oracle关联查询问题

--关联查询时爆了a.id无效然后测试了一下
--错误
select 
a.id,
b.id,
c.id
 from t1 a ,t2 b left join t3 c on 
c.id=a.id
where a.id=b.id
--正确
select 
a.id,
b.id,
c.id
 from t1 a left join t3 c on 
c.id=a.id ,
t2 b
where a.id=b.id