1? label:
2 CASE
3 simple-when-clause
3 searched-when-clause
4? else-clause
5 END CASE 

simple-when-clause

1 expression
2+ 
2 WHEN expression THEN
2.1+ 
2.1 SQL-procedure-statement  ; 

searched-when-clause

1+ 
1 WHEN search-condition THEN
1.1+ 
1.1 SQL-procedure-statement  ; 

else-clause

1 ELSE
2+ 
2 SQL-procedure-statement  ;