Oracle系列八高级子查询

子查询

子查询 (内查询) 在主查询执行之前执行
主查询(外查询)使用子查询的结果

多列子查询

主查询与子查询返回的多个列进行比较

多列子查询中的比较分为两种:

成对比较

问题：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

                      (SELECT manager_id, department_id

                       FROM   employees

                       WHERE  employee_id IN (141,174))

AND    employee_id NOT IN (141,174);

不成对比较

SELECT  employee_id, manager_id, department_id

FROM    employees

WHERE   manager_id IN                   (SELECT  manager_id

                   FROM    employees

                   WHERE   employee_id IN (174,141))

AND     department_id IN                   (SELECT  department_id

                   FROM    employees

                   WHERE   employee_id IN (174,141))

AND    employee_id NOT IN(174,141);

在 FROM 子句中使用子查询

问题：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
方法一：

select last_name,department_id,salary,

(select avg(salary)from employees e3

where e1.department_id = e3.department_id

group by department_id) avg_salary

from employees e1

where salary >

         (select avg(salary)

          from employees e2

          where e1.department_id = e2.department_id

          group by department_id

          );

方法二：

SELECT  a.last_name, a.salary,

        a.department_id, b.salavg

FROM    employees a, (SELECT   department_id,

                      AVG(salary) salavg

                      FROM     employees

                      GROUP BY department_id) b

WHERE   a.department_id = b.department_id

AND     a.salary > b.salavg;

单列子查询表达式

单列子查询表达式是在一行中只返回一列的子查询

Oracle8i 只在下列情况下可以使用

例如:
SELECT 语句 (FROM 和 WHERE 子句)
INSERT 语句中的VALUES列表中

Oracle9i中单列子查询表达式可在下列情况下使用:

DECODE 和 CASE
SELECT 中除 GROUP BY 子句以外的所有子句中

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,

    (

        CASE

            WHEN department_id = (

                SELECT

                    department_id

                FROM

                    departments

                WHERE

                    location_id = 1800

            ) THEN 'Canada'

            ELSE 'USA'

        END

    ) location

FROM

    employees;

使用 EXISTS 和 NOT EXISTS 操作符

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

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

SELECT employee_id, last_name, job_id, department_id

FROM   employees outer

WHERE  EXISTS ( SELECT 'X'

                 FROM   employees

                 WHERE  manager_id =

                        outer.employee_id);

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

SELECT department_id, department_name

FROM departments d

WHERE NOT EXISTS (SELECT 'X'

                  FROM   employees

                  WHERE  department_id

                         = d.department_id);

使用 WITH 子句

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

问题：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

WITH dept_costs  AS (

   SELECT  d.department_name, SUM(e.salary) AS dept_total

   FROM    employees e, departments d

   WHERE   e.department_id = d.department_id

   GROUP BY d.department_name),

avg_cost    AS (

   SELECT SUM(dept_total)/COUNT(*) AS dept_avg

   FROM   dept_costs)

SELECT *

FROM   dept_costs

WHERE  dept_total >

        (SELECT dept_avg

         FROM avg_cost)

ORDER BY department_name;