Re: combining tables

--- On Sun, 10/26/08, Cliff Nieuwenhuis <cliff(at)nieusite(dot)com> wrote:

> G. J. Walsh writes:
>
> > > 1 of the 4 tables has the complete range of keys,
> in this case about
> > > 1,000. The other 3 tables have data representing
> most but not all of
> > > those keys, and in different sets. I want to end
> up with a new
> > > 'combined' table which will allow me to
> immediately 'see' missing
> > > data from the 3 smaller tables and take the
> necessary steps to
> > > 'fill in the blanks'. I realize I would
> of course have to create
> > > that table without constrictions other than the
> primary key.
>
> and
>
> > My <<< shaky >>> understanding from
> the manual is that I could use
> > something like:
> >
> > SELECT * FROM tmain,tsub1,tsub2,tsub3 NATURAL INNER
> JOIN tcombo
> >
> > Am I on the right track?
>
> I asked
>
> > Do you mean you want to find keys that exist in the
> 'main' table but
> > have no match in the other three tables?
>
> .. and you said 'yes'. There is more to it on your
> side, but (to me)
> those details are unique to what you are doing with your
> application.
>
> For me (and I'm a novice as well and speak under
> correction here), I
> will start will a query I can easily understand and then
> work towards
> one that is more efficient. In your case I might try
> something like:
>
> SELECT * FROM tmain WHERE tmain.primarykey NOT IN (
> SELECT DISTINCT foreignkey FROM tsub1 );
>
> ..to get records that are in the 'main' table but
> have no related
> information in tsub1. You would need to repeat this for the
> other
> tables of course.
>
> If you wanted a big table with all 'missing'
> fields, I would try an
> OUTER JOIN instead of the NATURAL INNER JOIN, because you
> want the
> records in the 'main' table to appear in the
> results even if there is
> no match in one of the 'sub' tables.
>
> Perhaps someone on this list has a better idea or
> understands your
> situation better than I do.
>

I'm no expert either but it seems to me the situation calls for a view. It will always show up-to-date data.

Unless you want to store additional data for the lines in with no matches. Then you would indeed need another table. You may have to handle updates to tmain and the tsubs in this case, probably with triggers.

Bruce