Codd's Rules

Codd's Rules for a Truly Relational Database System

1. Information is represented logically in tables.
2. Data must be logically accessible by table, primary key, and column.
3. Null values must be uniformly treated as "missing information," not as empty strings, blanks, or zeros.
4. Metadata (data about the database) must be stored in the database just as regular data is.
5. A single language must be able to define data, views, integrity constraints, authorization, transactions, and data manipulation.
6. Views must show the updates of their base tables and vice versa.
7. A single operation must be able to retrieve, insert, update, or delete data.
8. Batch and end user operations are logically separate from physical storage and access methods.
9. Batch and end user operations can change the database schema without having to re-create it or applications built upon it.
10. Integrity constraints must be available and stored in the metadata, not in an application program.
11. The data manipulation language of the relational system should not care where or how the physical data is distributed and should not require alteration if the physical data is centralized or distributed.
12. Any row processing done in the system must obey the same integrity rules and constraints that set-processing operations do.

Redundancy methods

Hot Standby
A method of redundancy in which the primary and secondary (i.e., backup) systems run simultaneously. The data is mirrored to the secondary server in real time so that both systems contain identical information.

Warm Standby
A method of redundancy in which the secondary (i.e., backup) system runs in the background of the primary system. Data is mirrored to the secondary server at regular intervals, which means that there are times when both servers do not contain the exact same data.

Cold Standby
A method of redundancy in which the secondary (i.e., backup) system is only called upon when the primary system fails. The system on cold standby receives scheduled data backups, but less frequently than a warm standby. Cold standby systems are used for non-critical applications or in cases where data is changed infrequently.