Arranging Elements in a Query
The best arrangement of elements within your query, particularly in the WHERE clause, really depends on the order of the processing steps in a specific implementation. The arrangement of conditions depends on the columns that are indexed, as well as on which condition will retrieve the fewest records.
You do not have to use a column that is indexed in the WHERE clause, but it is obviously more beneficial to do so. Try to narrow down the results of the SQL statement by using an index that returns the fewest number of rows. If you recall, the condition that returns the fewest records in a table is said to be the most restrictive condition. As a general statement, you should place the most restrictive conditions last ...
Get Sams Teach Yourself SQL in 21 Days, Fourth Edition now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.
