Re: Constraints, anyone?

[Charles Wilt <charles.wilt@xxxxxxxxx>]

James,

Primarily because the constraint term comes from the SQL world.  In
fact, if you defined a table using SQL DDL and give it a primary key
constraint, you'll get what looks like a regular uniquely keyed
physical *FILE object on the i.

There is one difference, a DDS uniquely keyed physical file or a file
with a unique key constraint allows NULLable fields as part of the
key.  So you could have multiple records with the same NULL key value.

Whereas fields that are part of a primary key constraint must also be
defined NOT NULL.

HTH,
Charles


On Mon, Dec 14, 2009 at 4:22 PM, James H. H. Lampert
<jamesl@xxxxxxxxxxxxxxxxx> wrote:
> Tommy.Holden@xxxxxxxxxxxxxxxxxxxxx wrote:
> > primary key = unique keyed PF
>
> So why is it available as a constraint, as opposed to simply creating
> the PF as keyed? And conversely, assuming the PF is already keyed, is it
> necessary to add a primary key constraint to the file in order to use it
> in referential constraints?
>
> --
> JHHL
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