Re: What is the use of a primary key?

--CELKO-- wrote:

>>>the candidate keys must be defined as not null and unique (just the
>>>
>>>
>same that with the primary key), so... what's the difference? <<
>
>None. When Dr. Codd first defined his model, he was still under the
>influence of the file model of data. A sequential file must have a
>sorted order on a subset of it columns to work -- merging and
>searching without it are impossible for practical purposes.
>

I greatly suspect that is the case. See e.g.

http://www.acm.org/classics/nov95/s1p2p1.html http://www.acm.org/classics/nov95/s1p3.html

This in 1969.

Candidate keys were introduced during the 70's.

>But he was too late! SQL had picked up the idea of a PRIMARY KEY, and
>given it special properities - it is automatically NOT NULL, it is the
>default target of a REFERENCES clause and some products store index on
>it differently.
>

If I can recall correctly DB2 was the first SQL-database system to implement primary keys and that was
not until version 1.3, in 1997 - 18 years later! The standard did not have primary keys until
SQL-92!! Can hardly blame Codd on that one.

regards,
Lauri Pietarinen Received on Sun Dec 01 2002 - 16:44:51 CET