select单表查询和多表查询

数据表

1).学生表: Student

字段: (SID,Sname,Sage,Ssex) -SID学生编号,Sneme学生姓名,Sage出生年月，Ssex学生性别

2).课程表: Course

字段: (CD,Cname,TI) –CID课程编号,Cname课程名称,TID教师编号

3).教师表: Teacher

字段: (TID,Tname) -ID教师编号,Tname教师姓名

4).成绩表: SC

字段: (SID,CID,score) -SID学生编号,CID课程编号,score分数

select查询

1.单表查询

问题：查询SID为1的学生信息

查询语句：select Sname,Sage,Ssex from student where SID=1

2.多表查询

问题：查询"01"课程比”02”课程成绩高的学生的信息及课程分数（这里01和02课程指的是CID）

方法一：from多表查询

1）第一步先查出所有学生的01课程的分数，查询结果作为a表

查询语句：select SID,CID,score from sc where CID=01

2）第二步再查出所有学生的02课程的分数，查询结果作为b表

查询语句：select SID,CID,score from sc where CID=02

3）第三步查询同一个学生的01课程分数大于02课程分数的学生SID以及课程分数，查询结果作为c表

查询语句：select a.SID,a.score as score01,b.score as score02 from (select SID,CID,score from sc where CID=01) as a,(select SID,CID,score from sc where CID=02) as b 

where a.SID = b.SID and a.score>b.score

4）第四步根据c表和student表查询出学生具体信息

查询语句：select d.SID,d.Sname,d.Sage,d.Ssex,score01,score02 from student as d, 

(

select a.SID,a.score as score01,b.score as score02 from (select SID,CID,score from sc where CID=01) as a,(select SID,CID,score from sc where CID=02) as b 

where a.SID = b.SID and a.score>b.score

) as c 

where d.SID=c.SID

方法二：使用join连接

1）第一步先查出所有学生的01课程的分数，查询结果作为a表

查询语句：select SID,CID,score from sc where CID=01

2）第二步再查出所有学生的02课程的分数，查询结果作为b表

查询语句：select SID,CID,score from sc where CID=02

3）第三步使用join连接student表、a表和b表

查询语句：select c.SID, c.Sname,c.Sage, c.Ssex,a.score as score01, b.score as score02 from student as c

right join (select SID,CID,score from sc where CID=01) as a on a.SID=c.SID

right join (select SID,CID,score from sc where CID=02) as b on b.SID=c.SID

where a.SID=b.SID and a.score>b.score