在SELECT语句中使用条件逻辑

select ename, sal,
       case when sal <= 2000 then 'UNDERPAID'
                 when sal >= 4000 then 'OVERPAID'
                 else 'OK'
       end as status
  from emp 

ENAME               SAL     STATUS
--------------------------------------------------
SMITH                 800     UNDERPAID
ALLEN              1600     UNDERPAID
WARD               1250     UNDERPAID
JONES              2975    OK
MARTIN            1250     UNDERPAID
BLAKE              2850     OK
CLARK              2450     OK
SCOTT             3000     OK
KING                 5000     OVERPAID
TURNER          1500     UNDERPAID
ADAMS              1100     UNDERPAID
JAMES                 950     UNDERPAID
FORD                3000     OK
MILLER             1300     UNDERPAID