Just add the decode

SELECT * FROM table WHERE field = value

In most cases in SQL, if you want to add a condition, you should use a WHERE clause as @StupidGirl points out. This is true in SELECT, UPDATE, and DELETE statements. The exception is aggregate functions, but that’s a different subject entirely.

If you’re new to SQL, you might consider a book on the subject. SQL isn’t terribly difficult, but it’s not necessarily intuitive. I hear this is a pretty good book for getting started with SQL.