I ran your code in VFP 7 and got the same results. Looks like a bug to me. You can report it at http://support.microsoft.com/Support/vfoxpro/report/report.asp

>We have found a discrepancy in the results from SQL Selects in VFP7, depending on whether the files being queried have indexes or not. Try running the following code and see for yourself. I am using VFP7 with the 1st service pack (VFP version: 07.00.0000.9262).
>
>We also tried this with VFP6 SP5 and found even different results. (see comments in following code).
>
>The purpose of these select statements is to locate records that exist in the "left" table that are not in the "right" table.
>
>Can anyone explain? It seems to be a bug.
>
>************* copy this code and run it. *************

>CLOSE all
>
>* create 2 dummy tables in current directory and add 2 records to each
>* note: one record is different in the tables.
>CREATE TABLE tbl1 (userid c(10))
>INSERT INTO tbl1 (userid) VALUES ("mark")
>INSERT INTO tbl1 (userid) VALUES ("ed")
>
>CREATE TABLE tbl2 (userid c(10))
>INSERT INTO tbl2 (userid) VALUES ("john")
>INSERT INTO tbl2 (userid) VALUES ("ed")
>
>* run query on our tables, which are not indexed.
>* As expected, in VFP7, this will return 1 row. (the row that doesn't exist in tbl2).
>* In VFP6 we get 0 rows.
>SELECT tbl1.*;
>	FROM tbl1;
>	LEFT JOIN tbl2;
>	ON tbl1.userid = tbl2.userid;
>	WHERE ISNULL(tbl2.userid)
>
>* now run same query, but on indexed files.
>* In VFP7, this will incorrectly return ALL rows.
>* In VFP6 we get 0 rows.
>SELECT tbl1
>INDEX ON userid TAG userid
>SELECT tbl2
>INDEX ON userid tag userid
>	
>SELECT tbl1.*;
>	FROM tbl1;
>	LEFT JOIN tbl2;
>	ON tbl1.userid = tbl2.userid;
>	WHERE ISNULL(tbl2.userid)
>
>* now run same query using the HAVING clause instead of the WHERE clause.
>* This will return correct results, in VFP7 or VFP6, whether the file is indexed or not.
>SELECT tbl1.*;
>	FROM tbl1;
>	LEFT JOIN tbl2;
>	ON tbl1.userid = tbl2.userid;
>	HAVING ISNULL(tbl2.userid)
>
>CLOSE ALL
>DELETE FILE tbl1.*
>DELETE FILE tbl2.*