Re: NOT IN vs. OUTER JOIN and NOT NULL

2010/9/9 Martín Marqués <martin.marques@gmail.com>:
> I was looking at rows in a table which are not referenced from another
> and found some discrepencies.
>
> These are the queries (with results):
>
> SELECT * from grupo_concursantes where codigo NOT IN (SELECT grupo
> FROM concursantes);
>  codigo | numero | evento | escuela
> --------+--------+--------+---------
> (0 filas)
>
> SELECT g.* FROM grupo_concursantes g left outer join concursantes c on
> (g.codigo=c.grupo)
> where c.codigo IS NULL;
>  codigo | numero | evento | escuela
> --------+--------+--------+---------
>     25 |      1 |      1 |   69331
>     33 |      2 |      1 |   60233
>     53 |      2 |      1 |   60490
>     64 |      6 |      1 |   68861
>     73 |      1 |      1 |   69220
> (5 filas)
>
> Why aren't the 5 rows from the second query in the first?

These are doing different things.  The first one is matching rows in
grupo_concursantes where codigo doesn't appear in concursantes.grupo
(and bear in mind, NULL concursantes.grupo values won't match this
evaluation) .  The second one is returning every instance of
concursantes where where they *do* match, but where
concursantes.codigo is NULL, which isn't mentioned in the first query.

I'm wondering that if you used * instead of g.* in the second query
whether you'd get NULLs returned in the grupo column.

--
Thom Brown
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