一、内连接 inner join：只取两张表的共同部分
select * from employee inner join department on employee.dep_id = department.id;
二、左连接 left join：在内连接的基础上保留左表的记录
select * from employee left join department on employee.dep_id = department.id;
三、右连接 right join：在内连接的基础上保留右表的记录
select * from employee right join department on employee.dep_id = department.id;
四、全外连接：在内连接的基础上保留左右两表没有对应关系的记录
select * from employee left join department on employee.dep_id = department.id
union
select * from employee right join department on employee.dep_id = department.id;