SQL高级部分一(SET运算符 && 高级子查询)

一、SET运算符

select employee_id , department_id
from emp01
union all --相当于两个集合相加， union A并B ，intersect ，A交B，MINUS 差集，A-B
select employee_id , department_id
from emp02

SET操作的注意事项

在SELECT 列表中的列名和表达式在数量和数据类型上要相对应

括号可以改变执行的顺序

ORDER BY 子句:

只能在语句的最后出现

可以使用第一个查询中的列名, 别名或相对位置

注意：

除 UNION ALL之外，系统会自动将重复的记录删除

系统将第一个查询的列名显示在输出中

除 UNION ALL之外，系统自动按照第一个查询中的第一个列的升序排列

SELECT department_id, TO_NUMBER(null) location, hire_date
FROM   employees
UNION
SELECT department_id, location_id,  TO_DATE(null)

FROM   departments;

所查询的列一定要一一对应，没有的可以用null来代替

SELECT employee_id, job_id,salary
FROM   employees
UNION
SELECT employee_id, job_id,0 --salary可以用0代替
FROM   job_history;

指定列不显示column 指定列的名字 noprint;

实现按

SELECT 'sing'  "My dream", 3 a_nu
FROM dual
UNION
SELECT 'I`d like to teach',1
FROM dual
UNION
SELECT 'the world to',1
FROM dual
order by 2

SQL> column a_nu noprint;--指定a_nu列不显示

二、高级子查询

多列子查询

主查询与子查询返回的多个列进行比较

举例：

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

1）成对比较举例

select manager_id , employee_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)

2）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);

注意主查询的列 和 内查询的列，一定要一一对应

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

1）已学知识解决：

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
          )

此方法进行了重复性操作

2）from字句

select employee_id , e1.department_id , salary ,e2.avg_sal
from employees e1 , (select department_id , avg(salary) avg_sal
                    from employees
                    group by department_id
                    ) e2 --以子查询的形式，形成了一个新表
where e1.department_id = e2.department_id
and e1.salary > e2.avg_sal

单列子查询应用举例

在 CASE 表达式中使用单列子查询

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 department_id
                                 when (select department_id
                                       from departments
                                       where location_id = 1800
                                       )
                                 then 'Canada' else 'Usa'
                                 end
                                 ) location
from employees
*/
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

在 ORDER BY 子句中使用单列子查询

问题：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
FROM     employees e
ORDER BY (SELECT department_name
	FROM departments d
	WHERE e.department_id = d.department_id
	);

三、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

外层的表在内层中使用，就是相关子查询

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

子查询中使用主查询中的列

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 employee_id , last_name , job_id
from employees e
where 2 <= (
               select count(*)
               from job_history j
               where e.employee_id = j.employee_id
           )

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 , salary
from employees e1
where e1.employee_id in(
                   select manager_id
                   from employees e2
                   )
*/
/*
select distinct e1.employee_id , e1.last_name , e1.salary
from employees e1,employees e2
where e1.employee_id = e2.manager_id
*/
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 d1.department_id
from departments d1
minus
SELECT department_id
 FROM   employees
*/
SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (
		SELECT 'X'
                  FROM   employees
                  WHERE  department_id  = d.department_id
		);