一、多表联合查询
二、子查询
三、动态条件查询
LESSON 4 Displaying Data from Multiple Tables
--------------------------------------------------------
查询s_emp表中最大的工资数,并且显示出这个最大工资数的员工名字
select last_name,max(salary)
from s_emp;
多表查询
查询多张表的时候会产生笛卡尔积
为了防止笛卡尔积的产生，我们需要使用某些条件把两张表或多张表连接起来（一般会使用两个表间主外键相等的关系）
1）等值连接
内连接
自连接，自己连接自己
2）不等值连接
三种不等值连接：
表名1 left outer join 表名2 on 连接条件
表名1 right outer join 表名2 on 连接条件
表名1 full outer join 表名2 on 连接条件
其中的outer可以省去不写
查询s_emp表中每一个员工的名字和部门编号
select last_name,dept_id
from s_emp
查询s_emp表中每一个员工的名字和部门编号,并且显示出部门的名字
select se.last_name,se.dept_id,sd.id,sd.name
from s_emp se,s_dept sd
order by se.last_name;
select se.last_name,se.dept_id,sd.name
from s_emp se,s_dept sd
where se.dept_id = sd.id
order by se.last_name;
查询s_emp表中每一个员工的名字和部门编号,并且显示出部门的名字以及部门所在地区的名字
select se.last_name,se.dept_id,sd.name,sd.region_id,sr.name
from s_emp se, s_dept sd, s_region sr
where se.dept_id = sd.id and sd.region_id = sr.id
order by last_name asc;
查询s_emp表中每一个员工的id、名字、领导人的id、领导人的名字，没有领导人的员工除外。
select s1.id,s1.last_name,s1.manager_id,s2.last_name
from s_emp s1,s_emp s2
where s1.manager_id = s2.id and s1.manager_id is not null
order by s2.id asc;
//向s_emp中插入一条测试数据
//这是一条DML语句，会产生事务
insert into s_emp(id,last_name)
values(26,'tom');
//所以插入之后的数据要提交
commit;
下面都是不等值连接的例子:
---------------------------------
查询员工的名字、部门号、部门名字
(即使某个员工还没有部门编号也要显示出来)
//如果还用这样的等值连接是查询不出来tom这个人的
select se.last_name,se.dept_id,sd.name
from s_emp se,s_dept sd
where se.dept_id = sd.id;
//哪边数据少在那边加上这个加号
select se.last_name,se.dept_id,sd.name
from s_emp se,s_dept sd
where se.dept_id = sd.id(+);
左连接（将左表作为主表）
select se.last_name,se.dept_id,sd.name
from s_emp se left join s_dept sd
on se.dept_id = sd.id;
右连接
select se.last_name,se.dept_id,sd.name
from s_emp se right join s_dept sd
on se.dept_id = sd.id;
insert into s_dept(id,name)values(51,'KunShan');
commit;
--------------------------------
查询员工的名字、部门号、部门名字
(没有员工存在的部门也要显示出来)
select se.last_name,se.dept_id,sd.name
from s_emp se,s_dept sd
where se.dept_id(+) = sd.id;
查询员工的名字、部门号、部门名字
(即使某个员工还没有部门编号也要显示出来、没有员工存在的部门也要显示出来)
//全连接
select se.last_name,se.dept_id,sd.name
from s_emp se full outer join s_dept sd
on se.dept_id = sd.id;
select se.last_name,se.dept_id,sd.name
from s_emp se full join s_dept sd
on se.dept_id = sd.id;
操作俩个结果集的关键字
---------------------------------------------
(这里说的结果集指的是一条sql语句查询后得到的结果)
union
union all
minus
intersect
第一条sql:
select id,last_name
from s_emp
where id < 7;
结果集一:
ID LAST_NAME
-- ---------------
1 Velasquez
2 Ngao
3 Nagayama
4 Quick-To-See
5 Ropeburn
6 Urguhart
第二条sql语句:
select id,last_name
from s_emp
where id < 6;
结果集二:
ID LAST_NAME
-- --------------
1 Velasquez
2 Ngao
3 Nagayama
4 Quick-To-See
5 Ropeburn
1 union 取得俩个结果集的并集
select id,last_name
from s_emp
where id < 7
union
select id,last_name
from s_emp
where id < 6;
2 union all 之间把俩个结果集放在一起,不管重复的数据
select id,last_name
from s_emp
where id < 7
union all
select id,last_name
from s_emp
where id < 6;
3 minus 去掉俩个结果集相同的部分
select id,last_name
from s_emp
where id < 7
minus
select id,last_name
from s_emp
where id < 6;
