语法: CASE selector WHEN exp1 then res1 WHEN exp2 then res2 WHEN exp3 then res3 ... [ELSE resN] end; SQL> declare 2 mygrade char(1):='A'; 3 res varchar2(20); 4 begin 5 res := 6 CASE mygrade 7 WHEN 'A' THEN 'The mark is 90-100' 8 WHEN 'B' THEN 'The mark