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 it works with any data type.
• It allows the programmer to avoid procedural code by replacing IF-THEN-ELSE control flow with CASE expression inside the query. This helps the optimizer.
• It makes SQL statements equivalent to primitive recursive functions. Only a math major cares about this, but it is important.
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.