SQL中Select后字段加别名可正常运行，不加则报错

select a1,a2,b1,b2 from aaa
as a left join bbb as b on b.b1=a.a1;
报错：error encountered near token 'b'

当把SQL改为(字段加上别名)正常：
select a.a1,a.a2,b.b1,b.b2 
from aaa as a
left join bbb as b on b.b1=a.a1;