[打卡]子查询 。

SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary)
FROM employees;

SELECT job_id,MAX(salary),MIN(salary),AVG(salary),SUM(salary)
FROM employees
GROUP BY job_id;

SELECT job_id,COUNT(*)
FROM employees
GROUP BY job_id;

SELECT MAX(salary),MIN(salary),MAX(salary)-MIN(salary) DIFFERENCE
FROM employees;

SELECT manager_id,MIN(salary)
FROM employees
WHERE manager_id IS NOT NULL
GROUP BY manager_id
HAVING MIN(salary) >= 6000;

SELECT department_name,location_id,COUNT(employee_id),AVG(salary)
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
GROUP BY department_name,location_id
ORDER BY AVG(salary) DESC;

SELECT employee_id,salary,department_name,location_id
FROM employees e RIGHT JOIN departments d
ON e.department_id=d.department_id;


# 第8章 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 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'
);

# 称谓的规范：外查询（主查询）、内查询（子查询）
/*
子查询（内查询）在主查询（外查询）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


*/
/*

子查询的分类

角度1：从内查询返回的结果条目数

单行子查询 VS 多行子查询（工资大于11000的姓名）

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

举例

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

*/