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1.4 DML StatementsIn this section, we introduce the four statements that comprise the DML portion of SQL. The information presented in this section should be enough to allow you to start writing DML statements. As is discussed at the end of the section, however, DML can look deceptively simple, so keep in mind while reading the section that there are many more facets to DML than are discussed here. 1.4.1 The SELECT StatementThe SELECT statement is used to retrieve data from a database. The set of data retrieved via a SELECT statement is referred to as a result set. Like a table, a result set is comprised of rows and columns, making it possible to populate a table using the result set of a SELECT statement. The SELECT statement can be summarized as follows: SELECT <one or more things> FROM <one or more places> WHERE <zero, one, or more conditions apply> While the SELECT and FROM clauses are required, the WHERE clause is optional (although you will seldom see it omitted). We therefore begin with a simple example that retrieves three columns from every row of the customer table: SELECT cust_nbr, name, region_id
FROM customer;
CUST_NBR NAME REGION_ID
---------- ------------------------------ ----------
1 Cooper Industries 5
2 Emblazon Corp. 5
3 Ditech Corp. 5
4 Flowtech Inc. 5
5 Gentech Industries 5
6 Spartan Industries 6
7 Wallace Labs 6
8 Zantech Inc. 6
9 Cardinal Technologies 6
10 Flowrite Corp. 6
11 Glaven Technologies 7
12 Johnson Labs 7
13 Kimball Corp. 7
14 Madden Industries 7
15 Turntech Inc. 7
16 Paulson Labs 8
17 Evans Supply Corp. 8
18 Spalding Medical Inc. 8
19 Kendall-Taylor Corp. 8
20 Malden Labs 8
21 Crimson Medical Inc. 9
22 Nichols Industries 9
23 Owens-Baxter Corp. 9
24 Jackson Medical Inc. 9
25 Worcester Technologies 9
26 Alpha Technologies 10
27 Phillips Labs 10
28 Jaztech Corp. 10
29 Madden-Taylor Inc. 10
30 Wallace Industries 10
Since we neglected to impose any conditions via a WHERE clause, our query returns every row from the customer table. If we want to restrict the set of data returned by the query, we could include a WHERE clause with a single condition: SELECT cust_nbr, name, region_id
FROM customer
WHERE region_id = 8;
CUST_NBR NAME REGION_ID
---------- ------------------------------ ----------
16 Paulson Labs 8
17 Evans Supply Corp. 8
18 Spalding Medical Inc. 8
19 Kendall-Taylor Corp. 8
20 Malden Labs 8
Our result set now includes only those customers residing in the region with a region_id of 8. But what if we want to specify a region by name instead of region_id? We could query the region table for a particular name and then query the customer table using the retrieved region_id. Instead of issuing two different queries, however, we could produce the same outcome using a single query by introducing a join, as in: SELECT customer.cust_nbr, customer.name, region.name
FROM customer, region
WHERE region.name = 'New England'
AND region.region_id = customer.region_id;
CUST_NBR NAME NAME
---------- ------------------------------ -----------
1 Cooper Industries New England
2 Emblazon Corp. New England
3 Ditech Corp. New England
4 Flowtech Inc. New England
5 Gentech Industries New England
Our FROM clause now contains two tables instead of one, and the WHERE clause contains a join condition that specifies that the customer and region tables are to be joined using the region_id column found in both tables. Joins and join conditions will be explored in detail in Chapter 3. Since both the customer and region tables contain a column called name, you must specify which table's name column you are interested in. This is done in the previous example by using dot-notation to append the table name in front of each column name. If you would rather not type the full table names, you can assign table aliases to each table in the FROM clause and use those aliases instead of the table names in the SELECT and WHERE clauses, as in: SELECT c.cust_nbr, c.name, r.name FROM customer c, region r WHERE r.name = `New England' AND r.region_id = c.region_id; In this example, we assigned the alias "c" to the customer table and the alias "r" to the region table. Thus, we can use "c." and "r." instead of "customer." and "region." in the SELECT and WHERE clauses. 1.4.1.1 SELECT clause elementsIn the examples thus far, the result sets generated by our queries have contained columns from one or more tables. While most elements in your SELECT clauses will typically be simple column references, a SELECT clause may also include:
While the first three items in this list are fairly straightforward, the last item merits further discussion. Oracle makes available several phantom columns, known as pseudocolumns, that do not exist in any tables. Rather, they are values visible during query execution that can be helpful in certain situations. For example, the pseudocolumn ROWID represents the physical location of a row. This information represents the fastest possible access mechanism. It can be useful if you plan to delete or update a row retrieved via a query. However, you should never store ROWID values in the database, nor should you reference them outside of the transaction in which they are retrieved, since a row's ROWID can change in certain situations, and ROWIDs can be reused after a row has been deleted. The next example demonstrates each of the different elements from the previous list: SELECT rownum,
