sl

SELECT last_name,department_name
FROM employees e,departments d
WHERE e.department_id=d.department_id;

SELECT * FROM employees;
#查询员工的employee_id,last_name,department_name,city
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
归纳式：总分

*/
#多表查询的分类
#角度一：等值连接vs非等值连接
SELECT * FROM job_grades;
#查询名字，月薪和对应的等级
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;