Agrupar por cláusula:
si las columnas de la lista de selección de selección tienen columnas ordinarias y columnas de función agregada, deben agruparse (es decir, se agrega la cláusula group by).
¿Cómo agrupar?
Respuesta: Todas las columnas ordinarias de la lista de selección de selección se utilizan como columnas de agrupación de la cláusula group by.
select *
from Student
order by Sdept,Sage desc
/*聚集函数*/
select COUNT(*) 学生总人数
from Student
select COUNT(distinct sno) 选修了课程的学生人数
from SC
select AVG(grade) 选修1号课程的学生的平均成绩
from SC
where Cno='1'
select MAX(grade) 选修1号课程的学生最高分数
from SC
where Cno='1'
select SUM(Ccredit) 学生201215121选修课程的总学分数
from Course,SC
where course.Cno=sc.Cno and Sno='201215121' and Grade>=60
select Cno 课程号,COUNT(*) 相应的选课人数
from SC
group by Cno
select Sno 选修了三门及以上课程的学生学号
from SC
group by Sno
having COUNT(*)>=3
/*求各个课程号，课程名及相应的选课人数*/
select SC.Cno,Cname,COUNT(*) 对应的选课人数
from Course,SC
where Course.Cno=SC.Cno
group by SC.Cno,Cname
select Sno,AVG(grade) 平均成绩
from SC
group by Sno
having AVG(grade)>=80
/*连接查询*/
select *
from Student,SC
where Student.Sno=SC.Sno
select Student.*,Cno,Grade
from Student,SC
where Student.Sno=SC.Sno
select SC.Sno,Sname
from Student,SC
where Student.Sno=SC.Sno and SC.Cno='2' and Grade>=90
select Student.*,Cno,Grade
from Student left outer join SC on (Student.Sno=SC.Sno)
select *
from SC right outer join Course on SC.Cno=Course.Cno
select *
from Course left outer join SC on SC.Cno=Course.Cno
select SC.Sno,Sname,Cname,Grade
from Student,SC,Course
where student.Sno=SC.Sno and SC.Cno=Course.Cno
/*查询每个学生的学号、姓名、选修的课程名及成绩,包括没有选修课程的学生，包括没有被选修的课程*/
select SC.Sno,Sname,Cname,Grade
from (Student full outer join SC on Student.Sno=SC.Sno)
full outer join Course on SC.Cno=Course.Cno
Ensayo con tesoro:
portal