SQL：使用来自两个不同表的计数

Posted 2023-03-29

【中文标题】SQL：使用来自两个不同表的计数

【英文标题】：SQL: Use count from two different tables

【发布时间】：2013-09-23 09:37:53

【问题描述】：

如何为以下场景构造查询：

我有三张桌子：

人 书籍（与peopleserno 上的人相关联） 视频（与peopleserno 上的人相关联）

我想创建一个 SQL，其中输出给出一行，其中包含特定人阅读/观看的书籍和视频的数量。

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。

【参考方案1】：

您需要 left join 来获取甚至没有阅读/观看任何内容的用户，然后您需要 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

【讨论】：

@491243：你说得对，我没有。现在修复了错误。谢谢。

实际上，如果该列上设置了标识并且您可以正确计数，则它会起作用，例如COUNT(DISTINCT x.identityColumn):)

【参考方案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

【参考方案3】：

除非表上存在 auto_incremented 列，否则在子查询中进行计算是安全的。假设表没有 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

【参考方案4】：

您必须对需要汇总的字段使用 GROUP BY 子句才能计数。

【参考方案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