oralce CASE WHEN 用法

如何将时间日期分隔成“上半月”“下半月”

select 
  (CASE WHEN TO_NUMBER(TO_CHAR(JYSJ,'DD'))<=15 THEN '上半月'
        WHEN TO_NUMBER(TO_CHAR(JYSJ,'DD'))>15 THEN '下半月'
        ELSE 'NULL'
   END) AS mon
from BFBHDD8.SKTXSJL