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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;