第07章_子查询-优快云博客

本文链接：https://blog.youkuaiyun.com/m0_56999722/article/details/147250083

1由一个具体的需求，引入子查询

需求：谁的工资比Abel高

#方式1:
SELECT 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
                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

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

SELECT employee_id,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
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：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

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：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：

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中聚合函数不能嵌套使用

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_dept_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 
                           #WHERE manager_id IS NOT NULL
												 );

6.1 相关子查询

回顾：查询员工中工资大于本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：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

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！

SELECT...,...,...(存在聚合函数)
FROM...(LEFT / RIGHT)JOIN...ON 多表的连接条件
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 *
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 和 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: 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 employee_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:
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2:
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
                   SELECT *
                   FROM employees e
                   WHERE d.department_id = e.department_id
              );