Occasionally, you will want to update all rows in a table. To do this, you use UPDATE, too—you just omit the WHERE clause, or specify a WHERE clause that matches multiple rows.
When updating multiple rows using a WHERE clause, always be sure to test that WHERE clause with a simple SELECT statement before executing the UPDATE. If the SELECT returns the correct data (i.e., the data you want updated), you’ll know that it is safe to use with UPDATE. If you don’t, you might update the wrong data!
Before executing INSERT, UPDATE, or DELETE operations that contain complex statements or WHERE conditions, you should test the statement or condition by using it in a SELECT statement. If SELECT returns incorrect statement results ...