MySql查詢：左加入

Question

我是新的Sql查詢。我想從兩個表的差異中創建一個表。
這裏是我的查詢：

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

請幫助。

Answer 1

移動第一WHERE子句結束，並移動其他where條件ON得到真正LEFT JOIN：

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

替代語法：

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

這篇文章將幫助你瞭解更好的連接。它也幫助我。

Explanation about Joins click here.