SQL：使用来自两个不同表的计数

Question

如何为以下场景构建查询：

我有三张桌子：

1. 人
2. 图书（与peopleserno上的人关联）
3. 视频（与peopleserno上的人关联）

我想创建一个SQL，其中输出给出一行包含多少本书和一本书。特定人士已阅读/观看的视频。

示例输出：

John | 3 (books) | 2 (videos)

我尝试过这样的事情，但它不起作用：

select a.name, 
       count(b.serno) as books, 
       count(c.serno) as videos
  from people a, 
       books b, 
       videos c
 where a.serno = b.peopleserno
   and a.serno = c.peopleserno

谢谢。

Answer 1

你需要一个left join来吸引那些没有阅读/观看任何内容的用户，然后你需要group by用户来获取特定的用户数

select a.name, 
       count(distinct b.title) as books, 
       count(distinct c.title) as videos 
from people a
left join books b on a.serno = b.peopleserno
left join videos c on a.serno = c.peopleserno
group by a.name

SQLFiddle demo

Answer 2

select a.name, 
       ( select count(b.serno) from books b where a.serno = b.peopleserno ) as books, 
       ( select  count(c.serno) from videos c where a.serno = c.peopleserno) as videos 
from people a

Answer 3

在子查询中进行计算是安全的，除非表中包含auto_incremented列。假设该表没有auto_incremented列，

SELECT  a.Name,
        COALESCE(b.TotalBook, 0) TotalBook,
        COALESCE(c.TotalVideo, 0) TotalVideo
FROM    People a
        LEFT JOIN
        (
            SELECT  peopleserno, COUNT(*) TotalBook
            FROM    Books
            GROUP   BY peopleserno
        ) b ON a.serno = b.peopleserno
        LEFT JOIN
        (
            SELECT  peopleserno, COUNT(*) TotalVideo
            FROM    Videos
            GROUP   BY peopleserno
        ) c ON a.serno = c.peopleserno

• SQLFiddle Demo

Answer 4

您必须使用GROUP BY子句来填充您需要计算的字段。

Answer 5

select a.name, 
       count(b.title)||' (books)'  as books, 
       count(c.title)||' (videos)'  as videos 
from people a
left join books b on a.serno = b.peopleserno
left join videos c on a.serno = c.peopleserno
group by a.name


OR

select a.name, 
       ( select count(b.serno) from books b where a.serno = b.peopleserno ) as books, 
       ( select  count(c.serno) from videos c where a.serno = c.peopleserno) as videos 
from people a