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Home -> Community -> Usenet -> comp.databases.theory -> Data independence
Codd 1970 ACM paper:
Then he discuss the following dependencies:
- Ordering Dependence
Taking into consideration that:
1) the access path to an item of data in a RDBMS "would take a form such as
R (g).r.d"
2) when someone write R1=R2 (where R1 and R2 are relationships) , he/she
usually mean "=" for all time
3) if someone change the key constraints of a relationship, actually he/she
define a new relationship
4) to access an "item of data" in a RDBMS, in addition to relationship name
and attribute name, one would need the value of some key
how the relational model solve the "Access Path Dependence" problem ?
In other words: It is possible to write applications that are independent of the change of key contraints in a RDBMS ? How ?
Citation ; Codd 1970 ACM Paper:
In previous work there has been a strong tendency to
treat the data in a data bank as consisting of two parts, one
part consisting of entity descriptions (for example, descriptions
of suppliers) and the other part consisting of relations
between the various entities or types of entities (for
example, the supply relation). This distinction is difficult
to maintain when one may have foreign keys in any relation
whatsoever.
In the USERS's relational model there appears to be no advantage to making such a distinction.
If the answer is : "By introducing entity domains", how such an relational model would compare to an ER model ? (Someone claimed that ER-modeling is less powerful than R-modeling)
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