假设有一个成绩表score，有id、学号stu_id、分数score三个字段，将此表跟上述学生表进

mysql 左连接

select * from tb_student(学生表) stu left join score sco on stu.id=sco.stu_id

右连接

select * from tb_student(学生表) stu right join score sco on stu.id=sco.stu_id

[SNO] 学号

,[SEX] 性别

,[Grade] 班级

select Grade as 班级,convert(decimal(18,2),sum(case when sex='男' then 1 else 0 end )*1.0/count(Sex)*100) as [男生比例],convert(decimal(18,2),sum(case when sex='女' then 1 else 0 end )*1.0/count(Sex)*100) as [女生比例]

from [student]

group by Grade

order by convert(decimal(18,2),sum(case when sex='男' then 1 else 0 end )*1.0/count(Sex)*100)

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