摘要：
--其中，如果员工部门_ ID和位置_ ID为1800的部门_如果ID相同，则位置为“加拿大”，其余为“美国”。语法：–对于具有tmpas的一个别名–对于具有tmpas、tmp2as、tmp3as的多个别名
一、概述
主要内容：
二、子查询介绍
1.简单子查询（WHERE子查询）
SELECT last_name
FROM employees
WHERE salary >
(SELECT salary
FROM employees
WHERE employee_id = 149) ;2.多列子查询
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);3.在FROM子句中使用子查询
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;4.单列子查询
在CASE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
--(CASE
--WHEN department_id =
--(SELECT department_id FROM departments
--WHERE location_id = 1800)
--THEN 'Canada' ELSE 'USA' END) location
FROM employees5.相关子查询
相关子查询就是每一次子查询都与外层主查询相关，
而非相关子查询就是每次都返回一致的结果，与主查询无关，例如只是提供类似一个固定的筛选条件
语法：
示例：
--问题：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
)6.EXISTS操作符
--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 1
FROM employees
WHERE manager_id = outer.employee_id
)NOT EXISTS同理
7.相关更新
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
语法：
UPDATE table1 alias1
SET column = (SELECT expression
FROM table2 alias2
WHERE alias1.column =
alias2.column);示例：
UPDATE employees e
SET department_name =
(SELECT department_name
FROM departments d
WHERE e.department_id = d.department_id);相关删除同理：
DELETE FROM employees E
WHERE employee_id =
(SELECT employee_id
FROM emp_history
WHERE employee_id = E.employee_id);8.WITH子句
有点类似ODPS的CTE
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
一般而言，可以用于UNION等操作，来提高可读性！
语法：
–针对一个别名
with tmp as (select * from tb_name)
–针对多个别名
with
tmp as (select * from tb_name),
tmp2 as (select * from tb_name2),
tmp3 as (select * from tb_name3),
…示例：
WITH TT
AS
(SELECT employee_id,salary FROM employees)
SELECT * FROM TT更多WITH示例，参考：https://www.cnblogs.com/linjiqin/archive/2013/06/24/3152667.html