HiveQL中的别名

我正在研究一个例子

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我猜av、ac和u是别名。如果是，为什么不需要AS？

在alias中没有强制使用AS，。如果一个表在FROM子句中出现多次，则需要使用表别名以保持其不同

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)

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)