--查询列
Select t.sname,t.ssex,t.class from student t       --t 别名;
Select *from student t;   --* 代表查询表内所有数据
Select *from student t where t.sclass=‘95031‘;  --规定判断条件t.sclass=‘95031‘，用where连接
Select t.sname as 姓名,t.ssex 性别,t.class 班级 from student t where t.sclass=‘95031‘;     --将列名翻译成汉文 
Select t.sname||t.ssex||t.class from student t where t.sclass=‘95031‘ or t.ssex=‘女‘;      --||代表将查询数据连接


--升降续查询 
select * from STUDENT t order by sclass;  --order by 升序排列，  默认的加上asc
select * from STUDENT t order by sclass desc;  --order by sclass desc   降序排列，
select * from STUDENT t order by sclass desc,sno asc;  --同时进行多个字段进行排序，先对sclass进行降序，在对sno升续排列
Select distinct t.depart t.prof from teacher t;    --distinct使depart字段与Prof字段不重复列查询出来


--关系查询
Select * from student t where t.sno in (select e.sno from score e where degree between 60 and 80);  --degree between 60 and 80在degree列，取在60到80之间成绩学生信息，
Select * from student t where t.sno in (select e.sno from score e where degree in(85,86,88));   --degree in(85,86,88)  在degree列取85,86,88的成绩学生信息 --in() 子查询


--模糊查询
select * from STUDENT t where sname like ‘王%‘;        --查询STUDENT表中性王的人的信息，%代表任意数量   关键字like
select * from STUDENT t where sname not like ‘王_‘;        --下划线_代表一个字符，查询姓王的两个字的名字，


--限制返回记录条件
select * from score where rownum=1;                    --rownum=1 伪列，在列表中是隐藏的
select * from (select * from score order by degree desc) where rownum=1;        --from后也可以加结果集，作为虚拟表


--分组查询 group by
--按照课程编号分组，在计算每组平均值
select cno,avg(degree),count(1) from score t group by cno;


--连接查询
--内连接查询
select Sname,cno,degree from score s,student t where t.sno=s.sno;         --将两个表score，student表创建练习。
select Sname,cno,degree from score s join student t on t.sno=s.sno;        --标准内连接格式
--外联接查询
select Sname,cno,degree from score s left join student t on t.sno=s.sno;         --left join 左外联，将左侧表内数据对不上的信息查询出来，
select Sname,cno,degree from score s left join student t on t.sno=s.sno;          --right join右外联，将右侧表内数据对不上的信息查询出来，
--全连接
select Sname,cno,degree from score s full join student t on t.sno=s.sno;          --full join全连接，将左，右两侧表内数据的信息全部查询出来，