The Relational Model — Relations, Tuples and Attributes

A database model describes the logical structure of a database — how the data is organised and how different pieces of data relate to one another. Several models exist (hierarchical, network, relational, object-oriented), but the one that matters most for business applications is the relational model, proposed by E. F. Codd.

In the relational model, data is stored in tables. It is worth learning the exact vocabulary, because these words are used constantly:

• A relation (table) is a collection of related data arranged in rows and columns, rather like a grid on paper. For example, a Student table stores data about students.
• An attribute (field / column) is a single column of the table. It represents one specific property of the data — for example, RollNo, Name, Class and Marks are attributes of a Student table. Every value in a column is of the same kind.
• A tuple (record / row) is a single row of the table. It holds one complete set of related values — for example, all the details of one particular student. If a table has 40 students, it has 40 tuples.
• A domain is the set of permitted values for an attribute — for example, the domain of a Marks attribute might be whole numbers from 0 to 100.
• The degree of a relation is the number of attributes (columns) it has, and the cardinality is the number of tuples (rows) it currently holds.

A small Student table might look like this: