NOT and AND problem

Dear All,

I am having some confusion over a query which is supposed to achieve the
following: To remove a record from a table if the one or both of the
columns containing references to other tables no longer point to table rows
which still exist. There are good reasons why I cannot use foreign keys to
maintain referential integrity, but I will not go into them, but they give
rise to the need to "clean-up" my database table from time to time. The
query that I have had most success with looks like this:

DELETE FROM myTable
WHERE (NOT myTable.item_id = item.item_id)
AND (NOT myTable.group_id = ep.group_id);

Which is odd, because logically it shouldn't work. What I find with the
above queries is that as follows:

let myTable.item_id = item.item_id be A
let myTable.group_id = ep.group_id be B

The derived and actual truth tables for the results of the where clause
follow:

Derived:
A | B | Result
1 | 1 | 0
1 | 0 | 0
0 | 1 | 0
0 | 0 | 1

Actual:
A | B | Result
1 | 1 | 0
1 | 0 | 0
0 | 1 | 1
0 | 0 | 1

This makes no sense to me, as effectively rows 2 and 3 of the Actual results
truth table are the same (unless there's some subtle difference with regards
to the order of the statements, otherwise just substitute A for B and vice
versa).

The result that I actually want from the operation is this:

A | B | Result
1 | 1 | 0
1 | 0 | 1
0 | 1 | 1
0 | 0 | 1

which would suggest a query like:

DELETE FROM myTable
WHERE NOT (myTable.item_id = item.item_id AND myTable.group_id =
ep.group_id);

which ought to provide the above output. Instead, using this query, the
output I get is as follows:

A | B | Result
1 | 1 | 1
1 | 0 | 1
0 | 1 | 1
0 | 0 | 1

I can only conclude that Postgres is doing something with regards to the
other two tables which I am unaware of. Can anyone help me understand what
is going on? Any suggestions gratefully received.

Cheers

Richard

Richard Jones
-----------------------
Systems Developer
Theses Alive! - www.thesesalive.ac.uk
Edinburgh University Library
r(dot)d(dot)jones(at)ed(dot)ac(dot)uk
0131 651 1611