Sams Teach Yourself Transact-SQL in 21 Days, Second Edition

Filtering Data with WHERE Clauses

The first half of today's lesson will show you how to use a WHERE clause to restrict the rows returned from a table. Learning to limit rows with WHERE will improve system performance by reducing the amount of data returned to (and processed by) your application programs.

Queries that don't include a WHERE clause operate on all rows in a table. Later, when you learn to UPDATE and DELETE data, you will use WHERE clauses to determine what data in your tables is deleted and/or updated. If you don't include a WHERE clause in a DELETE statement, you will delete every row in the table, which is not a good idea. An example of an update that affects all the rows in a table is

 Update Orders Set OrderDate = '1/1/2000'  ...