Question

sql 表如何多表合并查询（可能有空值）

假设有
学生表student 语文成绩chs 数学成绩math
id name chsid stid val maid stid val
1 sa 1 1 80 1 2 85
2 sb 2 2 90 2 3 95
3 sc

希望得到合并表st_course
id name chsvalue mathvalue
1 sa 80 <NULL>
2 sb 90 85
3 sc <NULL> 95

有简单的sql查询吗？

Answer 1

select a.id,b.val as chsvalue,c.val as mathvalue from student a
left join chs b on a.id=b.stid
left join math c on a.id=c.stid

Answer 2

语句如下：select s.id,c.val as chsvalue,m.val as mathvalue from student s
left join chs c on s.id=c.stid
left join math m ons.id=m.stid