MySQL学习之子查询

#第09章_子查询

#1.由一个具体的例子引入子查询
#需求：谁的工资比Abel的高
#方式1：
SELECT last_name,salary
FROM employees
WHERE last_name='Abel';

SELECT last_name,salary
FROM employees
WHERE salary>11000;

#方式2：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3：子查询
SELECT last_name,salary
FROM employees
WHERE salary>(
	SELECT salary   #这里结果要是一列才行	
	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：从内查询返回的条目树数
	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：查询工资大于本部门平均工资的员工信息
      不相关子查询：查询工资大于本公司平均工资的员工信息

*/

#4.单行子查询 
#4.1单行操作符：=  ！= > >=  < <=  

#题目：查询工资大于149号员工工资的员工的信息
#子查询的编写技巧（步骤）：①从里往外写 ②从外往里写

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

#题目：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 salary>(
	    SELECT salary
	    FROM employees
	    WHERE employee_id=143


)AND job_id=(
	    SELECT job_id
	    FROM employees
	    WHERE employee_id=141

);

#题目：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：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1：
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2：了解
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：查询最低工资大于50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=50
);

#题目：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;

#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：返回其它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：返回其它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使用的,
#解决方法：将查出来的结果看作是一张表，avg_sal作为其中的一个列名，对新表起个别名就可以查询
#方式1：
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_det_avg_sal

);
#方式2：
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
		#swhere manager_id is not null
);
#6.相关子查询
#6.1
#题目：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1：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2：在FROM中使用子查询
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`
)
#结论：在SLECT结构中，除了GROUP BY 和LIMIT之外，其他位置都可以声明子查询
/*
SELECT  ...,...,...(存在聚合函数)
FROM ...（LEFT/RIGHT）JOIN...ON多表连接条件
JOIN...ON...
WHERE 多表的连接条件 AND 不包含聚合函数的过滤条件
GROUP BY ...,...
HAVING 包含聚合函数的过滤条件
ORDER　BY　...，...（ASC，DESC）
LIMIT　...，...
*/
#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`
);
#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1：使用自连接
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2：子查询
SELECT 	employee_id,last_name,job_id,department_id
FROM employees
WHERE department_id IN(
			SELECT  DISTINCT manager_id
			FROM employees
			
);
#方式3：使用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，不存在于employees表中的部门的department_id和department_name
#方式1：使用左外连接
SELECT d.department_id,d.department_name
FROM departments d LEFT JOIN employees e
ON d.`department_id`=e.`department_id`
WHERE e.`department_id` IS NULL;
#方式2：
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS(
		SELECT *
		FROM employees e
		WHERE d.`department_id`=e.`department_id`
);
#课后题
#1.查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name,salary
FROM employees
WHERE department_id IN (
		    SELECT department_id
		    FROM employees
		    WHERE last_name='Zlotkey'
);
#2.查询工资比公司平均工资高的员工的员工号，姓名和工资。
SELECT employee_id,last_name,salary
FROM employees
WHERE salary >(
		SELECT AVG(salary)
		FROM employees
);
#3.选择工资大于所有JOB_ID = 'SA_MAN'的员工的工资的员工的last_name, job_id, salary
SELECT last_name,job_id,salary
FROM employees
WHERE salary>ALL(
		SELECT salary
		FROM employees
		WHERE job_id='SA_MAN'
);
#4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
SELECT employee_id,last_name
FROM employees
WHERE department_id IN (
			SELECT DISTINCT department_id
			FROM employees
			WHERE last_name LIKE '%u%'
);
#5.查询在部门的location_id为1700的部门工作的员工的员工号
SELECT employee_id
FROM employees
WHERE department_id IN( 
			  SELECT department_id
			  FROM departments
			  WHERE location_id=1700

