Question

我有一个mysql表private_messages（pm），如下所示：

userid --- sender --- text

虽然Userid和Sender都是表user中的ID。现在我想进行一个查询，其中用户获得他发送给另一个用户的pms数量与从这个pms收到的pms之间的差异 - 按Userid分组所以我想把这两个问题放在一起，得到不同的结论：

SELECT count(*) AS s_count, sender FROM pm WHERE userid=".$userid" GROUP BY sender
SELECT count(*) AS u_count, userid FROM pm WHERE sender=".$userid." GROUP BY userid

现在我想要在所有匹配的用户ID上区分s_count和u_count。

Answer 1

如果您想要与您对应的每个用户进行统计，则可以使用UNION ALL收集所有邮件，并在外部查询中对它们求和;

SELECT counterpart, SUM(sent) sent, SUM(received) received,
       SUM(sent) - SUM(received) difference
FROM (
  SELECT 1 sent, 0 received, userid counterpart 
  FROM messages WHERE sender = <your user id>
  UNION ALL
  SELECT 0 sent, 1 received, sender counterpart
  FROM messages WHERE userid = <your user id>
) a
GROUP BY counterpart;

An SQLfiddle to test with

Answer 2

您最好在选择列表中添加对结果进行分组的任何列：

SELECT sender, COUNT(*) AS s_count FROM pm WHERE userid = ? GROUP BY sender
SELECT userid, COUNT(*) AS u_count FROM pm WHERE sender = ? GROUP BY userid

然后，您将看到第一个查询返回给定userid的邮件数，按sender细分;而第二个查询返回给定sender的消息数，按userid细分。因此，将这些查询“并从中获取差异”组合起来没有多大意义。

Answer 3

您可以这样做：

SELECT
    (SELECT count(*) FROM pm WHERE userid=".$userid" GROUP BY sender) AS s_count,
    (SELECT count(*) FROM pm WHERE sender=".$userid." GROUP BY userid) AS u_count,
    (SELECT s_count - u_count) AS result

两个COUNTS之间的差异

3 个答案: