1 CASE 2 %simple-when-clause 2 %searched-when-clause 3! ELSE NULL 3? 3 ELSE result_expression 4 END simple-when 子句 1 source_value 2+ 2 WHEN test_value THEN 2.1 result_value 2.1 NULL searched-when 子句 1+ 1 WHEN search_condition THEN 1.1 result_value 1.1 NULL