);
#6.查询管理者是King的员工姓名和工资
SELECT last_name,salary,manager_id
FROM employees
WHERE manager_id IN (
		    SELECT employee_id
		    FROM employees
		    WHERE last_name='King'
);
#7.查询工资最低的员工信息: last_name, salary
SELECT last_name,salary
FROM employees
WHERE salary=(
		SELECT MIN(salary)
		FROM employees
);
#8.查询平均工资最低的部门信息
SELECT *
FROM departments
WHERE department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary)=(
				SELECT MIN(sal_avg)
				FROM (
					SELECT AVG(salary) sal_avg
					FROM employees
					GROUP BY department_id
				) t_avg_sal


		)

);
#方式2：
SELECT *
FROM departments
WHERE department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary)<= ALL(
					SELECT AVG(salary) sal_avg
					FROM employees
					GROUP BY department_id
		)

);

#方式3：LIMIT
SELECT d.* 
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
) t_avg_sal
WHERE d.`department_id`=t_avg_sal.`department_id`


#9.查询平均工资最低的部门信息和该部门的平均工资（相关子查询）
SELECT d.*,(SELECT AVG(salary)
	    FROM employees
	    WHERE department_id=d.`department_id`
) avg_sal
FROM departments d
WHERE department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary)=(
				SELECT MIN(sal_avg)
				FROM (
					SELECT AVG(salary) sal_avg
					FROM employees
					GROUP BY department_id
				) t_avg_sal


		)

);
#10.查询平均工资最高的 job 信息
#方式1：
SELECT *
FROM jobs
WHERE job_id=(
	SELECT job_id
	FROM employees
	GROUP BY job_id
	HAVING AVG(salary)=(
		SELECT MAX(avg_sal)
		FROM (
			SELECT AVG(salary) avg_sal
			FROM employees
			GROUP BY job_id
		) t_avg_sal

	)

);
#方式2：
SELECT *
FROM jobs
WHERE job_id=(
	SELECT job_id
	FROM employees
	GROUP BY job_id
	HAVING AVG(salary)>= ALL(
			SELECT AVG(salary) avg_sal
			FROM employees
			GROUP BY job_id
	)

);
#方式3：
SELECT *
FROM jobs
WHERE job_id=(
	SELECT job_id
	FROM employees
	GROUP BY job_id
	HAVING AVG(salary)=(
			SELECT AVG(salary) avg_sal
			FROM employees
			GROUP BY job_id
			ORDER BY avg_sal DESC
			LIMIT 0,1
			
	)

);

#方式4：
SELECT j.*
FROM jobs j,(
SELECT job_id,AVG(salary) avg_sal
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 0,1) t_max_sal
WHERE j.`job_id`=t_max_sal.`job_id`;

#11.查询平均工资高于公司平均工资的部门有哪些?
SELECT department_id
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING AVG(salary)>
(     SELECT AVG(salary)
      FROM employees
)
#12.查询出公司中所有 manager 的详细信息
#方式1：
SELECT employee_id,last_name,salary
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
);
#方式2：
SELECT DISTINCT e1.employee_id, e1.last_name, e1.salary
FROM employees e1 JOIN employees e2
WHERE e1.employee_id = e2.manager_id;
#方式3：
SELECT employee_id, last_name, salary
FROM employees e1
WHERE EXISTS ( SELECT *
FROM employees e2
WHERE e2.manager_id = e1.employee_id);
#13.各个部门中 最高工资中最低的那个部门的 最低工资是多少?
#方式1：
SELECT MIN(salary)
FROM employees
WHERE  department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING MAX(salary)=(
				SELECT MIN(max_sal)
				FROM(
				SELECT department_id,MAX(salary) max_sal
				FROM employees
				GROUP BY department_id
				) t_max_sal

		)


);

#方式2：
SELECT MIN(salary)
FROM employees
WHERE  department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING MAX(salary)<=ALL(
				SELECT MAX(salary) max_sal
				FROM employees
				GROUP BY department_id
				HAVING department_id IS NOT NULL
		)
);

