Question

假设我有以下表结构：

t1
-------------
id // row id
userID_follower // this user is a follows another member
userID_following  // other member that this user

是否可以运行单个查询以组合以下两者：

此人关注的用户数

从t1 WHERE userID_follower =“。$ myID”中选择COUNT（id）。。“
有多少用户关注此人

从t1选择COUNT（id）WHERE userID_following =“。$ myID。”

感谢。

Answer 1

在MySql中，您可以在条件上使用SUM()函数，因为错误条件等于0，而真条件等于1：

SELECT SUM(userID_follower = $myID) AS followerCount,
   SUM(userID_following = $myID) AS followingCount
FROM t1
WHERE userID_follower = $myID
   OR userID_following = $myID

Answer 2

更多Hoyle（ISO）解决方案将使用Case表达式：

Select Sum( Case When userID_follower = $myID Then 1 Else 0 End ) As followerCount
    , Sum( Case When userID_following = $myID Then 1 Else 0 End ) As followingCount
From t1
Where userID_follower = $myID
    Or userID_following = $myID

Answer 3

我建议每行返回两行，而不是两列：

SELECT 'follower', COUNT(*) AS count FROM t1 WHERE userID_follower = ?
UNION ALL
SELECT 'following', COUNT(*) FROM t1 WHERE userID_following = ?

这似乎是一种退化解决方案，但原因是如果对userID_follower和userID_following编制索引，则可以使用索引。如果您尝试将结果分成两列，如其他答案中所示，则无法使用索引并且必须执行表扫描。

与问题相关的其他提示：

在这种情况下使用COUNT（id）没有任何好处。
您应该使用SQL查询参数，而不是在查询中插入$ myID。

Answer 4

我认为这样的事情应该有效：

select ownerID, count( distinct userID_follow), count(distinct userID_following) from t1 group by ownerID

是否可以在同一查询中计算两列

4 个答案: