In the simple CASE, a value expression follows the CASE keyword. This expression is tested for equality against the value expression in each WHEN clause. If a truth condition is encountered, testing of any remaining conditions is halted and the value expression for the current THEN clause is returned. If no comparison evaluates to true, then the default value expression in the ELSE clause is returned. If there is no ELSE clause, a null value is returned. At least one of the THEN clauses must specify a value other than NULL. All comparisons must involve compatible datatypes.