SQL - Select and filtering whereby a date is not null.

[Kal]

I have a particular problem:

I am working at a client site where they have the following oracle
system:

Oracle7 Server Release 7.3.3.0.0 - Production Release
PL/SQL Release 2.3.3.0.0 - Production

I am trying to select Person_id's filtering by where a date is not
null see below:

SELECT Per_Id, Per_First_Names, Per_Surname
FROM Oss_Relationships, Oss_Persons
WHERE Rel_Source_Per_Gro_Id = Per_Id AND Rel_Rty_Code = 'CTE' AND
Rel_End_Date IS NOT NULL;

I have conducted a few tests and the above query is still pulling
people data with a null S.Rel_End_Date when it should not.

I have found a workaround using a nested SELECT but this not efficient
and sometimes cause SQL* to freeze and crash.

Any help would be appreciated. p.s. I am a relative newbie to oracle
so forgive my inexperience.

Regards,

Kal