mysql - Mysql中的多连接表只返回一行

Question

嗨，我正在为我的代码寻求帮助，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;

标签： mysqljoinselectrowmysql-workbench

Answer 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;