Sams Teach Yourself SQL in 10 Minutes, Second Edition

Guidelines for Updating and Deleting Data

The UPDATE and DELETE statements used in the previous section all have WHERE clauses, and there is a very good reason for this. If you omit the WHERE clause, the UPDATE or DELETE will be applied to every row in the table. In other words, if you execute an UPDATE without a WHERE clause, every row in the table will be updated with the new values. Similarly if you execute DELETE without a WHERE clause, all the contents of the table will be deleted.

Here are some important guidelines that many SQL programmers follow:

• Never execute an UPDATE or a DELETE without a WHERE clause unless you really do intend to update and delete every row.

• Make sure every table has a primary key (refer back to Lesson 12, “Joining ...