1 CASE
2 searched-when-clause
2 simple-when-clause
3! ELSE NULL
3?
3 ELSE result-expression
4 END

searched-when-clause:

1+ 
1 WHEN search-condition THEN
1.1 result-expression
1.1 NULL

simple-when-clause:

1 expression
2+ 
2 WHEN expression THEN
2.1 result-expression
2.1 NULL