The relational model presents an orderly, predictable and intuitive approach to organizing, manipulating and viewing data.
RDBMS Terminology
Relational data consists of relations. A relation (or relational table) is a two dimensional table with special properties. A relational table consists of a set of named columns and an arbitrary number of rows. The columns are called as attributes and rows are called as tuples. Each attribute is associated with a domain. A domain is a set of values that may appear in one or more columns.
Properties of Relational Data Structures
Relational tables have six properties, which must be satisfied for any table to be classified as relational. These are:
- Entries of attributes are single valued: Entry in every row and column position in a table must be single valued. This means columns do not contain repeating groups.
- Entries of attribute are of the same kind: Entries in a column must be of same kind. A column supposed to store salary of an employee should not store commission.
- No two rows are identical: Each row should be unique. This uniqueness is ensured by the values in a specific set of columns called the primary key.
- The order of attributes is unimportant: There is no significance attached to order in which columns are stored in the table. A user can retrieve columns in any order.
- The order of rows is unimportant: There is no significance attached to the order in which rows are stored in the table. A user can retrieve rows in any order.
- Every column can be uniquely identified: Each column is identified by its name and not its position. A column name should be unique in the table.
