Re: Database design, Keys and some other things

Dr. Codd: "..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 ..."(Dr. Codd in ACM TODS, pp 409-410) and Codd, E. (1979), Extending the database relational model to capture more meaning. ACM Transactions on Database Systems, 4(4). pp. 397-434.

This means that a surrogate ought to act like an index; created by the user, managed by the system and NEVER seen by a user. That means never used in queries, DRI or anything else that a user does.

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 entities. A solution - proposed in part [4] and more fully in [14] - is to introduce entity domains which contain system-assigned surrogates. 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).

References

Codd, E. (1979), Extending the database relational model to capture more meaning. ACM Transactions on Database Systems, 4(4). pp. 397-434 Received on Wed Sep 28 2005 - 08:29:42 CDT