4 intersect 只保留俩个结果集相同部分(交集)
select id,last_name
from s_emp
where id < 7
intersect
select id,last_name
from s_emp
where id < 6;
子查询
------------------------------------------------------
查询Ngao所在部门的编号
select dept_id from s_emp
where last_name='Ngao';
查询41号部门的平均工资
select avg(salary)
from s_emp
where dept_id = 41
group by dept_id;
select avg(salary)
from s_emp
where dept_id = 41;
AVG(SALARY)
-----------
1247.5
sql语句的基本格式:
-------------
select ...
from ..
where ...
group by ...
having ...
order by ..
**执行顺序： ---> where ---> group by --->组函数 ---> having ---> order by
select/having 后面出现了组函数，那么么有被组函数修饰的列就一定要写在group by 后面
-----------------------------------------------------------------------------
查询Ngao所在部门的平均工资
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
);
练习1：
---------------------------------------------------------------------------
查询比41号部门平均工资高的部门里面的员工信息(显示员工的 名字 部门号 工资)
select last_name,dept_id,salary
from s_emp
where dept_id=any(
select dept_id from s_emp
group by dept_id
having avg(salary) >
(
select avg(salary) from s_emp
where dept_id = 41
)
);
分析：
1.查询的是员工信息
2.员工所在部门的平均工资比41号部门高
41部门的平均工资
select avg(salary)
from s_emp
where dept_id = 41;
AVG(SALARY)
-----------
1247.5
比41部门平均工资高的部门
select dept_id
from s_emp
group by dept_id
having avg(salary)>1247.5
order by dept_id;
select dept_id
from s_emp
group by dept_id
having avg(salary)>
(
select avg(salary)
from s_emp
where dept_id = 41
)
order by dept_id;
DEPT_ID
----------
10
31
32
33
35
50
select last_name,dept_id,salary
from s_emp
where dept_id in(10,31,32,33,35,50);
select last_name,dept_id,salary
from s_emp
where dept_id in
(
select dept_id
from s_emp
group by dept_id
having avg(salary)>
(
select avg(salary)
from s_emp
where dept_id = 41
)
);
练习2：
-------------------------------------------------------------------------------------------
查询比Ngao所在部门的平均工资高的员工信息,并且这些员工所在部门平均工资也要比Ngao所在部门的平均工资要高.
select last_name,dept_id,salary
from s_emp
where dept_id in
(
select dept_id
from s_emp
group by dept_id
having avg(salary)>
(
select avg(salary)
from s_emp
where dept_id = 41
)
)
and salary >
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
);
分析：
1.查询的是员工信息
2.这些员工的工资比Ngao所在部门平均工资高
3.这些员工所在部门的平均工资也要比Ngao所在部门平均工资高
Ngao所在的部门编号
select dept_id
from s_emp
where last_name = 'Ngao';
DEPT_ID
--------
41
41号部门的平均工资
select avg(salary)
from s_emp
where dept_id = 41;
AVG(SALARY)
-----------
1247.5
比41部门平均工资高的部门
select dept_id
from s_emp
group by dept_id
having avg(salary)>1247.5
order by dept_id;
DEPT_ID
----------
10
31
32
33
35
50
这个例子最终要执行的sql其实为:
select last_name,dept_id,salary
from s_emp
where salary > 1247.5
and dept_id in(10,31,32,33,35,50);
所以最终把sql语句中的条件替换为子查询为:
select last_name,dept_id,salary
from s_emp
where salary >
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
and dept_id in
(
select dept_id
from s_emp
group by dept_id
having avg(salary)>
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
);
练习3：
----------------------------------------------------------------
查询比Ngao所在部门的平均工资高员工信息,并且这些员工所在部门平均工资也要比Ngao所在部门的平均工资要高.
