The relational model

The most widely used kind of database is the relational model. In it, data is stored in tables, called relations, and related tables are linked by common attributes.

The terms:

• Attribute: a column of a relation, such as Name.
• Tuple: a row: one record.
• Domain: the set of allowed values (the data type) for an attribute.
• Degree: the number of attributes (columns).
• Cardinality: the number of tuples (rows).

For example, the GUARDIAN relation, with 4 attributes and 5 rows, has degree 4 and cardinality 5.

A relation has these properties:

• Every attribute has a unique name, and the order of the attributes does not matter.
• Every tuple is distinct (no two rows are exactly the same), and the order of the tuples does not matter.
• All the values in one column come from the same domain.
• Each value is atomic: a single value that cannot be broken down further, and each tuple has only one value for each attribute.
• A special value, NULL, is used when a value is unknown or not applicable.