1  –  CASE

    An expression that evaluates to a single value. SQL compares the
    CASE clause value expression with each WHEN clause literal value
    in the WHEN clauses until it finds a match.

    The value expression cannot contain a column specification that
    is not part of a column select expression.

    See the Value_Expressions HELP topic for a complete description
    of the variety of value expressions that SQL provides.

2  –  ELSE

    Executes a set of SQL statements when SQL cannot find a WHEN
    clause that matches the value expression in the CASE clause.

    See the Compound_Statement HELP topic for a description of the
    SQL statements that are valid in a compound statement.

3  –  THEN

    Executes the set of SQL statements associated with the first
    WHEN clause in which its argument value matches the CASE value
    expression.

4  –  WHEN

    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.