Re: NOT and AND problem - Mailing list pgsql-sql

I can't help you explain what is going on with this query - like you, I 
am puzzled by the fact that it actually works, and have no idea how it 
is interpreted, and what it is doing...
The right way to do what you want, I think, would be something like:

delete from mytable where not exists (select 1 from item where item_id = 
mytable.item_id) or not exists (select 1 from ep where 
group_id=mytable.group_id);

I replaced your AND with OR, because that's what you seem to be saying 
in the description of your problem...

I hope, it helps..

Dima

Richard Jones wrote:

>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.d.jones@ed.ac.uk
>0131 651 1611
>
>
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