How to refactor this subquery to be written only once?

Question

procedure foo(p1 IN INTEGER,
    p2 IN INTEGER,
    ooo out sys_refcursor)
is
begin

open ooo for
select T1.*
from T1
where T1.x=p1
    and (   
        p2 in (SELECT z from T2 where T2.someId=T1.SomeId and T2.y=12)
    or
        not exists (SELECT z from T2 where T2.someId=T1.SomeId and T2.y=12)
        );
end;

The subquery is the exact same both times. Is there a way to refactor it to be specified only once? On Oracle at least.

One way would be to use it as a join:

procedure foo(p1 IN INTEGER,
    p2 IN INTEGER,
    ooo out sys_refcursor)
is
begin

open ooo for
select unique T1.*
from T1
left join T2 on T2.someId=T1.SomeId and T2.y=12
where T1.x=p1
    and (   
        p2 = T2.z
    or
        T2.z is null -- T2.z is a NOT NULL column
        );
end;

But this is not equivalent (I added UNIQUE to filter out rows multiplied due to join, but this also removes possible correct duplicate rows from T1).

PS: Made an SQL Fiddle with the two queries from above and from the two answers below.

Answer 1

I came up with this:

procedure foo(p1 IN INTEGER,
    p2 IN INTEGER,
    ooo out sys_refcursor)
is
begin

open ooo for
with sq as (SELECT z, someId from T2 where T2.y=12)
select T1.*
from T1
where T1.x=p1
    and (   
        (p2,T1.someId) in (select * from sq)
    or
        not exists (select * from sq where sq.someId=T1.someId)
        );
end;