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Relational Myths

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Taken from C.J. Date's article titled "Some Relational Myths Exploded (An examination of some popular misconceptions concerning relational database management systems)", InfoMIS 4 No. 2 (2nd Quarter 1984) and in InfoMIS 4 No. 3 (3rd Quarter 1984). Also reprinted in C. J. Date, "Relational Database Selected Writings", Addison - Wesley, Rading MA, 1986 revised 1989 ISBN 0201141965
Myth Explanation
A relational database is a database in which the data is physically stored in tables A fundamental misconception.  One of the major tenets of the relational system is the distinct separation between the logical model and the physical model.  The type of storage is the physical model.
A relation is just a flat file A relation in a relational database is a disciplined file in which:
  • All rows are of the same type
  • The columns have no particular order from left to right
  • The rows have no particular ordering top to bottom - or at most are ordered according to values of some field or field combination.
  • Every field is single valued
  • All rows have a unique identifier called the primary key


The relational model is "just theory" The relational model obviously is theoretical, but that is not a weak point,  in fact it is precisely the sound theoretical underpinnings that makes the relational approach so attractive.
Relational databases require Third Normal Form Relational systems do not require any level of normalization other than first.

Each successive level of normal forms add additional constraints on the design and address potential problems with the design, but they are not
required for a database to qualify as a relational model.

SQL is a panacea SQL provides database functions only, all of the standard programming language features, control structures, subroutine calls, screen I/O features, etc. are also needed.
The Third Normal Form is a panacea 3nF optimizes for update at the expense of retrieval. 3NF is not a "bad" level to reach but it can be compromised for retrieval performance considerations.  3NF states "one fact in one place", while this can improve the update process it can hurt the retrieval process.  Controlled redundancy can be a desired design goal.  Uncontrolled redundancy is unacceptable in any situation.
The relational approach is a panacea The relational design approach is a methodology for dealing with certain real world problems.  Any methodology has its limitations and cannot deal with every situation ever encountered and relational database design is no exception to this rule.

Category Data Modeling
( Topic last updated: 1999.09.13 11:24:25 AM )