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进阶7:子查询

时间:2021/6/2 23:59:07|来源:|点击: 次

进阶7:子查询

含义:出现在其他语句中的select语句,称为子查询或内查询
外部的查询语句,称为主查询或外查询

分类:
按子查询出现的位置:
select后面:仅仅支持标量子查询

    from后面:支持表子查询

    where或having后面(重点):标量子查询、列子查询、  行子查询

    exists后面(相关子查询):表子查询

按结果集的行列数不同:
标量子查询(结果集只有一行一列)
列子查询(结果集只有一列多行)
行子查询(结果有一行多列)
表子查询(结果集一般为多行多列)

一.where或having后面

1.标量子查询(单行子查询)

2.列子查询(多行子查询)

3.行子查询(多行多列)

特点:
a.子查询放在小括号内
b.子查询一般放在条件的右侧
c.标量子查询,一般搭配着单行操作符使用
> < >= <= = <>
d.列子查询,一般搭配着多行操作符使用
in any/some all
e.子查询的执行优先于主查询执行,主查询的条件用到了子查询的结果

1.标量子查询

案例1:谁的工资比Abel高?

p1 查询Abel的工资
select salary from employees where last_name = "Abel";
P2 查询员工的信息,满足salary>p1的结果
select * from employees where salary>(select salary from employees where last_name = "Abel");

案例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

select last_name,job_id,salary from employees where salary >(select salary from employees where employee_id = 143) and job_id =(select job_id from employees where employee_id = 141);

案例3:返回公司工资最少的员工的last_name,job_id和salary

select last_name,job_id,salary from employees where salary =(select min(salary) from employees);

案例4: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

p1:查询50号部最低工资、
select min(salary) from employees group by department_id where department_id = 50;
p2:查询最低工资大于p1结果的部门
select department_id,min(salary) from employees group by department_id having min(salary) > (select min(salary) from employees group by department_id where department_id = 50);

#非法使用标量子查询

子查询中返回值为多列或者为空,则主查询无法执行

2.列子查询(多行子查询)

案例1:返回location_id是1400或1700的部门中的所有员工姓名

select last_name from employees where department_id = any(select department_id from departments where location_id between 1400 and 1700);

select last_name from employees where department_id in (select department_id from departments where location_id between 1400 and 1700);

案例2:返回其他部门中比job_id为’IT_PROG’部门任一工资低的员工的员工号、姓名、job_id以及salary

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';

select employee_id,last_name,job_id,salary from employees where salary < some (select salary from employees where job_id='IT_PROG') and job_id <> 'IT_PROG';

案例3:返回其他部门中比job_id为’IT_PROG’部门所有工资都低的员工的员工号、姓名、job_id以及salary

select employee_id,last_name,job_id,salary from employees where salary < all (select salary from employees where job_id='IT_PROG') and job_id <> 'IT_PROG';

3.行子查询(结果集一行多列或者多行多列)

案例1:查询员工编号最小并且工资最高的员工信息

p1.查询最小的员工编号
select min(employee_id) from employees;
p2.查询最高工资
select max(salary) from employees;
p3.查询员工信息
select * from employees where employee_id=(select min(employee_id) from employees) and salary=(select max(salary) from employees);

select * from employees where (employee_id,salary)=(select min(employee_id),max(salary) from employees);

二.select后面

注意:仅仅支持标量子查询。

案例1.查询每个部门的员工个数

select d.*,(select count(*) from employees e where e.department_id=d.department_id) 个数 from departments d;

select d.*,count(*) from employees e left outer join departments d on e.department_id=d.department_id group by e.department_id;

案例2.查询员工号=102的部门名

select (select deaprtmen_name from departments d inner join employees e on e.department_id=d.department_id where d.department_id=102) 部门名;

三.from后面

将子查询结果充当一张表,要求必须起别名。

案例1:查询每个部门的平均工资的工资等级

p1.查询每个部门的平均工资
select avg(salary),department_id from employees group by department_id;
p2.连接p1的结果集和job_grade表,筛选条件平均工资between lowest_sal and highest_sal
select ag_dep.* from (select avg(salary),department_id from employees group by department_id) ag_dep inner join job_grades g on ag_dep.avg(salary) between lowest_sal and highest_sal;

四.exists后面(相关子查询)

语法:exists(完整的查询语句);

结果:1或0

select exists(select employee_id from employees where salary=300000);

案例1.查询有员工的部门名

select department_name from departments d where exists(select * from employees e where d.department_id=e.department_id);

#用in实现
select department_name from departments d where department_id in (selcet department_id from employees);

案例2.查询没有女朋友的男神信息

#exists
select * from boys bo where not exists(select * from beauty b where bo.id=b.boyfriend.id);

#in
select * from boys where id not in (select boyfriend_id from beauty);

1.查询和zlotkey相同部门的员工的姓名和工资

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。

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和姓名

select e.employee_id,e.last_name from (select *,avg(salary) avg from employees group by department_id ) av right outer join employees e on av.department_id = e.department_id where e.salary >av.avg;

4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名

select employee_id,last_name from employees where department_id in (select department_id from employees where last_name like '%u%');

5.查询所在部门的location_id为1700的部门工作的员工的员工号

select employee_id from employees where department_id in (select department_id from departments where location_id = 1700);

6.查询管理者是King的员工姓名和工资

select last_name,salary from employees where manager_id = (select employee_id from employees where last_name = 'King');

select e.last_name,e.salary from employee e inner join employees m on e.manager_id = m.employee_id where m.last_name = 'King';

7.查询工资最高的员工的姓名,要求first_name和last_name显示为一列,列名为姓,名

select concat(first_name,'',last_name) as '姓,名' from employees where salary = (select max(salary) from employees);

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