Joe Celko's SQL for Smarties, 3rd Edition

Chapter 11. CASE Expressions

THE CASE EXPRESSION IS probably the most useful addition in SQL-92. This is a quick overview of how to use the expression, but you will find other tricks spread throughout the book.

The reason it is so important is that:

1.	It works with any data type.
2.	It allows the programmer to avoid procedural code by replacing IF-THEN-ELSE control flow with CASE expression inside the query.
3.	It makes SQL statements equivalent to primitive recursive functions. You can look up what that means in a book on the theory of computation, but it is a nice mathematical property that guarantees certain kinds of problems can be solved.