Re: Weirdness with "not in" query

On 8 June 2017 at 17:27, greigwise <greigwise(at)comcast(dot)net> wrote:
> So, I'm using postgres version 9.6.3 on a mac and the results to this series
> of queries seems very strange to me:
>
> db# select count(*) from table1 where id in
> (1706302,1772130,1745499,1704077);
> count
> -------
> 4
> (1 row)
>
> db# select count(*) from table2 where table1_id in
> (1706302,1772130,1745499,1704077);
> count
> -------
> 0
> (1 row)
>
> db# select count(*) from table1 where id not in (select table1_id from
> table2);
> count
> -------
> 0
> (1 row)
>
> I would expect the "not in" query to return a result of at least 4. Am I
> totally misunderstanding how this should work (I really don't think so) or
> is something wrong?

You probably have table1_id's that are NULL in table2. In that case
the result of not in is null as well.

Not exists is perhaps a better candidate in this case.

--
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