#方式3：
SELECT MIN(salary)
FROM employees
WHERE  department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING MAX(salary)=(
				SELECT MAX(salary) max_sal
				FROM employees
				GROUP BY department_id
				ORDER BY max_sal ASC
				LIMIT 0,1
		)
);
#方式4：
SELECT MIN(salary)
FROM employees e,(
				SELECT department_id,MAX(salary) max_sal
				FROM employees
				GROUP BY department_id
				ORDER BY max_sal ASC
				LIMIT 0,1) t_max_sal
WHERE e.`department_id`= t_max_sal.`department_id`;
#14.查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary
#方式1：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN(
	SELECT DISTINCT manager_id
	FROM employees
	WHERE department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary)=(
			SELECT MAX(avg_sal)
			FROM(
			SELECT AVG(salary) avg_sal
			FROM employees
			GROUP BY department_id
			) t_avg_sal

		)
	)

);

#方式2：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN(
	SELECT DISTINCT manager_id
	FROM employees
	WHERE department_id=(
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary)>= ALL(
			SELECT AVG(salary) 
			FROM employees
			GROUP BY department_id
		)
	)

);
#方式3：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN(
SELECT DISTINCT manager_id 
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal DESC
LIMIT 0,1) t_avg_sal
WHERE e.`department_id`=t_avg_sal.`department_id`);
#15. 查询部门的部门号，其中不包括job_id是"ST_CLERK"的部门号
#方式1：
SELECT department_id
FROM departments
WHERE department_id NOT IN(
		SELECT DISTINCT department_id
		FROM employees
		WHERE job_id='ST_CLERK'

);
#方式2：
SELECT department_id
FROM departments d
WHERE NOT EXISTS (
		SELECT *
		FROM employees e
		WHERE e.`department_id`=d.`department_id`
		AND e.`job_id`='ST_CLERK'

	);

#16. 选择所有没有管理者的员工的last_name
SELECT last_name
FROM employees emp
WHERE NOT EXISTS(
	SELECT *
	FROM employees mgr
	WHERE emp.`manager_id`=mgr.`employee_id`
);
#17．查询员工号、姓名、雇用时间、工资，其中员工的管理者为 'De Haan'
#方式1：
SELECT employee_id,last_name,hire_date,salary
FROM employees
WHERE manager_id IN(
		SELECT employee_id
		FROM employees
		WHERE last_name='De Haan'
);

#方式2：
SELECT employee_id,last_name,hire_date,salary
FROM employees emp
WHERE EXISTS(
	    SELECT *
	    FROM employees mgr
	    WHERE emp.`manager_id`=mgr.`employee_id`
	    AND mgr.`last_name`='De Haan'
);
#18.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资（相关子查询）
#方式一：相关子查询
SELECT employee_id,last_name,salary
FROM employees e1
WHERE salary > (
# 查询某员工所在部门的平均
SELECT AVG(salary)
FROM employees e2
WHERE e2.department_id = e1.`department_id`
);
#方式二：
SELECT employee_id,last_name,salary
FROM employees e1,
(SELECT department_id,AVG(salary) avg_sal
FROM employees e2 GROUP BY department_id
) dept_avg_sal
WHERE e1.`department_id` = dept_avg_sal.department_id
AND e1.`salary` > dept_avg_sal.avg_sal;
#19.查询每个部门下的部门人数大于 5 的部门名称（相关子查询）
SELECT department_name
FROM departments d
WHERE 5<(
	SELECT COUNT(*)
	FROM employees e
	WHERE d.`department_id`=e.`department_id`
);
#20.查询每个国家下的部门个数大于 2 的国家编号（相关子查询）
SELECT country_id
FROM locations l
WHERE 2<(
	SELECT COUNT(*)
	FROM departments d
	WHERE l.`location_id`=d.`location_id`
);
 /*
	子查询的编写技巧（或步骤）：①从里往外写 ②从外往里写
	如何选择？
	①如果子查询较简单，建议从外往里写，如果较复杂，从里往外写
	②如果是相关子查询，通常是从外往里写
 */