一.单行子查询

1.1 单行操作符

二. 多行子查询

2.1 多行比较操作符

 2.2 例子 

2.3 空值问题 

 三. 相关子查询

3.1  EXISTS 与 NOT EXISTS关键字 

四. 子查询练习 

#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 = (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 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
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
# 方式一(把平均工资做成一个新表,新表必须起个别名)
SELECT *
FROM departments
WHERE department_id = (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_dep_avg))

#方式二
SELECT *
FROM departments
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
					   ))
#方式三(用limit求出最低平均工资)
SELECT *
FROM departments
WHERE department_id = (SELECT department_id
                       FROM employees
                       GROUP BY department_id
                       HAVING AVG(salary) = (SELECT AVG(salary)
                                             FROM employees
                                             GROUP BY department_id
                                             ORDER BY AVG(salary)
                                             LIMIT 0,1
                                             ))
 #方式四
 SELECT *
 FROM departments d,(SELECT department_id,AVG(salary)
                     FROM employees
                     GROUP BY department_id
                     ORDER BY AVG(salary)
                     LIMIT 0,1) t_dep_avg
 WHERE d.department_id = t_dep_avg.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）
#方式一
SELECT *
 FROM departments d,(SELECT department_id,AVG(salary)
                     FROM employees
                     GROUP BY department_id
                     ORDER BY AVG(salary)
                     LIMIT 0,1) t_dep_avg
 WHERE d.department_id = t_dep_avg.department_id
#方式二
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(dept_avgsal) 
					FROM (
					SELECT AVG(salary) dept_avgsal 
					FROM employees 
					GROUP BY department_id ) avg_sal ) );
#10.查询平均工资最高的 job 信息(四种写法)
SELECT *
FROM jobs
WHERE  job_id =(SELECT job_id
               FROM employees
               GROUP BY job_id
               HAVING AVG(salary) >= ALL (SELECT AVG(salary)
                                          FROM employees
                                          GROUP BY job_id
                                          ))

#11.查询平均工资高于公司平均工资的部门有哪些?
#方式一
SELECT department_id 
FROM employees 
WHERE department_id IS NOT NULL 
GROUP BY department_id 
HAVING AVG(salary) > ( SELECT AVG(salary) FROM employees );
#方式二
SELECT t_dep_avg.department_id
FROM (SELECT department_id,AVG(salary) 'avg1'
FROM employees
GROUP BY department_id) t_dep_avg
WHERE t_dep_avg.avg1>(SELECT AVG(salary)
        FROM employees)
#12.查询出公司中所有 manager 的详细信息
#方式1： 
SELECT  DISTINCT e1.last_name,e1.salary
FROM employees e1 JOIN employees e2
WHERE e1.employee_id = e2.manager_id
#方式二
SELECT employee_id,last_name,salary 
FROM employees 
WHERE employee_id IN ( 
			SELECT DISTINCT manager_id 
			FROM employees );
#方式三
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?
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 ) );



#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
#方式一
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN (SELECT 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_dep_avg

						)
						)
			      )

 #方式二
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN (SELECT manager_id 
FROM employees
WHERE department_id =(SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING AVG(salary) >= ALL (
						SELECT AVG(salary) avg_sal
						FROM employees
						GROUP BY department_id
						) 
						)
			      )     
# 方式三
SELECT *
FROM employees
WHERE employee_id IN (

			SELECT manager_id
			FROM employees e,(SELECT department_id
					FROM employees
					GROUP BY department_id
					ORDER BY AVG(salary)
					LIMIT 0,1
					) t_dep_avg
			WHERE e.department_id = t_dep_avg.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
#方式一:
SELECT department_id
FROM employees
WHERE department_id NOT IN (SELECT DISTINCT department_id
                  FROM employees
                  WHERE job_id='ST_CLERK')
#方式二
SELECT department_id
FROM departments d
WHERE NOT EXISTS (SELECT *
                  FROM employees e   
                  WHERE e.department_id = d.department_id
                  AND  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 e1
WHERE NOT EXISTS(
                 SELECT *
                 FROM employees mgr
                 WHERE mgr.employee_id = e1.manager_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'
#方式一
SELECT employee_id,last_name,hire_date,salary
FROM employees
WHERE manager_id IN (
                     SELECT employee_id
		     FROM employees
		     WHERE last_name='De Haan')
#方式二
SELECT employee_id,last_name,hire_date,salary
FROM employees e1
WHERE EXISTS (
               SELECT *
               FROM employees mgr
               WHERE e1.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）
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` );
#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 d.location_id = l.location_id)

#相关子查询,从外往里写