内链接：select * from t_student t inner join t_hours s on id = sid
等价于select * from t_student t ，t_hours s where id = sid
左连接:select * from t_student t left join t_hours s on id = sid
右连接 select * from t_student t right join t_hours s on id = sid
全连接 select * from t_student t full join t_hours s on id = sid
