SQL parse string to get dependent info
patindex can do want you expect:

select a.dummy_Id, b.dummy_Id
from testmain a
join testmain b on left(a.memberNo,patindex('%/%', a.memberNo)-1) = left(b.memberNo,patindex('%/%', b.memberNo)-1)
                    and b.memberNo not like '%/1'
                    and a.dummy_Id &lt;&gt; b.dummy_Id
and a.memberNo like '%/1'