Question

我有这个MySQL查询，看起来很简单：

SELECT users.nick, users.id, COUNT(pm.id) AS pms 
FROM users LEFT JOIN pm ON users.id = pm.touser  
WHERE users.id = :id AND pm.read = 0
GROUP BY users.id

我想在这里做的只是计算所有未读取的PM（“read”col中没有“1”）。如果有，查询工作正常，但当没有任何行满足该条件时，它不返回任何内容。

我花了最后一小时寻找解决方案，但它似乎很奇怪，它以这种方式工作。它只应在回显“pms”时打印“0”。

Answer 1

简单地：

SELECT users.nick, users.id, COUNT(pm.id) AS pms 
FROM users LEFT JOIN pm ON (users.id = pm.touser AND pm.read = 0)
WHERE users.id = :id
GROUP BY users.id;

Answer 2

试试这个

SELECT nick,id, COUNT(id) AS pms 
FROM users LEFT JOIN (Select touser as id from pm where read =0) as pms1  
WHERE id = :id GROUP BY id

Answer 3

尝试撤消表格顺序：

SELECT users.nick, users.id, COUNT(pm.id) AS pms 
FROM pm 
LEFT JOIN users 
ON users.id = pm.touser  
WHERE users.id = :id AND pm.read = 0
GROUP BY users.id

Answer 4

SELECT  users.nick, users.id, COUNT(pm.id) AS pms 
FROM    users
LEFT JOIN
        pm
ON      (pm.touser, pm.read) = (users.id, 0)
WHERE   users.id = :id
GROUP BY
        users.id

pm.read = 0应该转到ON条款。

WHERE子句过滤掉NULL未命中所产生的所有pm值。

MySQL计数应返回0，不返回任何内容

4 个答案: