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  • May 21 / 2008
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Data Modeling and Database Design, DBA Interview questions, dbDigger, T-SQL Interview Questions

Codd’s 12 rules for relational database design

These rules are defined by Dr. Codd, the pioneer of relational databases. The rules primarily address implementation requirements for relational database management system (RDBMS) vendors.

1. Information Rule: All information in the database should be represented in one and only one way — as values in a table.

2. Guaranteed Access Rule: Each and every datum (atomic value) is guaranteed to be logically accessible by resorting to a combination of table name, primary key value, and column name.
3. Systematic Treatment of Null Values: Null values (distinct from empty character string or a string of blank characters and distinct from zero or any other number) are supported in the fully relational DBMS for representing missing information in a systematic way, independent of data type.
4. Dynamic Online Catalog Based on the Relational Model: The database description is represented at the logical level in the same way as ordinary data, so authorized users can apply the same relational language to its interrogation as they apply to regular data.
5. Comprehensive Data Sublanguage Rule: A relational system may support several languages and various modes of terminal use. However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings and whose ability to support all of the following is comprehensible:
  • data definition
  • view definition
  • data manipulation (interactive and by program)
  • integrity constraints
  • authorization
  • transaction boundaries (begin, commit, and rollback).
6. View Updating Rule: All views that are theoretically update able are also update able by the system.
7. High-Level Insert, Update, and Delete: The capability of handling a base relation or a derived relation as a single operand applies not only to the retrieval of data, but also to the insertion, update, and deletion of data.
8. Physical Data Independence: Application programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representation or access methods.
9. Logical Data Independence: Application programs and terminal activities remain logically unimpaired when information preserving changes of any kind that theoretically permit un impairment are made to the base tables.
10. Integrity Independence: Integrity constraints specific to a particular relational database must be definable in the relational data sub language and storable in the catalog, not in the application programs.
11. Distribution Independence: The data manipulation sublanguage of a relational DBMS must enable application programs and terminal activities to remain logically unimpaired whether and whenever data are physically centralized or distributed.
12. Non subversion Rule: If a relational system has or supports a low-level (single-record-at-a-time) language, that low-level language cannot be used to subvert or bypass the integrity rules or constraints expressed in the higher-level (multiple-records-at-a-time) relational language.
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