t1表内容如下：
t2表内容如下：
下面来简述join和left join/right join的区别：
inner join
select * from t1 inner join t2 on t1.id = t2.id;
公共部分的数据才会被查询出来；
left join
select * from t1 left join t2 on t1.id = t2.id;
查询出来的结果和前表记录数一样多；
right join
select * from t1 right join t2 on t1.id = t2.id;
能转化为
select * from t2 left join t1 on t1.id = t2.id;