Question

我正在调查this示例

SELECT u.id, actions.date
FROM (
    SELECT av.uid AS uid
    FROM action_video av
    WHERE av.date = '2008-06-03'
    UNION ALL
    SELECT ac.uid AS uid
    FROM action_comment ac
    WHERE ac.date = '2008-06-03'
 ) actions JOIN users u ON (u.id = actions.uid)

我猜av，ac和u是别名。如果是这样，为什么不需要AS？

Answer 1

没有强制要求在别名中使用。如果表在FROM子句中出现多次，则需要表别名以使它们保持不同

SELECT u.id, actions.date
FROM (
    SELECT av.uid  uid
    FROM action_video av
    WHERE av.date = '2008-06-03'
    UNION ALL
    SELECT ac.uid AS uid
    FROM action_comment ac
    WHERE ac.date = '2008-06-03'
 ) actions JOIN users u ON (u.id = actions.uid)

HiveQL中的别名

1 个答案: