The Model of a Relational Database and SQL Components

The model of a relational database was created by E. F. Codd in 1969. The relational model is based on the set theory which was introduced in mathematics by the German mathematician Georg Cantor (1845-1918) who was considered the father of set theory.

The concept of the relational model is that an unordered set of tables which are related to each other could be the content of a database. Then the tables could be called relations and could be modified by the user. Each table is a sub-set of the relational database which is the hyper-set and represents a unique entity which contains a set of objects with common characteristics (attributes). Each entity or object of the table is a row (record) of the set of attributes (columns) of the relation (table). Relational Data Model is based on a simple data structure which is the relationship. Thus, a relational database is presented as a collection of relationships each of which can be stored separately. Every relation is represented by a two dimensional table. Each table from the collection of database tables is a sub-set of it and is composed by an unlimited (theoretically) number of rows and from a specific number of columns which are its attributes or data items or elements or fields. Each attribute has a specific set of values. Thus, tables are relations under which entities and their attributes as well as relationships among them are stored. As we have mentioned earlier an entity or object is represented by a row in the structure of the table. Then, the specific table keeps information for a set of entities or objects of this category as much as the rows (records) of it. Thus, in a relational database we have entities or objects that contain data which can be modified by the user and they are grouped as sets in tables which represent relations with common characteristics. In terms of set theory, the attributes or characteristics of a table (table columns) are elements of a set that contains sub-sets (table rows). In the following graphic representation we show the three levels of a typical database structure based on the set theory. In the first circle we present the database set of tables (root set) which contains all tables of the data model as elements (three element-tables). In the second circle we give the structure of the table as a sub-set of the database set. In this evel the elements of the table set are the attributes (columns) of the table. In the third circle we present the structure of the sub-set of an attribute-element of a table set. In this case each element of this set represents an entity (row) and inherits its attributes from its parent table set. The relations among the database tables are defined during the phase of database schema design which is the data model that the database will be based on. The phase of the database schema design takes place after the phase of data model analysis and its requirements. Thus, the model of the relational database must be based on a database where all data visible to the user is organized strictly as tables of data values and all database operations work on those tables. During the next sections we are going to discuss the more essential components of relational databases.