1 CASE
2 %simple-when-clause
2 %searched-when-clause
3! ELSE NULL
3?
3 ELSE  result_expression
4 END

simple-when-clause

1 source_value
2+ 
2 WHEN  test_value THEN
2.1 result_value
2.1 NULL

searched-when-clause

1+ 
1 WHEN  search_condition THEN
1.1 result_value
1.1 NULL