mysql - 查询连接

Question

这周我正在准备考试，我看到过去的试卷问题是这样的：

讲师

lec_id
lec_name
phone
dept

要求

lec_id
book_id 
qty 
date

书

book_id
title
author
cost
supplier

我需要编写一个 MySQL 命令，该命令将找到每个请求 id#123 书籍的讲师的姓名和电话号码。

我尝试创建查询，如下所示：

SELECT lec_name, phone FROm lecturer 
  LEFT JOIN requests ON requests.lec_id = lecturer.lec_id 
  LEFT JOIN requests ON requests.book_id = book.book_id
WHERE book.book_id = 123

但是我不断收到错误 Not unique table/alias: 'requests'

Answer 1

SELECT lec_name, phone FROm lecturer 
  LEFT JOIN requests ON requests.lec_id = lecturer.lec_id 
  AND requests.book_id = book.book_id
WHERE book.book_id = 123

Answer 2

最好从书开始，这样 WHERE 子句更清晰，也许更直观地适应搜索另一本特定书和/或所有书：

select book.book_id, book.title, lecturer.lec_name, lecturer.phone 
from book
left join requests on book.book_id = requests.book_id
left join Lecturer on requests.lec_id = Lecturer.lec_id 
where book.book_id = 123