1  –  ELSE

    Specifies the set of SQL statements to be executed when the WHEN
    clause evaluates to FALSE or UNKNOWN.

2  –  THEN

    Specifies the set of SQL statements to be executed when the WHEN
    clause evaluates to TRUE.

3  –  WHEN

    Determines whether the compound use statements in the THEN clause
    are to be executed or the compound use statements in the ELSE
    clause are to be executed. If the predicate evaluates to TRUE,
    then the compound use statements in the THEN clause are executed.
    If the predicate evaluates to FALSE or UNKNOWN, then the compound
    use statements in the ELSE clause are executed.