一、高级子查询
1.多列子查询
定义：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；不成对比较
实例：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
成对比较：
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);
2.在from子句中使用子查询
实例：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;
3.单列子查询表达式
实例：
在 CASE 表达式中使用单列子查询
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;
在 ORDER BY 子句中使用单列子查询
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);
4.相关子查询
定义：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
格式：
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
实例：
查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id
SELECT last_name, salary, department_id
FROM employees outer
WHERE salary > (SELECT AVG(salary)
FROM employees
WHERE department_id = outer.department_id);
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);
5.exists操作符
定义：
EXISTS 操作符检查在子查询中是否存在满足条件的行，NOT EXISTS则相反。
如果在子查询中存在满足条件的行：
停止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
实例：
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);
注：子查询里select子句无论写什么内容都没关系，只需要返回条件的True或False。
6.相关更新
定义：
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
格式：
UPDATE table1 alias1
SET column = (SELECT expression
FROM table2 alias2
WHERE alias1.column =
alias2.column);
实例：
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);
7.相关删除
定义：
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
格式：
DELETE FROM table1 alias1
WHERE column operator
(SELECT expression
FROM table2 alias2
WHERE alias1.column = alias2.column);
实例：
DELETE FROM employees E
WHERE employee_id =
(SELECT employee_id
FROM emp_history
WHERE employee_id = E.employee_id);
8.with子句
作用：
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
实例：
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
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;
