SQL有关问题,哪位高手能帮小弟我讲清楚,多谢大家了!

SQL问题,谁能帮我讲清楚,谢谢大家了!!!
SELECT   CUSTOMERMST.CUSTOMERCD   CUSTOMERMST.CUSTOMERNM,SUBUNITCD   =   CASE   CUSTOMERMST.CSTKNDCD       WHEN   '5 '   THEN   '001 '   ELSE   SUBUNITMST.SUBUNITCD   END   ,CUSTOMERMST.TRADEMARKCD   FROM   CUSTOMERMST   INNER   JOIN   SUBUNITMST   ON   CUSTOMERMST.SUBUNITCD   =   SUBUNITMST.SUBUNITCD   WHERE   1=1


其中   SUBUNITCD   =   CASE   CUSTOMERMST.CSTKNDCD       WHEN   '5 '   THEN   '001 '   ELSE   SUBUNITMST.SUBUNITCD   END   什么意思,谢谢大家了帮帮忙

------解决方案--------------------
如果
CUSTOMERMST.CSTKNDCD = 5
SUBUNITCD = 001
否则
SUBUNITCD = SUBUNITMST.SUBUNITCD