Re: Negative Numbers in "Identity" or" Autonumber" fields
Date: 20 Mar 2007 10:11:28 -0700
Message-ID: <1174410687.965463.59270_at_l75g2000hse.googlegroups.com>
>> Whose definition? <<
Dr. E. F. Codd, the inventor of RDBMS.
Codd also wrote the following:
"There are three difficulties in employing user-controlled keys as
permanent surrogates for entities.
(1) The actual values of user-controlled keys are determined by users
and must therefore be subject to change by them (e.g. if two companies
merge, the two employee databases might be combined with the result
that some or all of the serial numbers might be changed.).
(2) Two relations may have user-controlled keys defined on distinct
domains (e.g. one uses social security, while the other uses employee
serial numbers) and yet the entities denoted are the same.
(3) It may be necessary to carry information about an entity either
before it has been assigned a user-controlled key value or after it
has ceased to have one (e.g. and applicant for a job and a retiree).
These difficulties have the important consequence that an equi-join on
common key values may not yield the same result as a join on common
[emphasis begin] Database users may cause the system to generate or delete a surrogate, but they have no control over its value, nor is its value ever displayed to them....." (Codd in ACM TODS, pp 409-410) [emphasis end].
That means if IDENTITY were a surrogate, we would not see its values, print them out as invoice numbers and so forth. That means if IDENTITY were a surrogate, we could drop it and the schema would still work. Like an index. But IDENTITY fails on both those points. Received on Tue Mar 20 2007 - 18:11:28 CET