(显示 员工的名字、部门号、工资)
增加显示这个员工所在部门名字
select se.last_name,se.dept_id,se.salary,sd.name,sr.name,se2.last_name
from s_emp se,s_dept sd,s_region sr,s_emp se2
where se.salary >
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
and se.dept_id in
(
select dept_id
from s_emp
group by dept_id
having avg(salary)>
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
)
and se.dept_id = sd.id
and sd.region_id = sr.id
and se.manager_id = se2.id;
1.查询的是员工信息
2.这些员工的工资比Ngao所在部门平均工资高
3.这些员工所在部门的平均工资也要比Ngao所在部门平均工资高
select se.last_name,se.dept_id,se.salary,sd.dept_name
from s_emp se join s_dept sd
on se.dept_id = sd.id;
select last_name,dept_id,salary
from s_emp
where salary >
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
and dept_id in
(
select dept_id
from s_emp
group by dept_id
having avg(salary)>
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
);
select se.last_name,se.dept_id,se.salary,sd.name
from s_emp se join s_dept sd
on ((se.dept_id=sd.id) and
(se.salary >
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
and se.dept_id in
(
select dept_id
from s_emp
group by dept_id
having avg(salary)>
(
select avg(salary)
from s_emp
where dept_id =
(
select dept_id
from s_emp
where last_name = 'Ngao'
)
)
)
)
);
练习4：
------------------------------------------
查询每一个部门中比本部门平均工资高的员工信息
显示出员工的名字、工资、部门号、这个部门的平均工资
//这个sql语句的查询结果将来可以当做要查询的一张表
select dept_id,avg(salary) avgSal
from s_emp
group by dept_id;
DEPT_ID avgSal
------- -----------
42 1081.66667
43 900
34 1160
44 1050
31 1400
32 1490
35 1450
50 2025
41 1247.5
45 1089
33 1515
10 1450
select se.last_name,se.salary,se.dept_id,temp.avgSal
from s_emp se,(
select dept_id,avg(salary) avgSal
from s_emp
group by dept_id
) temp
where se.dept_id = temp.dept_id
and se.salary > temp.avgSal;
1.查询的是员工信息
2.例如要把41号部门中比41号部门(本部门)平均工资高的员工信息查询出来
select avg(salary),dept_id
from s_emp
group by dept_id;
select last_name,salary,dept_id
from s_emp;
select s1.last_name,s1.salary,s1.dept_id,s2.avg_sal
from s_emp s1
join (select avg(salary) avg_sal,dept_id from s_emp group by dept_id) s2
on ((s1.dept_id = s2.dept_id) and (s1.salary > s2.avg_sal))
order by s1.dept_id asc;
sql语句执行的时候输入参数(运行时参数)
------------------------------------------------
select last_name,salary,dept_id
from s_emp
where last_name=&name;
select last_name,salary,dept_id
from s_emp
where id = 2;
select last_name,salary,dept_id
from s_emp
where id = &id;
//运行sql语句的时候给字符串参数赋值
select last_name,salary,dept_id
from s_emp
where last_name = &last_name;
select last_name,salary,dept_id
from s_emp
where last_name = '&last_name';
select last_name,salary,dept_id
from s_emp
where id > &id or last_name = '&last_name';
//运行时再给出具体的筛选条件
select last_name,salary,dept_id
from s_emp
where &condition;
select last_name,salary,dept_id
from s_emp
&condition;
insert into student(id,name,age)
values(&id,'&name',&age);