[打卡]联系+子查询——sql

#随堂练习
SELECT max(salary),MIN(salary),AVG(salary),SUM(salary)
FROM employees


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

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

#SELECT(MAX(salary)-MIN(salary))
#AS salary FROM employees
SELECT MAX(salary),MIN(salary),MAX(salary)-MIN(salary)
FROM employees;

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

SELECT department_name,location_id,COUNT(*),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;

#子查询

#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'
);

#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

*/

SELECT last_name,salary
FROM employees
WHERE (
SELECT salary
FROM employees
WHERE last_name='Abel'
)
/*
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6000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

比如
相关子查询的需求：查询工资大于本部门平均的员工信息
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

*/