Friday, 29 January 2016

Codd rules

Rule 1: The information rule:

    All information in the database is to be represented in one and only one way,
ie in the form of rows and columns.

Rule 2: The guaranteed access rule:

    All data must be accessible.
   If you are able to store,  you should able to retrieve.

Rule 3: Systematic treatment of null values:

    The DBMS must allow each field to remain null (or empty). Specifically, it must support a representation of "missing information and inapplicable information" that is systematic.

Treatment of null values should be same irrespective of datatype.

Rule 4: Active online catalog based on the relational model:

     That is, users must be able to access the database's structure (catalog) using the same query language that they use to access the database's data.

Rule 5: The comprehensive data sublanguage rule:

Rule 6: The view updating rule:

    All views that are theoretically updatable must be updatable by the system.

Rule 7: High-level insert, update, and delete:

    The system must support set-at-a-time insert, update, and delete operators.

 This rule states that insert, update, and delete operations should be supported for any retrievable set rather than just for a single row in a single table.

Rule 8: Physical data independence:

    Changes to the physical level (how the data is stored, whether in arrays or linked lists etc.) must not require a change to an application based on the structure.

Rule 9: Logical data independence:

    Changes to the logical level (tables, columns, rows, and so on) must not require a change to an application based on the structure. Logical data independence is more difficult to achieve than physical data independence.

Rule 10: Integrity independence:

    Integrity constraints must be specified separately from application programs and stored in the catalog. It must be possible to change such constraints as and when appropriate without unnecessarily affecting existing applications.

Rule 11: Distribution independence:

Rule 12: The nonsubversion rule:

    If the system provides a low-level (record-at-a-time) interface, then that interface should not subvert the system,

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