MySql 查询 : Left Join

Question

我是新的 Sql 查询。我想从两个表的差异中创建一个表。
这是我的查询:

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 子句移动到末尾，并将另一个 where 条件移动到 ON 以获得 true 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