-多表查询
1.交叉连接
select * from t_class for update;
select * from t_student for update;
select for update 是为了在查询时,避免其他用户以该表进行插入,修改或删除等操作,造成表的不一致性.
查询学生信息及对应的班级信息
select t1.,t2.
from t_student t1,t_class t2
–交叉连接获取的结果是一个笛卡尔乘积
–也就是表1中的数据都要和表2中的每条数据连接一次 是数据条数时两个表数据量的乘积
2.等值连接
select t1.,t2. – 100000 2（两者相乘）
from t_student t1,t_class t2 – 10000 100 100W 获取的结果集可能非常大 效率很低
where t1.classid = t2.cid – 10条
3.内连接
select t1.,t2.
from t_student t1 inner join t_class t2 on t1.classid = t2.cid
select t1.,t2.
from t_class t2 inner join t_student t1 on t1.classid = t2.cid
–先比较过滤，再保存到内存中 左表的数据一条一条的和右表的数据进行连接比较
– 左边的数据和右边的数据满足 on 关键字后面的条件保留
–在连接的时候一般将数据量小的表放在连接符合的左侧
–但不满足条件就会过滤掉，有时会出错
查询出学生表中的所有的学生信息及对应的班级信息
4.左连接:在内连接的基础上保留左侧不满足条件的数据
左表是主表，不满足条件也会保留
select t1.,t2.
from t_student t1 left outer join t_class t2
on t1.classid = t2.cid
select t2.,t1.
from t_class t1 left join t_student t2
on t1.cid = t2.classid
5.右连接:在内连接的基础上保留右侧不满足条件的数据
select t1.,t2.
from t_student t1 right join t_class t2
on t1.classid = t2.cid
6.全连接:在内连接的基础上保留左右两侧不满足条件的数据
全连接是左连接和右连接的并集
select t1.,t2.
from t_student t1 full join t_class t2
on t1.classid = t2.cid
例题：
--drop table student；
create table student (
id number(3) PRIMARY key,
name VARCHAR2(20) not null,
sex varchar2(4),
birth number(4),
department varchar2(20),
address VARCHAR2(50))
--创建score表。SQL代码如下：
create table score(
id number(3) PRIMARY key,
stu_id number(3) not null,
c_name VARCHAR(20) ,
grade number(3)
)
-- 向student表插入记录的INSERT语句如下：
insert into student values(901,'张老大','男',1985,'计算机系','北京市海淀区');
insert into student values(902,'张老二','男',1986,'中文系','北京市昌平区');
insert into student values(903,'张三','女',1990,'中文系','湖南省永州市');
insert into student values(904,'李四','男',1990,'英语系','辽宁省阜新市');
insert into student values(905,'王五','女',1991,'英语系','福建省厦门市');
insert into student values(906,'王六','男',1988,'计算机系','湖南省衡阳市');
-- 向score表插入记录的INSERT语句如下：
insert into score values(1,901,'计算机',98);
insert into score values(2,901,'英语',80);
insert into score values(3,902,'计算机',65);
insert into score values(4,902,'中文',88);
insert into score values(5,903,'中文',95);
insert into score values(6,904,'计算机',70);
insert into score values(7,904,'英语',92);
insert into score values(8,905,'英语',94);
insert into score values(9,906,'计算机',90);
insert into score values(10,906,'英语',85);
SELECT * from student;
select * from score;
--1、查询student表的第2条到4条记录
select s2.*,rownum
from (select s.*,rownum num from student s where rownum <=4) s2
where s2.num >=2;
--2、从student表查询所有学生的学号（id）、
姓名（name）和院系（department）的信息
select id 学号,name 姓名,department 院系
from student;
--3、从student表中查询计算机系和英语系的学生的信息
select *
from student
where department='计算机系' or department = '英语系';
--4、从student表中查询年龄25~30岁的学生信息
select *
from student
where (extract(YEAR from sysdate)-birth) between 25 and 30;
--5、从student表中查询每个院系有多少人
select department,count(1)
from student
group by department;
--6、从score表中查询每个科目的最高分
select c_name,max(grade)
from score
group by c_name;
--7、查询李四的考试科目（c_name）和考试成绩（grade）
注意： '=' 只有在确定结果是一个的情况下使用，不确定的使用用 'in'
select s.c_name,s.grade
from score s
where stu_id = (select id from student where name='李四');
--in 的效率会比较低
select s.c_name,s.grade
from score s
where exists (select id from student s2 where name='李四' and s2.id = s.stu_id);
select t1.* ,t2.* --内连接
from student t1 inner join score t2
on t1.id = t2.stu_id and t1.name='李四';
select t1.* ,t2.* --效率比较高
from (select * from student where name='李四') t1 inner join score t2
on t1.id = t2.stu_id;
--8、用内连接的方式查询所有学生的信息和考试信息
select s.*,s2.*
from student s inner join score s2 on s.id = s2.stu_id;
--9、计算每个学生的总成绩
select stu_id,sum(grade)
from (select stu_id,grade from score s1)
group by stu_id;
--10、计算每个考试科目的平均成绩
select c_name,avg(grade)
from score
group by c_name
--11、查询计算机成绩低于95的学生信息
select * from student
where id in (
select stu_id
from score s
where s.c_name ='计算机' and s.grade < 95)
select s.* ,s2.* --内连接
from student s inner join score s2
on s2.c_name ='计算机' and s2.grade < 95 and s2.stu_id = s.id
select s.* ,s2.* --比之前的内连接效率要高，先查出计算机低于95的信息减少数据条数
from (select * from score where c_name ='计算机' and grade < 95) s2 inner join student s
on s2.stu_id = s.id
--12、查询同时参加计算机和英语考试的学生的信息
select * from student
where id in(--或者用exists
select stu_id
from score
where c_name='英语' and stu_id in(select stu_id
from score s1
where c_name='计算机')
)
select * from student s
where exists(--exists
select stu_id
from score s2
where c_name='英语' and stu_id in(select stu_id
from score s1
where c_name='计算机')
and s.id = s2.stu_id
)
select * --完全的内连接
from (select s1.stu_id
from (select * from score where c_name='英语') s2 inner join
(select * from score where c_name='计算机') s1
on s1.stu_id = s2.stu_id
) s3 inner join student s4 on s4.id = s3.stu_id;
select *
from student --内连接配合子查询
where id in (select s1.stu_id
from (select * from score where c_name='英语') s2 inner join
(select * from score where c_name='计算机') s1
on s1.stu_id = s2.stu_id
)
--13、将计算机考试成绩按从高到低进行排序
select *
from score
where c_name='计算机'
order by grade desc
--14、从student表和score表中查询出学生的学号，
然后合并查询结果 UNION与union all
select stu_id
from score
union
select id
from student
--15、查询姓张或者姓王的同学的姓名、院系和考试科目及成绩
select s.name,s.department,s2.c_name,s2.grade
from (select id,name,department from student
where name like '张%' or name like '王%') s inner join score s2
on s.id = s2.stu_id;
--16、查询都是湖南的学生的姓名、年龄、院系和考试科目及成绩
select s.name,s.age,s.department,s.address,s2.c_name,s2.grade
from (select id,name,extract(YEAR from sysdate)-birth age,department,address from student
where address like '湖南%') s inner join score s2
on s.id = s2.stu_id;