select a.sn,nvl(b.username,a.createuser) as createuser,nvl(c.username,a.updateuser) as updateuser
from tb1 a,tb2 b,tb2 c
where a.createuser=b.userid(+)
and a.updateuser=c.userid(+);
select sn,
nvl((select username from 表2 where userid=createUser),createUser) createUser,
nvl((select username from 表2 where userid=updataUser),updataUser) updataUser
from 表1