内连接定义：仅将两个表中满足连接条件的行组合起来作为结果集。

关键词：INNER JOIN

select * from employees e inner join department d 
on e.employee_id = d.department_id  where e.employee_id = "1";

等价于

select * from employees e,department d 
where e.employee_id = d.department_id and e.employee_id = "1";