mysql - 如何使用 UNION 对字段求和？

Question

我想计算字段 status 等于 1 的行，仅对于 UNION 之前的一个子查询:

SELECT sender, login, sex, avatar, status, 
    SUM(case when status = 1 AND receiver = 1 then 1 else 0 end) AS unread_messages 
FROM messages 
LEFT JOIN users 
ON users.idUser = messages.sender 
WHERE receiver = 1

它工作正常，但是当我为第一个查询添加 UNION 时:

... UNION SELECT receiver, login, sex, avatar, status, idMessage 
FROM messages 
LEFT JOIN users 
ON users.idUser = messages.receiver 
WHERE sender = 1 
GROUP BY receiver, sender

它打破了计数的结果。因此，对于 receiver

，我只需要为第一个查询计算行数

我尝试添加附加条件:

case when (status = 1 and receiver = 1)

Answer 1

您的第一个和第二个 SELECT 具有不同的字段。

您必须在两者中添加字段:

首先选择:

SELECT sender, login, sex, avatar, status, null idMessage
       SUM(case when status = 1 AND receiver = 1
                then 1 else 0 end) AS unread_messages

第二次选择:

SELECT receiver, login, sex, avatar, status, idMessage, null unread_messages