Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!ncrlnk!ncratl!jgoodwin
From: jgoodwin@ncratl.Atlanta.NCR.COM (John Goodwin)
Newsgroups: comp.databases
Subject: Re: SQL Question
Summary: Better SQL Solution
Message-ID: <671@ncratl.Atlanta.NCR.COM>
Date: 11 Oct 90 20:26:48 GMT
References: <391@ulticorp.UUCP> <1990Oct10.013533.949@ingres.Ingres.COM>
Reply-To: jgoodwin@ncratl.Atlanta.NCR.COM (John Goodwin)
Organization: NCR Engineering & Manufacturing Atlanta -- Atlanta, GA
Lines: 8

Why not try the following:
    select unique PID
       from patients a
       where 100 >= (select max(b.WEIGHT)
                        from patients b
                        where b.pid = a.pid)

This is one quick and dirty solution.  Good Luck.