Mysql中的多连接表仅返回一行[重复]

Posted 2023-03-23

【中文标题】Mysql中的多连接表仅返回一行[重复]

【英文标题】：Multi Join Table in Mysql returning only one row [duplicate]

【发布时间】：2021-12-05 21:19:23

【问题描述】：

您好，我正在就我的代码寻求帮助，目前它只返回一行，我完全被这一行困住了，似乎无法修复它。我希望其他人的见解可以帮助我。

use library;

#3
Select title, count(loanId) as 'Number of loans'
From book
     Join bookCopy ON bookCopy.isbn = book.isbn
     Join loan ON bookCopy.copyId = loan.copyId
Where dateBack IS NULL
Order By title;

【问题讨论】：

你忘了GROUP BY title

如果没有group by，mysql 是 allowing you to get away with，则您的查询是无效的 SQL

【参考方案1】：

那是因为 COUNT。

使用 GROUP BY 标题

Select title, count(loanId) as 'Number of loans'
From book
     Join bookCopy ON bookCopy.isbn = book.isbn
     Join loan ON bookCopy.copyId = loan.copyId
Where dateBack IS NULL
GROUP BY title
Order By title;