where、from、exists子查询、分页查询
- [1 where子查询](#1 where子查询)
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- [1.1 where后面的标量子查询](#1.1 where后面的标量子查询)
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- [1.1.1 having后的标量子查询](#1.1.1 having后的标量子查询)
- [1.2 where后面的列子查询](#1.2 where后面的列子查询)
- [1.3 where后面的行子查询(了解即可)](#1.3 where后面的行子查询(了解即可))
- [2 from子查询](#2 from子查询)
- [3 exists子查询(相关子查询)](#3 exists子查询(相关子查询))
- [4 分页查询](#4 分页查询)
- [5 联合查询](#5 联合查询)
- [6 练习](#6 练习)
1 where子查询
1.1 where后面的标量子查询
1.谁的工资比Abel高?
sql
select *
from employees
where salary > (
select salary
from employees
where last_name = 'Abel'
);
2.返回job_id与141号员工相同,salary比143号员工多的员工姓名,job_id 和工资
sql
select last_name,job_id,salary
from employees
where job_id = (
select job_id
from employees
where employee_id = 141
)
and salary > (
select salary
from employees
where employee_id = 143
);
3.返回公司工资最少的员工的last_name,job_id和salary
sql
select last_name,job_id,salary
from employees
where salary = (
select min(salary)
from employees
);
1.1.1 having后的标量子查询
查询最低工资大于50号部门最低工资的部门id和其最低工资
可以拆分去考虑
1.查询50号部门最低工资
sql
select min(salary)
from employees
where department_id = 50;
2.查询每个部门的最低工资
sql
select department_id,min(salary)
from employees
group by department_id;
3.合并
sql
select department_id,min(salary)
from employees
group by department_id
having min(salary) > (
select min(salary)
from employees
where department_id = 50
);
1.2 where后面的列子查询
单列多行
IN/NOT IN 任意一个
ANY/SOME 某一个
ALL 所有
IN 等于 = ANY
1.返回location_id是1400或1700的部门中的所有员工姓名
sql
select last_name
from employees
where department_id in (
select distinct department_id
from departments
where location_id in (1400,1700)
);
2.返回其它工种中比job_id为'IT_PROG'工种任一工资低的员工的,工号、姓名、job_id 以及salary
sql
select employee_id,last_name,job_id,salary
from employees
where salary < any (
select salary
from employees
where job_id = 'IT_PROG'
) and job_id <> 'IT_PROG';
1.3 where后面的行子查询(了解即可)
一行多列或者多行多列
查询员工编号最小并且工资最高的员工信息
sql
select *
from employees
where (employee_id,salary) = (
select min(employee_id),max(salary)
from employees
);
2 from子查询
查询每个部门的平均工资的工资等级
sql
select avg_dep.department_id,avg_dep.avg,g.grade_level
from job_grades g
inner join (
select department_id,avg(salary) as avg
from employees
group by department_id
)as avg_dep
on avg_dep.avg between g.lowest_sal and g.highest_sal;
3 exists子查询(相关子查询)
只关心有没有这个值
查询有员工的部门名
sql
select department_name
from departments d
where exists(
select *
from employees e
where d.department_id = e.department_id
);
4 分页查询
limit offset,size
offset 要显示条目的索引
1.查询前五条的员工信息
sql
select *
from employees
limit 0,5; #起始索引是0,一共显示5条
2.查询第11条~第25条的员工信息
sql
select *
from employees
limit 10,15;
3.有奖金的员工信息,并且工资较高的前10名显示出来
sql
select *
from employees
where commission_pct is not null
order by salary desc
limit 10;
5 联合查询
查询中国用户中男性的信息以及外国用户中年男性的用户信息
sql
select * from t_ca where csex='男'
union
select * from t_ua where tGender = 'male';
联合查询的特点:
1.要求多条查询语句的查询列数是一致的
2.要求多条查询语句查询的每一列的类型和顺序最好一致
3.union会去重
不想去重的话,可以使用union all
关键字
6 练习
1.查询和zlotkey相同部门的员工姓名和工资
sql
select last_name,salary
from employees
where department_id = (
select department_id
from employees
where last_name = 'Zlotkey'
);
2查询工资比公司平均工资高的员工的员工号,姓名和工资。
sql
select employee_id,last_name,salary
from employees
where salary > (
select avg(salary)
from employees
);
3查询各部门中工资比本部门平均工资高的员工的员工号,姓名和工资
sql
select employee_id,last_name,salary
from employees e
inner join (
select department_id,avg(salary) as ag
from employees
group by department_id
) avg_dep
on avg_dep.department_id = e.department_id
where salary > avg_dep.ag;
4.查询管理者是king的员工姓名和工资
sql
select last_name,salary
from employees
where manager_id in(
select employee_id
from employees
where last_name = 'K_ing'
);
5.查询工资最高的员工的姓名,要求first_name和iast_name显示为一列,列名为 姓.名
sql
select concat(first_name,last_name) as '姓.名'
from employees
where salary = (
select max(salary)
from employees
);