连接操作

内连接

select Student.Sno, Sname, Ssex, Sage, Sdept, Cno, Grade
from Student inner join SC
on Student.Sno = SC.Sno;

左外连接

select Student.Sno, Sname, Cno, Grade
from Student left jouter oin SC
on Student.Sno = SC.Sno;

右外连接

select Student.Sno, Sname, Cno, Grade
from SC right outer join Student
on Student.Sno = SC.Sno;

复合条件连接查询，添加限制条件
查询选修课平均成绩超过85分的学生信息和平均成绩

select Student.*, avg(Grade) '平均成绩'
from Student inner join SC
on Student.Sno = SC.Sno
group by Sno
having avg(Grade) > 85;