The relational database is relatively new. The first database system was based on either a network model or hierarchical model. This model greatly improves the flexibility of the database management system. It has established itself as a primary data model for commercial data processing application. In relational model, a relation is a two dimensional table as shown below; A relation/table in relational model has the following features: Relation is a two-dimensional table. Attribute (i.e. field) is a column in the table. Each column in the table has a unique name within that table. Each column is homogeneous. Thus the entries in any column are all of the same type (e.g. age, name, employee-number, etc.). Each column has a domain, the set of possible values that can appear in that column. A Tuple (i.e. record) is a row in the table. The order of the rows and columns is not important. Values of a row describe the attributes related to the same entry. Duplicate rows are not allowed (candidate keys are designed to prevent this). Cells must be single-valued (but can be of variable length). Single valued means that the multivalued attributes are not allowed. The relational model is today the primary model for commercial data processing applications. It has attained its primary position because of its simplicity as compared to earlier data models such as the network model or the hierarchical model. It is a lower level model that uses a collection of tables (also called relations) to represent both data and the relationship among those data. A table of values is called relation. A relation may be thought of as a set of rows. Each row represents a fact that corresponds to a real world entity or relationship. Each row has a value of an item or set of items that uniquely identifies that row in the table. Sometimes row-ids or sequential numbers are assigned to identify the rows in the table. Each column
typically is called by its column name or column header or attribute name. Each table has multiple columns and each column has a unique name. Fig: Relationship Between Employee and Department Formally, a relation schema can be expressed in the form R(A1, A2, …. , An), where R is the Relation which is defined in terms of attributes A1, A2 .. etc. Thus above Employee and Department relations can be expressed as: Employee (eid, ename, salary, city, dno) Department( dno, dname, head) The set of valid values for each attribute in a relation is called its domain. The main difference between relation and relationship in DBMS is that relation refers to a table in a relational model based database while relationship refers to how two tables are connected together. Relational Database Management System (RDBMS): A Relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd. Data is represented in terms of tuples (rows) in RDBMS. Relational database is most commonly used database.