A database is a collection of data for one or more multiple uses. Databases are usually integrated and offers both data storing and retrieval.
Codd's 12 rules are a set of thirteen rules (numbered zero to twelve) proposed by Edgar F. Codd, a pioneer of the relational model for databases.
The system must qualify as relational, as a database, and as a management system.
For a system to qualify as a relational database management system (RDBMS), that system must use its relational facilities (exclusively) to manage the database.
The information rule
All information in the database is to be represented in one and only one way, namely by values in column positions within rows of tables.
The guaranteed access rule
All data must be accessible. This rule is essentially a restatement of the fundamental requirement for primary keys. It says that every individual scalar value in the database must be logically addressable by specifying the name of the containing table, the name of the containing column and the primary key value of the containing row.
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, distinct from all regular values (for example, "distinct from zero or any other number", in the case of numeric values), and independent of data type. It is also implied that such representations must be manipulated by the DBMS in a systematic way.
Active online catalog based on the relational model
The system must support an online, inline, relational catalog that is accessible to authorized users by means of their regular query language. 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.
The comprehensive data sublanguage rule
The system must support at least one relational language that
- Has a linear syntax
- Can be used both interactively and within application programs,
- Supports data definition operations (including view definitions), data manipulation operations (update as well as retrieval), security and integrity constraints, and transaction management operations (begin, commit, and rollback).
The view updating rule
All views that are theoretically updatable must be updatable by the system.
High-level insert, update, and delete
The system must support set-at-a-time insert, update, and delete operators. This means that data can be retrieved from a relational database in sets constructed of data from multiple rows and/or multiple tables. 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.
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.
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.
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.
The distribution of portions of the database to various locations should be invisible to users of the database. Existing applications should continue to operate successfully :
- when a distributed version of the DBMS is first introduced; and
- when existing distributed data are redistributed around the system.
The nonsubversion rule
If the system provides a low-level (record-at-a-time) interface, then that interface cannot be used to subvert the system, for example, bypassing a relational security or integrity constraint.
Database ACID Property
ACID(atomicity, consistency, isolation, durability) is a set of properties that guarantee that database transactions are processed reliably.
Atomicity requires that database modifications must follow an all or nothing rule. Each transaction is said to be atomic if when one part of the transaction fails, the entire transaction fails and database state is left unchanged
Consistency property ensures that the database remains in a consistent state; more precisely, it says that any transaction will take the database from one consistent state to another consistent state. The consistency rule applies only to integrity rules that are within its scope. Thus, if a DBMS allows fields of a record to act as references to another record, then consistency implies the DBMS must enforce referential integrity: by the time any transaction ends, each and every reference in the database must be valid.
Isolation refers to the requirement that other operations cannot access or see data that has been modified during a transaction that has not yet completed. Each transaction must remain unaware of other concurrently executing transactions, except that one transaction may be forced to wait for the completion of another transaction that has modified data that the waiting transaction requires.
Durability is the DBMS's guarantee that once the user has been notified of a transaction's success, the transaction will not be lost. The transaction's data changes will survive system failure, and that all integrity constraints have been satisfied, so the DBMS won't need to reverse the transaction. Many DBMSs implement durability by writing transactions into a transaction log that can be reprocessed to recreate the system state right before any later failure.