Combining Records

I have a database table tblobjects like this:

object_name, reference_id

a 1
a 2
a 3
a 4

b 2
b 3
b 1
b 4

c 2
c 4
c 5
c 6

d 2
d 4
d 5
d 6


I now would like to have a SQL query which gives me the number of
unique object and reference combinations, like this:

a
b
c

d shouldn't be displayed, because it's equal to c. The problem is also that a sequence of object references is
also important. So, for instance, object a shouldn't be equal to object b. The solution should also work is MS SQL and Mysql.

Any ideas how can I do this?

Thanks!