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"RPG400-L" <rpg400-l-bounces@xxxxxxxxxxxxxxxxxx> wrote on 07/27/2019
01:23:04 PM:
This is interesting and maybe a database expert will chime in, but I've
always thought of the identity key, the one assigned by the database, as
the surrogate key. The customer number (in this thread) is the natural
key, or business key, and is the primary key and it must be unique.
The answer is that it depends upon how you define it. But,
generally, an identity column usually has the following constraint
accompanying it. This makes it the primary key. This key cannot be
changed without a delete and re-add.
Alter Table ... Add Constraint ... Primary Key ...;
After that, a company number, customer number, item number, etc.,
might have the following constraint associated with it. This makes it a
"surrogate" key and can be changed without having to delete and re-add the
row.
Alter Table ... Add Constraint ... Unique ...;
Lastly, there is a foreign key that might be used to avoid
carrying things like hard-coded category codes in a company row, customer
row, item row, etc. A separate table (usually refered to as a "lookup"
table) would be used to carry the codes as surrogate keys and have an
identity (primary) key that is carried in the tables referencing these
codes.
Alter Table ... Add Constraint ... Foreign Key ...;
Sincerely,
Dave Clark
midrange.com
