SQL CASE WHEN THEN ELSE END用法

 <sql id="SQL.common.SonyWorkDayCheck">
    SELECT
      <expr var="COMPAY_PLAN_DATE">
          CASE EXTRACT( day from ?)
      </expr>
      WHEN 1 THEN DAY_1
      WHEN 2 THEN DAY_2
      WHEN 3 THEN DAY_3
      WHEN 4 THEN DAY_4
      WHEN 5 THEN DAY_5
      WHEN 6 THEN DAY_6
      WHEN 7 THEN DAY_7
      WHEN 8 THEN DAY_8
      WHEN 9 THEN DAY_9
      WHEN 10 THEN DAY_10
      WHEN 11 THEN DAY_11
      WHEN 12 THEN DAY_12
      WHEN 13 THEN DAY_13
      WHEN 14 THEN DAY_14
      WHEN 15 THEN DAY_15
      WHEN 16 THEN DAY_16
      WHEN 17 THEN DAY_17
      WHEN 18 THEN DAY_18
      WHEN 19 THEN DAY_19
      WHEN 20 THEN DAY_20
      WHEN 21 THEN DAY_21
      WHEN 22 THEN DAY_22
      WHEN 23 THEN DAY_23
      WHEN 24 THEN DAY_24
      WHEN 25 THEN DAY_25
      WHEN 26 THEN DAY_26
      WHEN 27 THEN DAY_27
      WHEN 28 THEN DAY_28
      WHEN 29 THEN DAY_29
      WHEN 30 THEN DAY_30
      ELSE
          DAY_31
      END WORK_DAY
    FROM
        T_M_WORKDAY_CALENDAR
   <where joint="and">
     <cond var="PRIMO_COMPANY_CODE"> PRIMO_COMPANY_CODE =?</cond>
     <cond var="COMPAY_PLAN_DATE">TO_CHAR(CALENDAR_YYYYMM,'YYYYMM')= TO_CHAR(?,'YYYYMM') </cond>
    </where>
 </sql>