The previous section introduced one of the most important SQL concepts, the WHERE clause. In SQL, a WHERE clause enables you to pick out specific rows in a table by specifying a value that must be matched by the column in question. For example:

UPDATE bands
SET lead_singer = 'Ian Anderson'
WHERE band_name = 'Jethro Tull'

This UPDATE specifies that you should change only the lead_singer column for the row where band_name is identical to Jethro Tull. If the band_name column is not a unique index, that WHERE clause may match multiple rows. Many SQL commands employ WHERE clauses to help pick out the rows on which you wish to operate. Because the columns in the WHERE clause are columns on which you search, you should generally have indexes created around whatever combinations you commonly use. We discuss the kinds of comparisons you can perform in the WHERE clause later in the chapter.