Oracle子查询之高级子查询

Oracle 高级子查询

　　高级子查询相对于简单子查询来说，返回的数据行不再是一列，而是多列数据。

1，多列子查询

主查询与子查询返回的多个列进行比较

查询与141号或174号员工的manager_id和department_id相同的其他员工的employee_id, manager_id, department_id

select employee_id, manager_id, department_id

from employees

where (manager_id,department_id) in ( --由于内查询返回两列，所以此处也必须使用两列来进行对应,并且列的顺序一致

select manager_id,department_id --内查询返回2列

from employees

where employee_id in (141,174)

)

and employee_id not in (141,174);

2，FROM子句中使用子查询

将子查询的结果集，作为一个虚表，然后从这个虚表中查询数据行

返回比本部门平均工资高的员工的last_name, department_id, salary及平均工资

select last_name, e.department_id, salary,avgsal

from employees e,(select department_id,round(avg(salary)) avgsal

from employees

group by department_id) avg_sal

where e.department_id = avg_sal.department_id

and e.salary > avg_sal.avgsal

order by e.department_id;

3，单列子查询的其他应用

在主查询的select列表中使用

问题：显式员工的employee_id,last_name和location。其中，若员工department_id与location_id为1800的department_id相同，

则location为’Canada’,其余则为’USA’。

select employee_id,last_name,department_id,

(case when department_id = (select department_id

from departments

where location_id = 1800

)

then 'USA'

else 'Canada' end) location

from employees

order by department_id;

或

select employee_id,last_name,department_id,

(case department_id when (select department_id

from departments

where location_id = 1800

)

then 'USA'

else 'Canada' end) location

from employees

order by department_id;

或

select employee_id,last_name,department_id,

decode(department_id,(select department_id

from departments

where location_id = 1800

),'USA',

'Canada') location

from employees

order by department_id;

4，相关子查询，

子查询语句中使用了主查询语句中的表中的数据（这个数据不一定在主查询的select语句中）

相关子查询按照一行接一行的顺序执行，即子查询取主查询表中的每一行数据值，主查询的每一行都执行一次子查询。

使用相关子查询，要考虑是否必须使用、是否合理，否则在不需要使用相关子查询的情况下就能轻易得到查询结果，使用相关子查询反而会使查询的效率下降。

用法：

get：从主查询所用的表中获取候选列

execute：子查询使用主查询的数据，并进行相关的筛选

use：如果满足内查询的条件则返回值

例1：查询员工的employee_id,last_name,要求按照员工的department_name排序

select employee_id,last_name,e.department_id,department_name

from employees e,departments dd

where e.department_id = dd.department_id(+) --该查询返回employees表中的所有行，没返回一行，都会取一个employee_id到子查询中

order by (select department_name

from departments d

where e.department_id = d.department_id); --子查询根据主查询返回的employee_id，在departments表中进行查找，有则返回数据。

desc;

例2：查询员工中工资大于本部门平均工资的员工的last_name,salary,department_id和本部门的平均工资

select last_name,salary,e1.department_id,ss.avgsal

from employees e1,(select department_id,round(avg(salary)) avgsal

from employees

group by department_id) ss --该from字句同前例

where e1.department_id = ss.department_id

and salary > (

select round(avg(salary))

from employees e2

where e1.department_id = e2.department_id --将外查询中的department_id，查找该处的department_id，并将所有的返回结果分组

group by e2.department_id)

order by e1.department_id;

例3：若employees表中employee_id与job_history表中employee_id相同且job_history表中employee_id的数目不小于2，

输出employees中这些相同id的员工的employee_id,last_name和其job_id

select e1.employee_id ,last_name,e1.job_id

from employees e1

where (select count(job_id)

from job_history j1

where e1.employee_id = j1.employee_id) >= 2

5，exists

EXISTS 操作符检查在子查询中是否存在满足条件的行

如果在子查询中存在满足条件的行:

不在子查询中继续查找

条件返回 TRUE

如果在子查询中不存在满足条件的行:

条件返回 FALSE

继续在子查询中查找

查询公司管理者的employee_id,last_name,job_id,department_id信息

子查询

select employee_id,last_name,job_id,department_id

from employees e1

where e1.employee_id in (

select distinct(manager_id)

from employees);

自连接

select distinct e1.employee_id,e1.last_name,e1.job_id,e1.department_id

from employees e1,employees e2

where e1.employee_id = e2.manager_id;

相关子查询

select employee_id,last_name,job_id,department_id

from employees e1

where e1.employee_id in (

select manager_id

from employees e2

where e1.employee_id = e2.manager_id);

使用exists相关子查询

select employee_id,last_name,job_id,department_id

from employees e1

where exists (

select 'A' --根据e1表中的每个 employee_id在e2中查找 manager_id,如果找到，子查询返回true，主查询返回该数据行

from employees e2

where e1.employee_id = e2.manager_id);

6.not exists

查询departments表中，不存在于employees表中的部门的department_id和department_name

select d.department_id,d.department_name

from departments d

where not exists ( --not exists 返回子查询中false的结果

select 'X' --根据e1表中的每个 employee_id在e2中查找 manager_id,如果找到，子查询返回true，即该部门在departments中存在，也有员工

from employees e

where d.department_id = e.department_id);

7.相关更新

使用相关子查询依据一个表中的数据更新另一个表的数据

例：向employees中添加一列 department_name ，并更具department_id填充

update employees e1

set department_name = (

select department_name

from departments

where e1.department_id = department_id);

8,相关删除

使用相关子查询依据一个表中的数据删除另一个表的数据

例：删除表employees中，其与emp_history表皆有的数据

delete from employees e1

where employee_id in (

select employee_id

from job_history

where e1.employee_id = employee_id);

9,使用 WITH 子句

可以避免在 SELECT 语句中重复书写相同的语句块

WITH 子句将该子句中的语句块执行一次并存储到用户的临时表空间中

使用 WITH 子句可以提高查询效率

例，查询公司中各部门的总工资,大于公司中各部门的平均总工资,的部门信息

使用普通方法：使用普通方法，汇总后的语句冗长不易理解

--各部门总工资

select department_id,sum(salary) sum_sal from employees group by department_id;

--各部门的平均总工资

select sum(to_sal.sum_sal)/count(*) to_avg_sal

from (select department_id,sum(salary) sum_sal

　　 from employees group by department_id) to_sal;

--根据前面两项，查询最终结果

select to_sal.department_id,sum_sal

from (select department_id,sum(salary) sum_sal from employees group by department_id) to_sal,

　　 (select sum(to_sal.sum_sal)/count(*) to_avg_sal from (select department_id,sum(salary) sum_sal from employees group by department_id) to_sal) avg_sal

where to_sal.sum_sal > avg_sal.to_avg_sal;

使用with字句

with to_sal as --各部门总工资

(select department_id,sum(salary) sum_sal from employees group by department_id),

avg_sal as --各部门的平均总工资

(select sum(to_sal.sum_sal)/count(*) to_avg_sal from to_sal)

select department_id,sum_sal

from to_sal

where to_sal.sum_sal > (

select avg_sal.to_avg_sal

from avg_sal);

with

to_sal as

(select department_id,sum(salary) sum_sal from employees group by department_id),

avg_sal as

(select sum(to_sal.sum_sal)/count(*) to_avg_sal from to_sal)

select department_id,sum_sal,to_avg_sal

from to_sal,avg_sal

where to_sal.sum_sal > avg_sal.to_avg_sal;

注明：本博文系学习尚硅谷网易云课堂教学课程总结输出。