二表&三表联查

s1,t2,b3 是别名    (studeng表1  -   s1 表一)   (stu表二  -  t2表二)     (st表三  - b3表三)
二表 select s1.id,s1.name,stu2.sex from student as s1 INNER JOIN stu as stu2 ON s1.id = stu2.id 

三表 select s1.id,s1.name,t2.sex,b3.sex from student as s1 INNER JOIN stu as t2 on s1.id = t2.id INNER JOIN st as b3 ON s1.id=b3.id

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