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A relation is defined as a set of tuples. By definition, a relation has no duplicate tuples. It means that no two tuples can have the same combination of values for all their attributes. An attribute (or combination of attributes) of a relation is selected as a key that uniquely identifies the tuples of a relation. The value of a key attribute can be used to identify uniquely each tuple in the relation.
A relation schema may have more than one key. In this case, each of the keys is candidate key. It is common to designate one of the candidate keys as the primary key of the relation. In this case, other candidate keys are called the secondary keys. A primary key is a candidate key whose values are used to identify tuples in the relation. The primary key is underlined in relation schema, Each relation schema must have a primary key. A primary key must not contain null values (undefined values).
A foreign key is an attribute or combination of attributes of a relation that is the primary key of another relation. For example, Roll-No may be an attribute of 'Marks' relation but not the primary key of 'Marks'. However, it is the primary key of the 'Student' relation. In 'Marks' relation, Roll-No attribute will be referred to as foreign key.
A relation schema may have more than one key. In this case, each of the keys is candidate key. It is common to designate one of the candidate keys as the primary key of the relation. In this case, other candidate keys are called the secondary keys. A primary key is a candidate key whose values are used to identify tuples in the relation. The primary key is underlined in relation schema, Each relation schema must have a primary key. A primary key must not contain null values (undefined values).
A foreign key is an attribute or combination of attributes of a relation that is the primary key of another relation. For example, Roll-No may be an attribute of 'Marks' relation but not the primary key of 'Marks'. However, it is the primary key of the 'Student' relation. In 'Marks' relation, Roll-No attribute will be referred to as foreign key.
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