[打卡]wuyy 多表查询

SELECT*from employees;

SELECT*from departments;

SELECT*from locations;

SELECT employee_id,last_name,department_name,city
from employees e,departments d,locations l
where e.department_id=d.department_id
AND d.location_id=l.location_id;

#非等值连接
SELECT*FROM job_grades;

#举例：查询名字，月薪和对应的等级

SELECT last_name,salary,grade_level
FROM employees e,job_grades j
where e.salary BETWEEN j.lowest_sal
AND j.highest_sal;

SELECT*FROM employees;

SELECT last_name,salary,grade_level
FROM employees e,job_grades j
where e.salary >= j.lowest_sal
AND e.salary<=j.highest_sal;

#角度2：自连接（自我引用）vs 非自连接

SELECT*FROM employees;

SELECT emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name
FROM employees emp,employees mgr
WHERE emp.manager_id=mgr.employee_id;