2.5 WHERE to Go from Here
This chapter has introduced the role of the WHERE clause in different
types of SQL statements as well as the various components used to
build a WHERE clause. Because the WHERE clause plays such an
important role in many SQL statements, however, the topic is far from
exhausted. Additional coverage of WHERE clause topics may be found
in:
Chapter 3, in which various flavors of join
conditions are studied in detail
Chapter 5, which probes the different types of
subqueries along with the appropriate operators for evaluating their
results
Chapter 6, in which various methods of handling
date/time data are explored
Chapter 14, which explores certain aspects of the
WHERE clause from the standpoint of performance and efficiency
Additionally, here are a few tips to
help you make the most of your WHERE clauses:
Check your join
conditions carefully. Make sure that
each data set in the FROM clause is properly joined. Keep in mind
that some joins require multiple conditions. See Chapter 3 for more information.
Avoid unnecessary joins. Just because two data sets in your FROM
clause contain the same column does not necessitate a join condition
be added to your WHERE clause. In some designs, redundant data has
been propagated to multiple tables through a process called
denormalization. Take the time to understand the
database design, and ask your DBA or database designer for a current
data model.
Use parentheses. Oracle
maintains both operator precedence
and condition precedence, meaning there are clearly defined rules for
the order in which things will be evaluated, but the safest route for
you and for those who will later maintain your code is to dictate
evaluation order using parentheses. For operators, specifying
(5 *
p.inventory_qty) +
2 rather than 5
* p.inventory_qty
+ 2 makes the order in which
the operations should be performed clear. For conditions, use
parentheses any time the OR operator is employed.
Use consistent
indentation. For example, if the previous
line contains a left parenthesis without a matching right
parenthesis, indent the current line to show that it is a
continuation of the previous line.
When using OR, put the
condition requiring the least
effort to evaluate first. If the first condition evaluates to TRUE,
Oracle won't bother evaluating the remaining
OR'd conditions, possibly saving significant
execution time. This strategy is useful with correlated subqueries,
which are generally executed once per candidate row.
Handle NULLs properly. After writing your WHERE clause, inspect each
condition with respect to its ability to properly handle NULL values.
Take the time to understand the table definitions in your database so
that you know which columns allow NULLs.
Pick up introductory books on logic and set theory at your local
library. While understanding these two topics won't
necessarily get you invited to more cocktail parties, it will
certainly make you a better SQL programmer.
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