Explain the characteristics of relations?
Database Management System > DBMS Relational Databases > Relational Database Design
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Answer:
A relation has certain characteristics, which are given here.
Ordering of tuples in a relation: Since a relation is a set of tuples and a set has no particular order among its elements, hence, tuples in a relation do not have any specified order. However, tuples in a relation can be logically ordered by the values of various attributes. In that case, information in a relation remains same, only the order of tuple varies. Hence, tuple ordering in a relation is irrelevant.
Ordering of values within a tuple: An n-tuple is an ordered set of attribute values that belongs to the domain D, so, the order in which the values appear in the tuples is significant. However, if a tuple is defined as a set of (<attribute> : <value>) pairs, the order in which attributes appear in a tuple is irrelevant. This is due to the reason that there is no preference for one attribute value over another.
Values and nulls in the tuples: Relational model is based on the assumption that each tuple in a relation contains a single value for each of its attribute. Hence, a relation does not allow composite and multivalued attributes. Moreover, it allows denoting the value of the attribute as null, if the value does not exist for that attribute or the value is unknown.
No two tuples are identical in a relation: Since a relation is a set of tuples and a set does not have identical elements. Therefore, each tuple in a relation must be uniquely identified by its contents. In other words, two tuples with the same value for all the attributes (that is, duplicate tuples) cannot exist in a relation.
Interpretation of a relation: A relation can be used to interpret facts about entities as a type of assertion. A relation can also be used to interpret facts about relationships.
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