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;

相关子查询

• 相关子查询按照一行接一行的顺序执行，主查询的每一行都执行一次子查询

SELECT column1, column2, ...
 FROM   table1  outer
 WHERE  column1 operator
              (SELECT  colum1, column2
                       FROM    table2
                       WHERE   expr1 = outer.expr2);

问题：若employees表中employee_id与job_history表中employee_id相同的数目不小于2，输出这些相同id的员工的employee_id,last_name和其job_id

SELECT e.employee_id, last_name,e.job_id
FROM   employees e
WHERE  2 <= (SELECT COUNT(*)
             FROM   job_history
             WHERE  employee_id = e.employee_id);

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);

相关更新

UPDATE table1 alias1
SET    column = (SELECT expression
                 FROM   table2 alias2
                 WHERE  alias1.column =    
                        alias2.column);

• 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

ALTER TABLE employees
ADD(department_name VARCHAR2(14));

UPDATE employees e
SET    department_name =
              (SELECT department_name
           FROM   departments d
           WHERE  e.department_id = d.department_id);

相关删除

DELETE FROM table1 alias1
 WHERE  column operator
    (SELECT expression
      FROM   table2 alias2
      WHERE  alias1.column = alias2.column);

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
问题：删除表employees中，其与emp_history表皆有的数据

DELETE FROM employees E
WHERE employee_id =  
           (SELECT employee_id
            FROM   emp_history
            WHERE  employee_id = E.employee_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;