Question

SELECT * FROM
(
SELECT messages.from AS `from`, messages.creation AS `creation`, account_images.profile AS `profile`, bio.user_full_name AS `user_full_name`
FROM messages
INNER JOIN account_images ON account_images.uuid=messages.from
INNER JOIN bio ON bio.uuid=messages.from
WHERE messages.to='{$user}'
ORDER BY `creation` DESC
)
GROUP BY `from`

为什么这个查询不起作用？嵌套的select语句有问题吗？请帮我解决这个问题...

Answer 1

FROM是一个mysql保留关键字，使用from作为列名，应该在backtics中，否则你会收到错误messages。from

SELECT * FROM
(
SELECT messages.`from` AS `from`, messages.creation AS `creation`, account_images.profile AS `profile`, bio.user_full_name AS `user_full_name`
FROM messages
INNER JOIN account_images ON account_images.uuid=messages.`from`
INNER JOIN bio ON bio.uuid=messages.`from`
WHERE messages.to='{$user}'
ORDER BY `creation` DESC
)
GROUP BY `from`

Reserved Words

Answer 2

除了不带引号的from问题之外，连接的顺序错误。在定义on子句之前，您不能引用表别名，并且您需要在子查询上使用表别名：

SELECT *
FROM (SELECT messages.from AS `from`, messages.creation AS `creation`,
             account_images.profile AS `profile`, bio.user_full_name AS `user_full_name`
      FROM account_images INNER JOIN
           bio
           ON bio.uuid = messages.`from` INNER JOIN
           messages
           ON account_images.uuid = messages.`from`
      WHERE messages.to='{$user}'
      ORDER BY `creation` DESC
     ) t
GROUP BY `from`;

最后，您正尝试使用明确描述为group by扩展的MySQL文档中不可靠的技术来获取组的最后一条消息（请参阅here）。

MySQL查询无法使用嵌套选择

2 个答案: