<P> The relational model also allowed the content of the database to evolve without constant rewriting of links and pointers . The relational part comes from entities referencing other entities in what is known as one - to - many relationship, like a traditional hierarchical model, and many - to - many relationship, like a navigational (network) model . Thus, a relational model can express both hierarchical and navigational models, as well as its native tabular model, allowing for pure or combined modeling in terms of these three models, as the application requires . </P> <P> For instance, a common use of a database system is to track information about users, their name, login information, various addresses and phone numbers . In the navigational approach, all of this data would be placed in a single record, and unused items would simply not be placed in the database . In the relational approach, the data would be normalized into a user table, an address table and a phone number table (for instance). Records would be created in these optional tables only if the address or phone numbers were actually provided . </P> <P> Linking the information back together is the key to this system . In the relational model, some bit of information was used as a "key", uniquely defining a particular record . When information was being collected about a user, information stored in the optional tables would be found by searching for this key . For instance, if the login name of a user is unique, addresses and phone numbers for that user would be recorded with the login name as its key . This simple "re-linking" of related data back into a single collection is something that traditional computer languages are not designed for . </P> <P> Just as the navigational approach would require programs to loop in order to collect records, the relational approach would require loops to collect information about any one record . Codd's suggestions was a set - oriented language, that would later spawn the ubiquitous SQL . Using a branch of mathematics known as tuple calculus, he demonstrated that such a system could support all the operations of normal databases (inserting, updating etc .) as well as providing a simple system for finding and returning sets of data in a single operation . </P>

Define database management system in terms of its purpose and use