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这很难解释，所以让我们设置舞台......

userActions         userGroupMap
+------+--------+   +------+-------+
| user | action |   | user | group |
+------+--------+   +------+-------+
| x    | acted! |   | x    | a     |
| y    | acted! |   | y    | a     |
| y    | acted! |   | z    | b     |
| z    | acted! |   +------+-------+
| y    | acted! |
| z    | acted! |
| x    | acted! |
| z    | acted! |
+------+--------+

我想选择a组的动作。我的想法是

SELECT actions, user FROM userActions
    WHERE user = (SELECT user, group FROM userGroupMap WHERE group = a)

但显然这个子查询返回多行。我应该使用JOIN吗？

Subquery returns more than 1 row

Answer 1

一种方法是：

SELECT actions,
       user
FROM   userActions
WHERE  user IN
               (SELECT user
               FROM    userGroupMap
               WHERE   [group] = 'a'
               );

但是，对于大型表，此查询往往效率低下并且更好地进行连接：

SELECT actions,
       userActions.user
FROM   userActions
       INNER JOIN
              (SELECT user
              FROM    userGroupMap
              WHERE   [group] = 'a'
              ) AS tmp
       ON     userActions.user = tmp.user;

或者，正如乔纳森所提到的那样，你可以做到这一点，而且效率非常高，如果不是更多的话：

SELECT actions,
       userActions.user
FROM   userActions
       INNER JOIN userGroupMap
       ON     userActions.user = userGroupMap.user
WHERE  [group] = 'a';

Answer 2

SELECT actions, user FROM userActions
    WHERE user IN (SELECT user FROM userGroupMap WHERE group = a)

SELECT actions, user FROM userActions
    WHERE user = ANY (SELECT user FROM userGroupMap WHERE group = a)

（修订：如其他人所述，只应退回用户专栏。）

Answer 3

难道你不能做这样的事情：

SELECT 
  a.actions, 
  a.user 
FROM 
  userActions a 
  INNER JOIN userGroupMap g 
    ON a.user = g.user
WHERE
  g.group = 'a'

Answer 4

实际上，此查询将为您提供所需内容：

SELECT actions, user 
FROM userActions 
WHERE user IN 
    (SELECT user FROM userGroupMap WHERE group = 'a')

Answer 5

SELECT actions, user FROM userActions
    WHERE user = (SELECT user FROM userGroupMap WHERE group = a)

子查询返回用户和组（两个字段）时应该只返回用户。

Answer 6

而是使用join而不是子查询：

SELECT
    userActions.action,
    userActions.user
FROM
    userActions
CROSS JOIN userGroupMap ON 
    userGroupMap.user = userActions.user AND
    userGroupMap.group = 'a'

在MySQL中选择子查询（带有ANY和IN的子查询）

6 个答案: