Question

我有以下数据库结构：

categories
 -id
 -name

videos
 -id
 -name
 -category

pictures
 -id
 -name
 -category

我想知道每个类别有多少视频和图片。

我尝试过从类别到图片和视频的简单左连接，但这会返回错误的值。是否可以仅在1个sql命令中获取每个的COUNT（）？

Answer 1

使用UNION应该可以工作：

SELECT c.name, count(*) 
FROM (SELECT * FROM videos
      UNION
      SELECT * FROM pictures) AS all
      LEFT JOIN categories c ON c.id=all.category
GROUP BY c.name

Answer 2

试试这个：

SELECT
 categories.name, COUNT(videos.id) AS video_count, COUNT(pictures.id) AS picture_count
 FROM categories
 INNER JOIN videos
 ON categories.id = videos.category
 INNER JOIN pictures
 ON categories.id = pictures.category
 GROUP BY categories.name;

Answer 3

可以在同一个查询中获取两者。试试这个：

SELECT c.name,COUNT(DISTINCT v.id) as Videos,count(DISTINCT p.id) as Pictures
FROM categories c 
LEFT JOIN videos v on v.category=c.id
LEFT JOIN Picture p on p.category=c.id
GROUP BY c.name

MySQL：组合不同的表

3 个答案: