Mastering the WHERE Clause - Placement of Join Conditions
(Page 8 of 8 )
Throughout this chapter, all examples that join multiple tables have had their join conditions included in the WHERE clause along with various filter conditions. Beginning with the Oracle9i release, you have the option of using the ANSI join syntax, which specifies that all join conditions be included in the FROM clause, as illustrated by the following:
SELECT p.part_nbr, p.name, p.supplier_id, p.status, p.inventory_qty,
s.supplier_id, s.name
FROM part p INNER JOIN supplier s
ON s.supplier_id = p.supplier_id
WHERE s.name NOT IN ('Acme Industries', 'Tilton Enterprises');
As you can see, the join condition s.supplier_id = p.supplier_id has been moved to the ON subclause, and the FROM clause specifies that the part and supplier tables be joined via an inner join. This syntax may look a bit strange at first, but it greatly improves the readability and maintainability of your queries. Therefore, for the remainder of this book, all examples will employ the ANSI join syntax.
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 15, 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 FROM/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.
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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