Question

我有两张桌子，用户和参赛者。我试图选择具有个人资料照片的最大参赛者ID（位于用户桌面上）

这是我可怕的SQL：

SELECT u.thumbnail, u.id FROM users AS u
INNER JOIN
(
 SELECT c.id, c.user_id FROM contestants AS c
 WHERE u.id = c.users_id
AND c.id = (select max(c.id))
) WHERE u.thumbnail IS NOT NULL

目前的错误是：＃1248 - 每个派生表都必须有自己的别名。

这让我感到困惑，因为用户的别名为u，参赛者的别名是c ..

我在这里做错了什么？我猜了很多，所以一些帮助会非常感激！

Answer 1

每当您执行连接操作时，您实际上正在加入两个表。例如，您在此处编写的子查询作为单独的表工作。因此，您必须使用此表的别名。这就是您的错误消息背后的原因。

您的查询：

    SELECT u.thumbnail, u.id FROM users AS u
    INNER JOIN
    (
    SELECT c.id, c.user_id FROM contestants AS c
    WHERE u.id = c.users_id
    AND c.id = (select max(c.id))
    ) WHERE u.thumbnail IS NOT NULL

它应该包含子查询的别名：

    SELECT c.id, c.user_id FROM contestants AS c
    WHERE u.id = c.users_id
    AND c.id = (select max(c.id))

让我们说，它是T。

因此，您的查询现在变为：

    SELECT u.thumbnail, u.id FROM users AS u
    INNER JOIN
    (
    SELECT c.id, c.user_id FROM contestants AS c
    WHERE u.id = c.users_id
    AND c.id = (select max(c.id)) 
    ) AS T
    WHERE u.thumbnail IS NOT NULL

但是你想要实现的目标实际上可以用一种更简洁的方式来实现：

    SELECT u.thumbnail, u.id, max(c.id),
    FROM users as u
    LEFT JOIN contestants as c
    on u.id = c.user_id
    WHERE u.thumbnail IS NOT NULL

为什么在你有更好更整洁的方法时会大惊小怪呢？

Answer 2

试试这个：

SELECT u.thumbnail, u.id 
FROM users AS u
INNER JOIN
(
 SELECT c.id, c.user_id FROM contestants AS c
 WHERE u.id = c.users_id
 AND c.id = (select max(c.id))
)A 
WHERE u.thumbnail IS NOT NULL

Answer 3

我认为这应该很简单，

SELECT  u.thumbnail, u.id
FROM    users u
        INNER JOIN contestants c
            ON u.id = c.users_id
WHERE   u.thumbnail IS NOT NULL
ORDER   BY c.id DESC
LIMIT   1

Answer 4

这很简单。

SELECT  user.thumbnail, user.id
FROM    users user
        INNER JOIN contestants cont ON cont.id = cont.users_id
WHERE   cont.thumbnail IS NOT NULL
ORDER   BY user.id DESC

SQL加入MAX（）。

4 个答案: