这个 MySQL 查询有什么问题？ SELECT * AS `x`，稍后如何再次使用x？

Question

以下 MySQL 查询：

select `userID` as uID,
(select `siteID` from `users` where `userID` = uID) as `sID`,
from `actions`
where `sID` in (select `siteID` from `sites` where `foo` = "bar")
order by `timestamp` desc limit 100

...返回错误：

Unknown column 'sID' in 'IN/ALL/ANY subquery'

我不明白我在这里做错了什么。 sID 东西不应该是一列，而是我通过执行 (select siteID from users where userID = uID) as sID< 创建的“别名”（这叫什么？） /代码>。它甚至不在 IN 子查询内。

有什么想法吗？

---

编辑： @Roland：感谢您的评论。我有三个表：actions、users 和 sites。表 actions 包含一个 userID 字段，该字段对应于 users 表中的条目。此表中的每个用户 (users) 都有一个 siteID。 我尝试从 actions 表中选择最新操作，并将它们链接到 users 和 sites 表以找出执行这些操作的人员操作以及在哪个网站上进行。希望这是有道理的:)

Answer 1

我不知道这是否不在11年前的SQL标准中，但我发现它是使用HAVING的最简单的方法：

select `userID` as uID,
(select `siteID` from `users` where `userID` = uID) as `sID`,
from `actions`
order by `timestamp` desc limit 100
HAVING `sID` in (select `siteID` from `sites` where `foo` = "bar")

Answer 2

您需要将其包含在子查询中：

SELECT  *
FROM    (
        SELECT  userID as uID, (select siteID from users where userID = actions.userID) as sID,
        FROM    actions
        ) q
WHERE   sID IN (select siteID from sites where foo = "bar")
ORDER BY
        timestamp DESC
LIMIT   100

，或者更好的是，将其重写为 JOIN

SELECT  a.userId, u.siteID
FROM    actions a
JOIN    users u
ON      u.userID = a.userID
WHERE   siteID IN
        (
        SELECT  siteID
        FROM    sites
        WHERE   foo = 'bar'
        )
ORDER BY
        timestamp DESC
LIMIT   100

创建以下索引：

actions (timestamp)
users (userId)
sites (foo, siteID)

Answer 3

在查询处理器完成 Select 子句并构建第一个中间结果集之前，不会建立列别名，因此如果需要，只能在 group By 中引用它（因为 group By 子句对该中间结果集进行操作）不要以这种方式使用它，将别名放入子查询中，然后它将出现在子查询生成的结果集中，因此可供外部查询访问。为了说明

（这不是执行此查询的最简单方法，但它说明了如何从子查询建立和使用列别名）

 select a.userID as uID, z.Sid
 from actions a
 Join  (select userID, siteID as sid1 from users) Z, 
     On z.userID = a.userID
 where Z.sID in (select siteID from sites where foo = "bar") 
 order by timestamp desc limit 100 

Answer 4

请尝试以下操作：

SELECT
       a.userID as uID
       ,u.siteID as sID
    FROM
       actions as a
    INNER JOIN
       users as u ON u.userID=a.userID
    WHERE
       u.siteID IN (SELECT siteID FROM sites WHERE foo = 'bar')
    ORDER BY
       a.timestamp DESC
    LIMIT 100

Answer 5

我认为错误的原因是别名不可用于 WHERE 指令，这就是我们有 HAVING 的原因。

select `userID` as uID,
(select `siteID` from `users` where `userID` = uID) as `sID`,
from `actions`
HAVING `sID` in (select `siteID` from `sites` where `foo` = "bar")
order by `timestamp` desc limit 100

尽管我也同意其他答案，但您的查询可以更好地结构化。

Answer 6

尝试以下操作

SELECT 
       a.userID as uID 
       ,u.siteID as sID 
    FROM 
       actions as a 
    INNER JOIN 
       users as u ON u.userID = a.userID 
    INNER JOIN
        sites as s ON u.siteID = s.siteID   
    WHERE 
       s.foo = 'bar'
    ORDER BY 
       a.timestamp DESC 
    LIMIT 100 

如果您希望稍后使用选择部分中的字段，您可以尝试子选择

SELECT  One,
        Two,
        One + Two as Three
FROM    (
            SELECT  1 AS One,
                    2 as Two
        ) sub