Guidelines for Updating and Deleting Data

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

Here are some best practices 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 to Chapter 15, "Joining Tables," if you have forgotten what this is), and use it as the WHERE clause whenever possible. (You may specify individual primary keys, multiple values, or value ranges.)

• Before you use a WHERE clause with an UPDATE or a DELETE, first test it with a SELECT to make sure it is filtering the right recordsit is far too easy to write incorrect WHERE clauses.

• Use database enforced referential integrity (refer to Chapter 15 for this one, too) so MySQL will not allow the deletion of rows that have data in other tables related to them.

Caution

Use with Caution The bottom line is that MySQL has no Undo button. Be very careful using UPDATE and DELETE, or you'll find yourself updating and deleting the wrong data.