aaa

SELECT first_name,department_name
FROM employees,departments
WHERE employees.department_id=departments.department_id;

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

/*
演绎法：发现一个问题---解决一个问题-----发现第二个问题
总--分
*/

#多表查询的分类
/*
角度1：等级连接vs非等值连接
角度二：自连接（自我引用）vs非自连接

*/
# 角度1：等级连接vs非等值连接
SELECT * FROM job_grades;
#查询名字，月薪和对应等级
SELECT last_name,salary,grade_level
FROM employees,job_grades
WHERE employees.salary BETWEEN job_grades.lowest_sal AND job_grades.highest_sal;

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

#角度二：自连接（自我引用）vs非自连接
SELECT *FROM employees;

#练习:查询员工id，员工姓名及管理者的id及姓名
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;