ID   email
A    a@gmail.com

ID   email2        email3   email4
A    a@gmail.com   null     ab@gmail.com

select T1.ID, T1.email,
case when T1.email<>T2.email2 then T2.email2 end email2,
case when T1.email<>T2.email3 and T2.email2<>T2.email3 then T2.email3 end email3,
case when T1.email<>T2.email4 and T2.email2<>T2.email4 and T2.email3<>T2.email4 then T2.email4 end email4
from t1
left join t2 on t1.id=t2.id

ID  email         email2   email3   email4
A   a@gmail.com   null     null     null

ID  email         email2   email3   email4
A   a@gmail.com   null     null     ab@gmail.com

select T1.ID, T1.email,
case when T1.email<>T2.email2  then T2.email2 end email2,
case when T1.email<>T2.email3 and T2.email2<>T2.email3 then T2.email3 end email3,
case when (T1.email<>T2.email4 OR T1.email IS NULL) and
            (T2.email2<>T2.email4  OR T2.email2 IS NULL)  and
            (T2.email3<>T2.email4  OR T2.email3 IS NULL)  then T2.email4 end email4
from t1
left join t2 on t1.id=t2.id