Aliias names in select criteria

To whom it may concern:

The following selection criteria causes my box to start a process that gives no result.

SELECT P.Last_Name, P.First_Name, S.Status, S.Entry_Year, S.Graduation_Year, S.Former_School, S.Day_or_Board,
S.Admissions_Id,P.Participant_Id, P.Birthday, P.Modification_Date 
FROM Participants P, Students S
WHERE Participant.Participant_Id = Students.Participant_Id

It also throws the linux box into a process that has to be killed even though I've exited the client.

I found the problem to be the non-use of the alias after the WHERE clause  If I rerun this as below without the full
tablename and using the alias it works.  Is this a normal occurrence.  Other systems I have used work either way. 

SELECT P.Last_Name, P.First_Name, S.Status, S.Entry_Year, S.Graduation_Year, S.Former_School, S.Day_or_Board,
S.Admissions_Id,P.Participant_Id, P.Birthday, P.Modification_Date 
FROM Participants P, Students S
WHERE P.Participant_Id = S.Participant_Id


Bob Powell