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
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;
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
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
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
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
/*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
SELECT employee_id, last_name
FROM employees e
ORDER BY (SELECT department_name
FROM departments d
WHERE e.department_id = d.department_id
);
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
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
)
/*
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
);