Working with the CASE clause

The CASE clause is an expression constructed that is used to transform results. There are two different forms of CASE expression and they are as follows:

• A simple CASE form to compare against multiple values
• A generic CASE form to express multiple conditional expressions

We will take a look at the simple CASE expression first.

Working with simple CASE expressions

In simple CASE expressions, the expression is evaluated and compared to the WHEN clauses. The corresponding expression is then evaluated and the resulting value is returned. If no value is found, the ELSE clause expression is evaluated and the corresponding value is returned. If there is no ELSE clause, then a null value is returned.

The syntactic representation of this looks like this:

CASE test
  WHEN value THEN result
  [WHEN ...]
  [ELSE default]
END

We can see from this syntax that the first CASE expression is evaluated and its...