Teradata 13：Join中的CASE语句

Question

CASE语句是否允许加入？有没有一个好方法来完成这里的任务？我的真实查询还有其他一些左连接。

我想在这种情况下加入T1和T2： 1.当T1.sub_service不为null时，则加入T2.type 2.当T1.sub_service为null时，使用T1.service与T2.type

连接

SELECT T1.service, T1.sub_service, T2.type 
FROM      TABLE1 T1
LEFT JOIN TABLE2 T2
ON T2.type LIKE
   CASE WHEN T1.sub_service IS NULL THEN T1.service
        WHEN T1.sub_service IS NOT NULL THEN T1.sub_service
   END

Answer 1

只需将 LIKE 替换为 = ：

ON T2.type = 
   CASE WHEN T1.sub_service IS NULL THEN T1.service
        WHEN T1.sub_service IS NOT NULL THEN T1.sub_service
   END

但你可以进一步将其简化为COALESCE：

ON T2.type = COALESCE(T1.sub_service, T1.service)