DB2 case when语句用法

[coff_cola]

我问一个动态控制表连接条件的

SELECT
    A.ORDER_NUMBER,
    A.ORIGINAL_ORDER_NUMBER,
    A.ORDER_TYPE,
    A.BULK_TRADE_FLAG,
    A.ORDER_STATUS_CODE,
    A.DEAL_LOG_CODE,
    A.LEGAL_ENTITY_CODE,
     B.STATUS_TYPE
FROM OMS.A A
    LEFT JOIN OMS.B B ON  ( A.ORIGINAL_ORDER_NUMBER = B.ORIGINAL_ORDER_NUMBER
    AND
    CASE  WHEN A.LEGAL_ENTITY_CODE in ('509','637')
    THEN
      A.CUSTOMER_NUMBER = B.CUSTOMER_NUMBER  AND A.SUB_ACCOUNT_NUMBER =   B.SUB_ACCOUNT_NUMBER
    ELSE
     1=1
    END
    )

意思就是当A.LEGAL_ENTITY_CODE in ('509','637') 时候A 和 B的连接条件是
A.ORIGINAL_ORDER_NUMBER = B.ORIGINAL_ORDER_NUMBER
AND
A.CUSTOMER_NUMBER = B.CUSTOMER_NUMBER
AND
A.SUB_ACCOUNT_NUMBER = B.SUB_ACCOUNT_NUMBER


否则就是 A.ORIGINAL_ORDER_NUMBER = B.ORIGINAL_ORDER_NUMBER

不过then后面好像不能用这样的表达式，请问有没有其他写法或者函数可以实现的？

[wwwwb]

分2条SQL语句执行