php - 合并两个mysql COUNT(*) 结果，不同的列

Question

我正在尝试合并以下查询的计数：

  SELECT recipient AS otheruser, COUNT(*) 
    FROM privatemessages 
   WHERE sender = '$userid' 
GROUP BY recipient

  SELECT sender AS otheruser, COUNT(*) 
    FROM privatemessages 
   WHERE recipient = '$userid' 
GROUP BY sender

如果无论如何要结合这两个查询，并将每个“其他用户”的计数（*）加在一起

Answer 1

最直接的方法是使用 SUM / Condition

SELECT
     CASE WHEN sender = '$userid' THEN recipient else sender END AS OtherUser,
     SUM(sender = '$userid') AS RCount,
     SUM(recipient = '$userid') AS SCount,
     COUNT(*) total
FROM
     privatemessages 
WHERE
     '$userid' IN (sender , recipient )
GROUP BY
     OtherUser

演示

更新

感谢 deltab 和 Michael Buen，我清理了 SUM 和 GROUP BY 语句

Answer 2

最直接的方法就是使用 2 个查询。
使用 2 个查询请求 2 个不同的数据集绝对没有害处，但没有一个理由这样做。

Answer 3

您可以像这样将它们与 UNION 结合起来：

SELECT otheruser, COUNT(*) FROM
(SELECT recipient AS otheruser
FROM privatemessages 
WHERE sender = '$userid' 
GROUP BY recipient
UNION
SELECT sender AS otheruser
FROM privatemessages 
WHERE recipient = '$userid' 
GROUP BY sender)

Answer 4

SELECT recipient AS r, sender AS s, COUNT(total) // can't use same alias
FROM privatemessages
WHERE sender = '$userid' AND recipient = '$userid'
GROUP BY s;

然后在你的结果函数中，调用 count

while ($row = $result) {
  echo $row['COUNT(total)'];
}

我相信这个查询应该按预期工作。您原来的唯一问题是双重别名和您对两个不同字段的分组。这在 2 个单独的查询中是可能的，但结合起来，它不能完成（至少是别名，对于 GROUP BY 不这么认为）。

希望这对你有用。

更新

一个很好的替代方法COUNT是简单地使用num_rows... 因此，上面的查询将是：

SELECT recipient AS r, sender AS s // can't use same alias
FROM privatemessages
WHERE s = '$userid' AND r = '$userid'
GROUP BY s;

然后在你的结果函数中，调用num_rows函数：

mysql_num_rows($result);