Joe Celko's SQL for Smarties, 3rd Edition

11.1. The CASE Expression

The CASE expression allows the programmer to pick a value based on a logical expression in his code. ANSI stole the idea and the syntax from the now-defunct Ada programming language. Here is the syntax for a <case specification>:

 <case specification> ::= <simple case> | <searched case> <simple case> ::= CASE <case operand> <simple when clause>... [<else clause>] END <searched case> ::= CASE <searched when clause>... [<else clause>] END <simple when clause> ::= WHEN <when operand> THEN <result> <searched when clause> ::= WHEN <search condition> THEN <result> <else clause> ::= ELSE <result> <case operand> ::= <value expression> <when operand> ::= <value expression> <result> ::= <result expression> | NULL <result expression> ...