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• 1985 Proposed to test DBMSs for confirmation to concept of Codd’s Relational model
• Hardly any commercial product follows all
• Oracle = 8 1 out of 12.

Codd’s Rule Zero

• For a system to qualify as an RDBMS it must be able to manage its databases entirely through its Relational capabilities
• The other 12 rules derive from this rule

Codd’s Rule 1: Information Rule

• All Information (inlcuding metadata) is to be represented as data stored in cells of tables.
• The rows and columns have to be strictly unordered.

Codd’s Rule 2: Guaranteed Access

• Each  unique  piece  of  data  (atomic  value)  should  be accessible  by  :Table Name  +  Primary  Key  (Row)  + Attribute (Column)
• Violation:  Ability to directly access via pointers

Codd’s Rule 3:  Systematic treatment of NULL

• NULLs may mean:  Missing data, not applicable, No value
• Should be handled consistently - Not Zero or Blank
• Primary keys — Not NULL
• Expressions on NULL should give NULL

Codd’s Rule 4: Active On-Line Catalog

• Database dictionary (Catalog) to have description of the Database
• Catalog  to  be  governed  by  same  rules  as  rest  of  the database
• The same query language to be used on catalog as on the application database

Codd’s Rule 5: Powerful language

• One well defined language to provide all manners of access to data
• Example:  SQL
• If file supporting table can be accessed by any manner except a SQL Interface, then a violation

Codd’s Rule 6:  View Updation Rule

• All views that are theoretically updatable should be updatable
• View =”Virtual table”, temporarily derived from base tables
• Example: If  a  view  is  formed  as  join  of  3  tables, changes to view should be reflected in base tables
• Not updatable:  View does not have NOT-NULL at- tribute of base table
• Problems with computed fields in view
E.g. Total Income = White income + Black income

Codd’s Rule 7: Relational level operations

• There must be insert, update, and delete operations at the level of Relations
• Set operations like Union, Intersection and minus should be supported

Codd’s Rule 8:  Physical Data Independence

• The physical storage of data should not matter to the system
• If say, some file supporting table was renamed or moved from one disk to another, it should not effect the applications.

Codd’s Rule 9: Logical Data Independence

• If there is change in the logical structure (table structures) of the database the user view of the data    should not change
• implemented through views.  Say, if a table is split into two tables, a new view should give result as the join of the two tables
• Difficult rule to satisfy

Codd’s Rule 10: Integrity Independence

• The database should be able to enforce its own integrity rather than using other programs
• Integrity rules = Filter to allow correct data, should be stored in Data Dictionary
• Key and check constraints, triggers etc should be stored in Data Dictionary
• This also makes RDBMS independent of front end

Codd’s Rule 11: Distribution Independence

• A database should work properly regardless of its distribution across a network
• This lays foundation of Distributed databases
• Similar to Rule8 only that applies to distribution on a local Disk

Codd’s Rule 12: Non subversion Rule

• If low level access is allowed to a system it should not be able to subvert or bypass integrity rules to change data
• This may be achieved by some sort of locking or encryption
• Some low level access tools are provided by vendors that violate these rules for extra speed



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