cust_nbr,
1 multiplier,
'cust # ' || cust_nbr cust_nbr_str,
'hello' greeting,
TO_CHAR(last_order_dt, 'DD-MON-YYYY') last_order
FROM customer;
ROWNUM CUST_NBR MULTIPLIER CUST_NBR_STR GREETING LAST_ORDER
------ -------- ---------- ------------ -------- -----------
1 1 1 cust # 1 hello 15-JUN-2000
2 2 1 cust # 2 hello 27-JUN-2000
3 3 1 cust # 3 hello 07-JUL-2000
4 4 1 cust # 4 hello 15-JUL-2000
5 5 1 cust # 5 hello 01-JUN-2000
6 6 1 cust # 6 hello 10-JUN-2000
7 7 1 cust # 7 hello 17-JUN-2000
8 8 1 cust # 8 hello 22-JUN-2000
9 9 1 cust # 9 hello 25-JUN-2000
10 10 1 cust # 10 hello 01-JUN-2000
11 11 1 cust # 11 hello 05-JUN-2000
12 12 1 cust # 12 hello 07-JUN-2000
13 13 1 cust # 13 hello 07-JUN-2000
14 14 1 cust # 14 hello 05-JUN-2000
15 15 1 cust # 15 hello 01-JUN-2000
16 16 1 cust # 16 hello 31-MAY-2000
17 17 1 cust # 17 hello 28-MAY-2000
18 18 1 cust # 18 hello 23-MAY-2000
19 19 1 cust # 19 hello 16-MAY-2000
20 20 1 cust # 20 hello 01-JUN-2000
21 21 1 cust # 21 hello 26-MAY-2000
22 22 1 cust # 22 hello 18-MAY-2000
23 23 1 cust # 23 hello 08-MAY-2000
24 24 1 cust # 24 hello 26-APR-2000
25 25 1 cust # 25 hello 01-JUN-2000
26 26 1 cust # 26 hello 21-MAY-2000
27 27 1 cust # 27 hello 08-MAY-2000
28 28 1 cust # 28 hello 23-APR-2000
29 29 1 cust # 29 hello 06-APR-2000
30 30 1 cust # 30 hello 01-JUN-2000
Interestingly, your SELECT clause is not required to reference columns from any of the tables in the FROM clause. For example, the next query's result set is composed entirely of literals: SELECT 1 num, 'abc' str
FROM customer;
NUM STR
---------- ---
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
1 abc
Since there are 30 rows in the customer table, the query's result set includes 30 identical rows of data. 1.4.1.2 Ordering your resultsIn general, there is no guarantee that the result set generated by your query will be in any particular order. If you want your results to be sorted by one or more columns, you can add an ORDER BY clause after the WHERE clause. The following example sorts the results from our New England query by customer name: SELECT c.cust_nbr, c.name, r.name
FROM customer c, region r
WHERE r.name = 'New England'
AND r.region_id = c.region_id
ORDER BY c.name;
CUST_NBR NAME NAME
-------- ------------------------------ -----------
1 Cooper Industries New England
3 Ditech Corp. New England
2 Emblazon Corp. New England
4 Flowtech Inc. New England
5 Gentech Industries New England
You may also designate the sort column(s) by their position in the SELECT clause. To sort the previous query by customer number, which is the first column in the SELECT clause, you could issue the following statement: SELECT c.cust_nbr, c.name, r.name
FROM customer c, region r
WHERE r.name = 'New England'
AND r.region_id = c.region_id
ORDER BY 1;
CUST_NBR NAME NAME
---------- ------------------------------ -----------
1 Cooper Industries New England
2 Emblazon Corp. New England
3 Ditech Corp. New England
4 Flowtech Inc. New England
5 Gentech Industries New England
Specifying sort keys by position will certainly save you some typing, but it can often lead to errors if you later change the order of the columns in your SELECT clause. 1.4.1.3 Removing duplicatesIn some cases, your result set may contain duplicate data. For example, if you are compiling a list of parts that were included in last month's orders, the same part number would appear multiple times if more than one order included that part. If you want duplicates removed from your result set, you can include the DISTINCT keyword in your SELECT clause, as in: SELECT DISTINCT li.part_nbr
FROM cust_order co, line_item li
WHERE co.order_dt >= TO_DATE('01-JUL-2001','DD-MON-YYYY')
AND co.order_dt < TO_DATE('01-AUG-2001','DD-MON-YYYY')
AND co.order_nbr = li.order_nbr;
This query returns the distinct set of parts ordered during July of 2001. Without the DISTINCT keyword, the result set would contain one row for every line-item of every order, and the same part would appear multiple times if it was included in multiple orders. When deciding whether to include DISTINCT in your SELECT clause, keep in mind that finding and removing duplicates necessitates a sort operation, which can add quite a bit of overhead to your query. 1.4.2 The INSERT StatementThe INSERT statement is the mechanism for loading data into your database. Data can be inserted into only one table at a time, although the data being loaded into the table can be pulled from one or more additional tables. When inserting data into a table, you do not need to provide values for every column in the table; however, you need to be aware of the columns that require non-NULL[3] values and the ones that do not. Let's look at the definition of the employee table:
