mysql - 多次计算带有外键的子表，有条件

Question

我有两个表，

1. 标签 -> id,name,description,user,status
2. 标签_连接。 -> id, Label_id, 类别

所以有多个类别，假设 1 => 新的，2 => 旧的。

我需要根据类别计算子表的计数。

这是我现在拥有的，

SELECT `L`.*, COUNT(DISTINCT LC1.id) as count1, COUNT(DISTINCT LC2.id) as count2 
FROM (`Labels` L) 
LEFT JOIN `Label_connection` LC1 ON `LC1`.`Label_id` = `L`.`id` AND LC1.categories = "1" 
LEFT JOIN `Label_connection` LC2 ON `LC2`.`Label_id` = `L`.`id` AND LC2.categories = "2" 
WHERE `L`.`status` = '0' AND `L`.`user` = 1
GROUP BY `L`.`id` 
ORDER BY `L`.`id` DESC 
LIMIT 20

这确实让我得到了正确的计数，但我担心多重连接，因为类别的数量会增加。

如果有人可以改进这一点，我将不胜感激，

提前致谢。

Answer 1

对于条件聚合，您还可以使用COUNT DISTINCT .. CASE..WHEN..END。

此外，您当前的查询 GROUP BY 无效(除非 L.id 是主键)。详情请阅读:Error related to only_full_group_by when executing a query in MySql

此外，请更喜欢在字符串文字周围使用 SQL 标准单引号而不是双引号。

SELECT L.id,  
       COUNT(DISTINCT CASE WHEN LC.categories = '1' THEN LC.id END) AS count1, 
       COUNT(DISTINCT CASE WHEN LC.categories = '2' THEN LC.id END) AS count2 
FROM Labels AS L
LEFT JOIN Label_connection AS LC 
  ON LC.Label_id = L.id
WHERE L.status = '0' AND
      L.user = 1
GROUP BY L.id 
ORDER BY L.id DESC 
LIMIT 20