[打卡]多表查询 2222222

-- 等值连接

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

-- 角度1：等值连接 VS 非等值连接

-- 非等值连接

SELECT * FROM job_grades;

-- 举例名字，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 非自我引用

-- 查询员工ID，员工姓名及其管理者ID和姓名

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;