加入了查询

Question

我这个星期准备我的考试，我看到这样一个过去的论文的问题：

讲师

lec_id 
lec_name 
phone 
dept 

请求

lec_id 
book_id 
qty 
date 

书

book_id 
title 
author 
cost 
supplier 

我需要写一个MySQL命令，将找到的姓名和电话号码，每个讲师的请求书编号123

我试图创建查询，如下图所示：

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 

不过，我不断收到错误不是唯一的表/别名： '请求'

Answer 1

最好从书所以WHERE子句是清洁的，也许更直观地适合于寻找另一个特定的书和/或所有书籍开始：

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 

Answer 2

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