Question

我有一个名为Languages的表，它存储了Persons表中存储的每个人能够说话的语言，然后我有一个名为Homework的表，它存储了每个人的所有作业必须这样做，但与语言有关。我能够选择1要为语言1（左连接）做的家庭作业，但也希望能够在同一查询中选择语言人1能够说话。有人能给我一个提示如何实现这一目标吗？左边的语言连接会搞乱结果，因为它会弄乱家庭作业表上的语言。

语言和人数很多，家庭作业和人员也是如此。

当我添加第二个left join：

时，它会起作用

SELECT p.id, l.id, h.id
FROM Persons AS p
LEFT JOIN Homework AS h ON h.person_id = p.id
#LEFT JOIN Languages AS l ON l.person_id = p.id #Screws the result multipliying by the number of available languages
WHERE p.id = 1
AND h.language_id = 1

我想结合上面的查询和以下内容：

 SELECT *
 FROM languages
 WHERE person_id=1

Answer 1

你想找到吗？

Person | Language | Homework 
------------------------------
Bob | English | 
Bob | French | 
Bob | Spanish | Geography

James | English |
James | French | History
James | Spanish |

使用此...

SELECT p.id, l.id, h.id
FROM Persons AS p

LEFT JOIN Languages AS l ON l.person_id = p.id 

LEFT JOIN Homework AS h ON h.language_id = l.id
    And h.person_id = p.id

或者

Person | Language | Homework 
------------------------------
Bob | Spanish | Geography
James | French | History

我相信已经提出的建议

更新：现在应该为您提供所有语言的人，但只显示与您想要的语言匹配的家庭作业

SELECT p.id, l.id, h.id
FROM Persons AS p

LEFT JOIN Languages AS l ON l.person_id = p.id 

LEFT JOIN Homework AS h ON h.language_id = l.id
    And h.person_id = p.id
    And h.language_id = 1

简单的SQL查询问题

1 个答案: