Re: Does entity integrity imply entity identity?
Date: Sun, 02 Aug 2009 07:04:09 GMT
"Mr. Scott" <do_not_reply_at_noone.com> wrote in message
> Since the entity integrity rule ensures that a relational table cannot
> have any duplicate rows, does that imply that each row in a table maps to
> a distinct entity?
Here's the way I learned it, back in 1984.
Each row in a table represents either an instance of an entity or an instance of a relationship between or among entities. I'm not sure whether or not "represents" in my wording is equivalent to "maps to" in your wording.
Also the way I learned it, entities and relationship among them are part of
what was called the "conceptual data model". The conceptual data model was
not a relational model as such, although it's very straight forward to start
with an ER model and transform it into a relational model that expresses the
The conceptual model was used for data analysis resulting in a clearer
definition of the information requirements. The conceptual model was NOT a
The conceptual model was used for data analysis resulting in a clearer definition of the information requirements. The conceptual model was NOT a design model.
The design model could be broken into two layers: logical model and physical model. I could go into more detail here, but it doesn't pertain to your question. Suffice it to say the logical model should be relational if the goal is to design a relational database. Back in 1984, they were quite loose about what was considered a relational DBMS. It was that looseness that led Codd to formulate the 12 rules, in order to distinguish between soi disant relational DBMSes and really relational DBMSes.
The entity integrity rule is generally presented as a subrule under Codd's rule 10. I do not know if Codd used the term "entity integrity" or not. And I also do not know if he explicitly made the rule that no part of a primary key can be missing. I started with practice rather than theory, and only learned enough of the theory to help me with the practice.
http://en.wikipedia.org/wiki/Entity_integrity http://en.wikipedia.org/wiki/Integrity_constraints http://en.wikipedia.org/wiki/Codd%27s_rules
(sorry this is reverse order). Received on Sun Aug 02 2009 - 09:04:09 CEST