLE READ is occurring on an affected table before executing a write statement.

SQL_MAX_JOIN_SIZE=value or DEFAULT

Prohibits MySQL from executing queries that will likely need more than the specified number of row combinations. If you set this value to anything other than the default, it will cause SQL_BIG_SELECTS to be reset. Resetting SQL_BIG_SELECTS will cause this value to be ignored.

SQL_QUERY_CACHE_TYPE=value

Tells MySQL not to cache or retrieve results (0 or OFF), to cache everything but SQL_NO_CACHE queries (1 or ON), or to cache only SQL_CACHE queries (2 or DEMAND).

SQL_SAFE_UPDATES=0 or 1

Prevents accidental executions of UPDATE or DELETE statements that do not have a WHERE clause or LIMIT set.

SQL_SELECT_LIMIT =number

The maximum number of records returned by a SELECT query. A LIMIT modifier in a SELECT statement overrides this value. The default behavior is to return all records.

SQL_UPDATE_LOG=0 or 1

When set to 0, turns off update logging for the current session. This does not affect standard logging or ISAM logging. You must have PROCESS LIST (SUPER as of MySQL 4.0.2) privileges to use this option. The default is 1, which enables update logging.

TIMESTAMP =value or DEFAULT

Determines the time used for the session. This time is logged to the update log and will be used if data is restored from the log. Specifying DEFAULT will return to the system time.

Example

# Turn off logging for the current connection.
SET OPTION SQL_LOG_OFF=1

SHOW [FULL] COLUMNS FROM table [FROM database] [LIKE clause]
SHOW DATABASES [LIKE clause]
SHOW FIELDS FROM table [FROM database] [LIKE clause]
SHOW GRANTS FOR userSHOW INDEX FROM table [FROM database]
SHOW KEYS FROM table [FROM database]
SHOW LOGS
SHOW MASTER STATUS
SHOW MASTER LOGS
SHOW [FULL] PROCESSLIST
SHOW SLAVE STATUS
SHOW STATUS [LIKE clause]
SHOW TABLE STATUS [FROM database [LIKE clause]]
SHOW [OPEN] TABLES [FROM database] [LIKE clause]
SHOW VARIABLES [LIKE clause]

Displays a lot of different information about the MySQL system. This statement can be used to examine the status or structure of almost any part of MySQL.

Examples

# Show the available databases
SHOW DATABASES;
# Display information on the indexes on table 'bigdata'
SHOW KEYS FROM bigdata;
# Display information on the indexes on table 'bigdata' 
# in the database 'mydata'
SHOW INDEX FROM bigdata FROM mydata;
# Show the tables available from the database 'mydata'
# that begin with the letter 'z'
SHOW TABLES FROM mydata LIKE 'z%';
# Display information about the columns on the table
# 'skates'
SHOW COLUMNS FROM stakes;
# Display information about the columns on the table
# 'people' that end with '_name'
SHOW FIELDS FROM people LIKE '%\_name';
# Show the threads
SHOW PROCESSLIST;
# Show server status information.
SHOW STATUS;
# Display server variables
SHOW VARIABLES;

TRUNCATE TABLE table

Drops and recreates the specified table.

Example

# Truncate the emp_data table
TRUNCATE TABLE emp_data;

UNLOCK TABLES

Unlocks all tables that were locked using the LOCK statement during the current connection.

Example

# Unlock all tables
UNLOCK TABLES

UPDATE [LOW_PRIORITY] [IGNORE] table 
SET column=value, ...
[WHERE clause]
[LIMIT n]

Alters data within a table. You may use the name of a column as a value when setting a new value. For example, UPDATE health SET miles_ran=miles_ran+5 would add five to the current value of the miles_ran column.

The WHERE clause limits updates to matching rows. The LIMIT clause ensures that only n rows change. The statement returns the number of rows changed.

You must have UPDATE privileges to use this statement.

Example

# Change the name 'John Deo' to 'John Doe' everywhere in 
the people table.
UPDATE people SET name='John Doe' WHERE name='John Deo'

USE database

Selects the default database. The database given in this statement is used as the default database for subsequent queries. Other databases may still be explicitly specified using the database.table.column notation.

Example

# Make db1 the default database.
USE db1