MySQL----多表查询（下）

# 左外连接
select 字段列表 from 表1 left [outer] join 表2 on 条件;
# 右外连接
select 字段列表 from 表1 right [outer] join 表2 on 条件;

# 查询emp表的所有数据，和应于的部门信息(左)
select emp.*, dept.* from emp left outer join dept on emp.dept_id = dept.id;
# 查询dept表的所有数据，和对于的员工信息(右)
select dept.*, emp.* from emp right outer join dept on emp.dept_id = dept.id;

select 字段列表 from 表a 别名a join 表a 别名b on 条件;

# 查询员工及其所属领导的名字
# 自连接可以看成两张一样的表进行连接查询
select a.name, b.name from emp a join emp b on a.managerid=b.id;

select 字段列表 from 表a
union [all]
select 字段列表 from 表b

# 将薪资低于5000的员工和年龄大于50的员工查询出来
select * from emp where salary>5000
union all
select * from emp where age>50;

# 没有all重复满足条件的只出现一次
# 将薪资低于5000的员工和年龄大于50的员工查询出来
select * from emp where salary>5000
union
select * from emp where age>50;

标量子查询：子查询的结果为单个值
列子查询：子查询的结果为一列
行子查询：子查询的结果为一行
表子查询：子查询的结果为多行多列

where之后
from之后
select之后

# 根据销售部门的id查询员工信息
# 先分开查询
# 查询销售部门的id
select id from dept where name='销售部'; #id为4
# 查询销售部门中员工的信息
select * from emp where dept_id=4;
# 合并为一个查询
select * from emp where dept_id=(select dept.id from dept where dept.name='销售部' );

# 列子查询
# 查询销售部和市场部的所有员工信息
# 查询销售部和市场部的id
select id from dept where name='销售部' or name='市场部'; #id为2 4
# 查询两个部门的所有员工
select * from emp where dept_id in (2,4);
# 合并
select * from emp where dept_id in (select id from dept where name='销售部' or name='市场部');

# 查询与张无忌的薪资及直属领导相同的员工信息
# 查询张无忌的薪资和直属领导
select salary, managerid from emp where name='张无忌';
# 查询与张无忌的薪资及直属领导相同的员工信息
select * from emp where (salary,managerid)=(select salary, managerid from emp where name='张无忌');

# 查询与鹿杖客和宋远桥的职位和薪资相同的员工信息
select * from emp where (job, salary) in ( select job, salary from emp where name in ('鹿杖客', '宋远桥'));

# 查询入职日期是’2006-01-01‘之后的员工信息和部门信息
# 先查询出入职在’2006-01-01‘之后员工的所有信息
# 与部门表左连接
select e.*, dept.* from (select * from emp where entrydate>'2006-01-01') e left outer join dept on e.dept_id=dept.id;

create table salgrade (
    grade int,
    losal int comment '本薪资等级的最低界限',
    hisal int comment '最高界限'
) comment '薪资等级表';
insert into salgrade values (1,0,3000);
insert into salgrade values (2,3001,5000);
insert into salgrade values (3,5001,8000);
insert into salgrade values (4,8001,10000);
insert into salgrade values (5,10001,15000);
insert into salgrade values (6,15001,20000);
insert into salgrade values (7,20001,25000);
insert into salgrade values (8,025001,30000);

select e.name, e.age, e.job, d.* 
from emp e, dept d 
where e.dept_id=d.id;

select e.name,e.age,e.job,d.*
from emp e
inner join dept d on e.dept_id = d.id
where e.age<30;

select distinct d.id,d.name
from emp e, dept d
where d.id=e.dept_id;

select e.*,d.name
from emp e
left outer join dept d on e.dept_id = d.id
where e.age>40;

select e.*,s.grade
from emp e, salgrade s
where e.salary between s.losal and s.hisal;

select e.*,s.grade
from emp e,dept d,salgrade s
where (e.dept_id=d.id) and (d.name='研发部') and (e.salary between s.losal and s.hisal);

select avg(e.salary)
from emp e, dept d
where e.dept_id=d.id and d.name='研发部';

select *
from emp
where emp.salary > (
                      select e.salary
                      from emp e
                      where e.name='灭绝'
                      );

select *
from emp
where salary> (
    select avg(e.salary)
    from emp e
    );

select *
from emp
where emp.salary<(
    select avg(salary)
    from emp e
    where e.dept_id=emp.dept_id
    );

select d.*, (
    select count(*)
    from emp
    where emp.dept_id=d.id
    )
from dept d;