Codd's Twelve Rules
- The information rule: This rule require all information to be represented as data values in the rows and column
- The guaranteed access rule:Every data
value in a relational database should be logically accessible by specifying a
combination of table name and column name.
- Systematic treatment of NULL values: DBMS support NULL values ,in case when user leave the column in the table.
- Active online catalog based on relational model: DBMS maintain system catalog. System catalog is a collection of system table.
- The comprehensive data sublanguage rule:
System must support following functions :
- Data definition
- View definition
- Data manipulation operation
- Security and integrity constraints
- Transaction management operation
- The view update rule: All views that are theoretically updated must be updated by the system.
- High level insert, update and delete: Means update at a time many rows values. Data are treated as set ,which are
easily delete ,& update and insert.
- Physical data independence: Application programs must remain unimpaired when any changes are made in storage
representation or across methods.
- Logical data independence: Changes should not affect the user's ability to work with the data.
- Integrity independence: Integrity constraints must be storable in the system catalog.
- Distribution independence: Database must allow manipulation of distributed data located on different computer
- Nonsubversion rule: This rule state that different language bypass the integrity rule and constraints.