MySql查询：左连接

Question

我是新的Sql Queries。我想根据两个表的不同来制作一个表 这是我的疑问：

SELECT * FROM `question` WHERE `relatedFields` = 'Math' LEFT JOIN  `answer` ON `question`.ques = `answer`.ques where `answer`.TeacherNumber=1111111111

请帮忙。

Answer 1

将第一个WHERE子句移到最后，并将其他条件移到ON以使其成为真LEFT JOIN：

SELECT *
FROM `question`
  LEFT JOIN  `answer` ON `question`.ques = `answer`.ques
     and `answer`.TeacherNumber=1111111111
where `question`.relatedFields = 'Math'

替代语法：

SELECT *
FROM
    (select * from `question` WHERE `relatedFields` = 'Math') as q
LEFT JOIN
    (select * from `answer` where TeacherNumber = 1111111111) as a
  ON q.ques = a.ques

Answer 2

这篇文章将帮助您更好地理解联接。它也对我很有帮助。

Explanation about Joins click here.