#第九章、子查询

#1、由一个具体的需求，引入子查询
#需求：谁的工资比Abel的高？
#方式一：
SELECT last_name,salary
FROM employees
WHERE last_name = 'Abel';

SELECT last_name,salary
FROM employees
WHERE salary > 11000;

#方式二：自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e2.salary > e1.salary	#多表的连接条件
AND e1.last_name = 'Abel';

#方式三：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (
								SELECT salary
								FROM employees
								WHERE last_name = 'Abel'
								);
								
#2、称谓的规范：外查询（或主查询）、内查询（或子查询）

/*
子查询（内查询）在主查询之前一次执行完成。
子查询的结果被主查询（外查询）使用。

注意事项：
子查询要包含在括号内
将子查询放在比较条件的右侧
单行操作符对应单行子查询，多行操作符对应多行子查询
*/
/*
3、子查询的分类
角度1：从内查询返回的结果的条目数
			单行子查询 vs 多行子查询
角度2：内查询是否被执行多次
			相关子查询 VS 不相关子查询
比如：相关子查询的需求：查询工资大于本部门平均工资的员工信息
			不相关子查询的需求：查询工资大于本公司平均工资的员工信息
*/

#子查询的编写技巧（或步骤）：（1）、从里往外写	（2）、从外往里写

#4、单行子查询
#4.1、单行操作符： =	 !=	 >		>=		<		<=
#题目1：查询工资大于149号员工工资的员工信息

SELECT employee_id,last_name,salary
FROM employees
WHERE salary > (
								SELECT salary
								FROM employees
								WHERE employee_id = 149
								);

#题目2：返回job_id与141号员工相同，salary比143号员工多的员工姓名，job_id和工资

SELECT last_name,job_id,salary
FROM employees
WHERE job_id = (
								SELECT job_id
								FROM employees
								WHERE employee_id = 141
								)
AND salary > (
								SELECT salary
								FROM employees
								WHERE employee_id = 143
								);

#题目3：返回公司工资最少的员工的last_name，job_id和salary。

SELECT last_name,job_id,salary
FROM employees
WHERE salary = (
								SELECT MIN(salary)
								FROM employees
								);
								
#题目4：查询与141号员工的manager_id和department_id相同的其他员工的employee_id，manager_id，department_id。

#方式一：适用性更好
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id = (
										SELECT manager_id
										FROM employees
										WHERE employee_id = 141
										)
AND department_id = (
										SELECT department_id
										FROM employees
										WHERE employee_id = 141
										)
AND employee_id <> 141;

#方式二
SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id)=(
																	SELECT manager_id,department_id
																	FROM employees
																	WHERE employee_id = 141
																	)
AND employee_id <> 141;

#题目5：查询最低工资大于110号部门最低工资的部门id和其最低工资

SELECT department_id,MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (
											SELECT MIN(salary)
											FROM employees
											WHERE department_id = 110
											);

#题目6：显示员工的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;

#4.2、子查询中的空值问题
SELECT last_name,job_id
FROM employees
WHERE job_id = (
								SELECT job_id
								FROM employees
								WHERE last_name = 'Haas'
								);
#4.3、非法使用子查询
#错误：Subquery returns more than 1 row
-- SELECT employee_id,last_name
-- FROM employees
-- WHERE salary = (
-- 								SELECT MIN(salary)
-- 								FROM employees
-- 								GROUP BY department_id
-- 								);

#5、多行子查询
#5.1、多行子查询的操作符：IN	ANY	ALL	SOME（同ANY）

#5.2、举例
# IN
SELECT employee_id,last_name
FROM employees
WHERE salary IN (
								SELECT MIN(salary)
								FROM employees
								GROUP BY department_id
								);

# ANY / ALL
#题目：返回其他job_id中比job_id为IT_PROG部门任一工资低的员工的员工号、姓名、
#job_id以及salary

SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ANY(
									SELECT salary
									FROM employees
									WHERE job_id = 'IT_PROG'
								);

#题目：返回其他job_id中比job_id为IT_PROG部门所有工资低的员工的员工号、姓名、
#job_id以及salary

SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ALL (
									SELECT salary
									FROM employees
									WHERE job_id = 'IT_PROG'
								);

#题目：查询平均工资最低的部门id
#MySQL中聚合函数是不能嵌套的
#方式一：

SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
											SELECT MIN(avg_sal)
											FROM (
														SELECT AVG(salary) avg_sal
														FROM employees
														GROUP BY department_id
														) t_dept_avg_sal
											);

#方式二：
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (											
											SELECT AVG(salary) avg_sal
											FROM employees
											GROUP BY department_id													
											);

#5.3、空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
											SELECT manager_id
											FROM employees
											WHERE manager_id IS NOT NULL
											);

#6、相关子查询
#6.1
#回顾：查询员工中工资大于公司平均工资的员工的last_name，salary和其department_id
SELECT last_name,salary,department_id
FROM employees
WHERE salary > (
								SELECT AVG(salary)
								FROM employees
								);

#题目2：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
#方式一：使用相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
								SELECT AVG(salary)
								FROM employees e2
								WHERE department_id = e1.department_id
								);

#方式二：在FROM中声明子查询

SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
								SELECT department_id,AVG(salary) avg_sal
								FROM employees
								GROUP BY department_id
								) t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal;

#题目：查询员工的id，salary，按照department_name排序

SELECT employee_id,salary
FROM employees e
ORDER BY (
					SELECT department_name
					FROM departments d
					WHERE e.department_id = d.department_id
					) ASC;

#结论：在SELECT中，除了GROUP BY和LIMIT之外，其他位置都可以声明子查询。

#题目：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job_id。

SELECT * FROM job_history;

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
					);

#6.2、EXISTS与NOT EXISTS关键字
#题目：查询公司管理者的employee_id，last_name，job_id，department_id信息
#方式一：自连接

SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp JOIN employees mgr
ON emp.manager_id = mgr.employee_id;

#方式二：子查询

SELECT DISTINCT employee_id,last_name,job_id,department_id
FROM employees emp
WHERE employee_id IN (
											SELECT DISTINCT manager_id
											FROM employees
											);

#我做的
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (
										SELECT manager_id
										FROM employees
										WHERE manager_id IS NOT NULL
										GROUP BY manager_id
										);

#方式三：EXISTS

SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (
							SELECT *
							FROM employees e2
							WHERE e1.employee_id = e2.manager_id
							);

#题目：查询departments表中，不存在与employees表中的部门的department_id和department_name。
#方式一：外连接

SELECT d.department_id,d.department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL;

#方式二：使用NOT EXISTS

SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
							SELECT *
							FROM employees e
							WHERE d.department_id = e.department_id
							);