php - MySQL 计数应返回 0，不返回任何内容

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

我在这里要做的只是计算所有未读取的 PM（“读取”列中没有“1”）。如果有，查询可以正常工作，但是当没有任何行满足该条件时，它什么也不返回。

我花了最后一个小时寻找解决方案，但它似乎真的很奇怪，它以这种方式工作。它应该只在回显“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

尝试颠倒表格顺序：

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应该去ON条款。

WHERE子句过滤掉所有未命中NULL产生的值。pm