Re: Constraints for grouping

--- PostgreSQL Admin <postgres(at)productivitymedia(dot)com> wrote:
> I have a question that I've thought in my head about using triggers, but
> I figure to ask people that do SQL more than I. So, I have a table that
> I want two columns.
>
> (1) A featured column which is for only 1 row, once it switched to
> another row than all other rows must be false
>
> title | author_id | categories | featured
> --------------------------------+-----------+------+-----
> Thierry Beta Release | 3 | 11 | True
> Different Approach | 3 | 11 |
> Ottawa Does Not Heart Hip-Hop | 3 | 11 |
>
> (2) A featured column by category and only allow category_feature equal
> to the number of categories.
>
> Is SQL the way to go (and how - ideas), or should I use python for the
> heavy lifting?

Are you saying that you have two featured columns? One column for category features and the other
for article feature?

If I understand you correctly, Item 1 can be achieved nicely with partial indexes.
for example, if you only want to allow one row in the entire table to have "feature"= true:

CREATE INDEX Only_one_row_true
ON Your_table ( featured )
WHERE featured = true;

Or if you want to only allow 1 featured article per catagory then:

CREATE INDEX Only_one_row_true_per_catagory
ON Your_table ( catigories, featured )
WHERE featured = true;

This will ensure that the sum( featured = true ) <= sum( unique( catagories )).

Regards,
Richard Broersma Jr.