在MySQL中使用子查询进行选择（使用ANY和IN进行子查询）

Question

感谢您的出色答案！

更多信息

• More info on MySQL IN comparison operator
• Joining Tables
• Subqueries with ANY, IN, and SOME

---

这是很难解释的，所以让舞台布景......

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! | 
+------+--------+ 

我想选择组a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个行。我应该使用JOIN吗？

Subquery returns more than 1 row 

Answer 1

一种方法是这样的：

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

然而，随着大表，此查询往往是低效的，做一个连接是更好：

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

另外，作为乔纳森提到，你可以做到这一点，它的效率非常高，甚至更高：

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 = (SELECT user FROM userGroupMap WHERE group = a) 

子查询返航用户和组（两个​​场）时，它应该返回只是用户。

Answer 3

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：只有用户栏应返回，如由他人注意）

Answer 4

你不能只是这样做：

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

Answer 5

事实上，这个查询将给你所需要的：

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

Answer 6

而是使用加入比子查询：

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