目录
一、子查询的例子和分类
1.举例需求：谁的工资比Abel的高？
#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。
子查询的编写技巧（或步骤）：① 从里往外写 ② 从外往里写
二、单行子查询
1.单行比较操作符
练习1
#题目：查询工资大于149号员工工资的员工的信息
SELECT employee_id,last_name,salary
FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE employee_id = 149
);
练习2
#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 job_id = (
SELECT job_id
FROM employees
WHERE employee_id = 141
)
AND salary > (
SELECT salary
FROM employees
WHERE employee_id = 143
);
练习3
#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
);
练习4
#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;
练习5
#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
);
练习6
#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;
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');
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);
三、多行子查询
1.多行子查询的操作符
IN ANY ALL SOME(同ANY)
IN
# 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。
#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
) 
2.空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM employees
);
四、相关子查询
1.基础相关子查询
#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：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;
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 ...,....#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`
)
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：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`
);
SELECT COUNT(*)
FROM departments;