describe employee Name Null? Type ----------------------------------------- -------- ------------ EMP_ID NOT NULL NUMBER(5) FNAME VARCHAR2(20) LNAME NOT NULL VARCHAR2(20) DEPT_ID NOT NULL NUMBER(5) MANAGER_EMP_ID NUMBER(5) SALARY NUMBER(5) HIRE_DATE DATE JOB_ID NUMBER(3) The NOT NULL designation for the emp_id, lname, and dept_id columns indicates that values are required for these three columns. Therefore, we must be sure to provide values for at least these three columns in our INSERT statements, as demonstrated by the following: INSERT INTO employee (emp_id, lname, dept_id) VALUES (101, 'Smith', 2); The VALUES clause must contain the same number of elements as the column list, and the data types must match the column definitions. In the example, emp_id and dept_id hold numeric values while lname holds character data, so our INSERT statement will execute without error. Oracle always tries to convert data from one type to another automatically, however, so the following statement will also run without errors: INSERT INTO employee (emp_id, lname, dept_id)
VALUES ('101', 'Smith', '2');
Sometimes, the data to be inserted needs to be retrieved from one or more tables. Since the SELECT statement generates a result set consisting of rows and columns of data, you can feed the result set from a SELECT statement directly into an INSERT statement, as in: INSERT INTO employee (emp_id, fname, lname, dept_id, hire_date)
SELECT 101, 'Dave', 'Smith', d.dept_id, SYSDATE
FROM department d
WHERE d.name = 'Accounting';
In this example, the purpose of the SELECT statement is to retrieve the department ID for the Accounting department. The other four columns in the SELECT clause are supplied as literals. 1.4.3 The DELETE StatementThe DELETE statement facilitates the removal of data from the database. Like the SELECT statement, the DELETE statement contains a WHERE clause that specifies the conditions used to identify rows to be deleted. If you neglect to add a WHERE clause to your DELETE statement, all rows will be deleted from the target table. The following statement will delete all employees with the last name of Hooper from the employee table: DELETE FROM employee WHERE lname = 'Hooper'; In some cases, the values needed for one or more of the conditions in your WHERE clause exist in another table. For example, your company may decide to outsource its accounting functions, thereby necessitating the removal of all Accounting personnel from the employee table: DELETE FROM employee WHERE dept_id = (SELECT dept_id FROM department WHERE name = 'Accounting'); The use of the SELECT statement in this example is known as a subquery and will be studied in detail in Chapter 5. 1.4.4 The UPDATE StatementModifications to existing data are handled by the UPDATE statement. Like the DELETE statement, the UPDATE statement includes a WHERE clause in order to specify which rows should be targeted. The following example shows how you might give a 10% raise to everyone making less than $40,000: UPDATE employee SET salary = salary * 1.1 WHERE salary < 40000; If you want to modify more than one column in the table, you have two choices: provide a set of column/value pairs separated by commas, or provide a set of columns and a subquery. The following two UPDATE statements modify the inactive_dt and inactive_ind columns in the customer table for any customer who hasn't placed an order in the past year: UPDATE customer SET inactive_dt = SYSDATE, inactive_ind = 'Y' WHERE last_order_dt < SYSDATE -- 365; UPDATE customer SET (inactive_dt, inactive_ind) = (SELECT SYSDATE, 'Y' FROM dual) WHERE last_order_dt < SYSDATE -- 365; The subquery in the second example is a bit forced, since it uses a query against the dual[4] table to build a result set containing two literals, but it should give you an idea of how you would use a subquery in an UPDATE statement. In later chapters, you will see far more interesting uses for subqueries.
1.4.5 So Why Are There 13 More Chapters?After reading this chapter, you might think that SQL looks pretty simple (at least the DML portion). At a high level, it is fairly simple, and you now know enough about the language to go write some code. However, you will learn over time that there are numerous ways to arrive at the same end point, and some are more efficient and elegant than others. The true test of SQL mastery is when you no longer have the desire to return to what you were working on the previous year, rip out all the SQL, and recode it. For one of us, it took about nine years to reach that point. Hopefully, this book will help you reach that point in far less time. While you are reading the rest of the book, you might notice that the majority of examples use SELECT statements, with the remainder somewhat evenly distributed across INSERT, UPDATE, and DELETE statements. This disparity is not indicative of the relative importance of SELECT statements over the other three DML statements; rather, SELECT statements are favored because we can show the query's result set, which should help you to better understand the query, and because many of the points being made using SELECT statements can be applied to UPDATE and DELETE statements as well. |
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