SQL按2個字段分組

Question

我得到那張桌子：

id, user_a, user_b, score
1,       3,      7,   19
2,       8,      3,   20
3,       3,      2,   10
4,       7,      6,   2
5,       3,      6,   7

我實現了用計算每個用戶的匹配數：

select user, count(*)
from
(
    select user_a as user from tablename
    union all
    select user_b from tablename
) dt
group by user

我現在想從usertable添加用戶電子郵件：

COUNT(*), user,  email
4,       3,      user3@mail.com
2,       7,      user7@mail.com
1,       8,      user8@mail.com
1,       2,      user2@mail.com
2,       6,      user6@mail.com

我已經試過了：

SELECT user, count(*) 
FROM ( 
SELECT user_a AS user, ut.email
   FROM tablename
   UNION ALL 
      SELECT user_b
      FROM tablename
) dt 
WHERE user IS NOT NULL
LEFT JOIN usertable ut ON ut.id = user
GROUP BY user

但存在語法錯誤：

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LEFT JOIN usertable ut ON ut.id = user
GROUP BY user'

任何想法 ？

Answer 1

在派生表中執行UNION ALL ，以獲取一列用戶。 GROUP BY結果。

select user, count(*)
from
(
    select user_a as user from tablename
    union all
    select user_b from tablename
) dt
group by user

回答更新的問題：

SELECT dt.user, ut.email, count(*) 
FROM
( 
    SELECT user_a AS user FROM tablename
    UNION ALL 
    SELECT user_b FROM tablename
) dt 
LEFT JOIN usertable ut ON ut.id = dt.user
GROUP BY dt.user, ut.email