A Relational Database Management System (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by Dr. Edgar F. Codd. Strictly speaking it should also satisfy Codd’s 12 rules, but in practice there is no DBMS that satisfies all these rules. In fact, most successful DBMS that are considered to be relational violate the relational model in several important ways, including the Structured Query Language (SQL).
Relational DataBase Management Systems (RDBMS) stores data in the form of related tables. RDBMS are powerful because they require few assumptions about how data is related or how it will be extracted from the database. As a result, the same database can be viewed in many different ways.
An important feature of relational systems is that a single database can be spread across several tables.
The first released RDBMS that was a relatively faithful implementation of the relational model was the Multics Relational Data store first sold in 1978. Others have been Berkeley Ingres, QUEL and IBM BS12.
Today, popular commercial RDMBS for large database include Oracle, Microsoft SQL Server, Sybase SQL server and IBM’s DB2. The most commonly used free RDBMS are MySQL, PostgreSQL.
Dr. E.F. Codd is an IBM researcher who first developed the relational data model in 1970. In 1985, Dr. Codd published a list of 12 rules that define an ideal relational database and has provided a guideline for the design of all relational database systems.
Dr. Codd has used the term guideline because till date no commercial relational database system fully conforms to all 12 rules. Today, the rules are not talked about as much but remain a goal for relational database design.
Rule 1: The Information Rule: All data should be presented in table form.
Rule 2: Guaranteed Access Rule: All data should be accessible without ambiguity. This can be accomplished through a combination of the table name, primary key and column name.
Rule 3: Systematic Treatment of Null Values: A field should be allowed to remain empty. This involves the support of a null value, which is distinct from an empty string or a number with a value of zero. Of course, this can't apply to primary keys. In addition, most database implementations support, the concept of a not null field constraint that prevents null values in a specific table, column.
Rule 4: Dynamic On Line Catalog based on the Relational Model: A relational database must provide access to its structure through the same tools that are used to access the data. This is usually accomplished by storing the structure definition within special system tables.
Rule 5: Comprehensive Data Sublanguage Rule: The database must support at least one clearly defined language that includes functionality for data definition, data manipulation, data integrity and database transaction control. All commercial relational databases use forms of standard SQL (i.e. Structured Query Language) as their supported comprehensive language.
Rule 6: View Updating Rule: Data can be presented in different logical combinations called views. Each view should support the same full range of data manipulation that has direct access to a table available. In practice, providing update and delete access to logical views is difficult and is not fully supported by any current database.
Rule 7: High level Insert, Update, and Delete: 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
Rule 8: Physical Data Independence: The user is isolated from the physical method of storing and retrieving information from the database. Changes can be I made to the underlying architecture (hardware, disk storage methods) without affecting how the user accesses it
Rule 9: Logical Data Independence: How data is viewed should not be changed when the logical ,structure (table's structure) of the database changes. This rule is particularly difficult to satisfy. Most databases rely on strong ties between the data viewed and the actual structure of the underlying tables
Rule 10: Integrity Independence: The database language (like SQL) should support constraints on user input that maintain database integrity. This rule is not fully implemented by most major vendors. At a minimum, all databases do preserve two constraints through SQL. No component of a primary key can have a null value. If a foreign key is defined in one table, any value in it must exist as a primary key in another table.
Rule 11: Distribution Independence: A user should be totally unaware of whether or not the database is distributed (whether parts of the database exist in multiple locations). A variety of reasons 'make this rule difficult to implement.
Rule 12: Non subversion Rule: There should be no way to modify the database structure other than through the multiple row database language (like SQL). Most databases today support administrative tools that allow some direct manipulation of the data structure.