--表连接
select Students .Name ,Students .Age,Classes .Name from Students
join Classes on Students .CId =Classes .Id
--查询年龄大于3的学生
select
Students .Name ,Students .Age ,Classes .Name from Students
join Classes on
Students .CId =Classes .Id where Students .Age >3
--left join
会把左边的表的数据全部查出来
select Students .Name ,Students .Age ,Students
.Birthday,classes.Name from Students
left join Classes on Students .CId
=Classes .Id
--right join
首先满足右表的数据，如果连接条件满足就显示左表的数据，如果不满足就显示null
select Students .Name ,Students .Age
,Students .Birthday,classes.Name from Students
right join Classes on
Students .CId =Classes .Id
原文：http://www.cnblogs.com/sumg/p/3649468.html