Você pode dar um nome ou alias a uma consulta com CTEs (Common Table Expressions) aka WITH cláusula aka por Oracle como Subquery Factoring:
WITH abc as (select client_ref_id, request from some_table where message_type = 1)
select *
from abc
inner join
(select client_ref_id, response from some_table where message_type = 2) defg
on abc.client_ref_id = def.client_ref_id;