【MySQL】第六章 子查询

角度1：从内查询返回的结果的条目数

1，单行子查询 vs 多行子查询

单行子查询是指内查询返回单行单列结果（一个标量值），然后外查询可以使用单行比较运算符（如 =、>、<、>=、<=、<>）与这个值进行比较

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
        );

多行子查询是指内查询返回多行单列的结果集，外查询使用多行比较运算符 （如 IN、ANY、ALL）或 EXISTS 运算符与这个结果集进行比较筛选

#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'
        );

角度2：内查询是否被执行多次

2，相关子查询 vs 不相关子查询

相关子查询：内层查询依赖外层查询 不相关子查询：内层查询不依赖外层查询

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。

#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
#6.1 
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
#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;

3，子查询可以放的位置

a，子查询放在 FROM 中

#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

b，子查询放在 ORDDER BY 中

#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;
    
​
SELECT : 用于聚合函数
FROM   : 用于查表操作
WHERE  : 用于过滤操作，如进行条件分组
ORDER BY : 用于按照表进行排序操作
HAVING   : 用于聚合函数，进行分组操作

# 按照返回的部门名称进行排序
#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
(LEFT / RIGHT)JOIN ... ON ....
WHERE 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 ...,....
*/