Syntax options:

    WHEN literal | WHEN NULL

    The literal or NULL value of the WHEN clause that SQL compares
    with the value expression of the CASE clause. Most CASE control
    statements include a set of WHEN clauses.

    When the values of the WHEN and CASE clauses match, SQL executes
    the SQL statements associated with that WHEN clause. Control then
    drops out of the CASE control statement and returns to the next
    SQL statement after the END CASE clause.