Different Types of Keys in Relational Database Model

Now we’ll take up another feature of relational tables. That is different type of keys. There are different types of keys, namely Primary keys, alternate keys, etc. The different types of keys are described below.

Primary key

Within a given relation, there can be one attribute with values that are unique within the relation that can be used to identify the tuples of that relation. That attribute is said to be primary key for that relation.

Composite primary key

Not every relation will have single-attribute primary key. There can be a possibility that some combination of attribute when taken together have the unique identification property. These attributes as a group is called composite primary key. A combination consisting of a single attribute is a special case.

Existence of such a combination is guaranteed by the fact that a relation is a set. Since sets don’t contain duplicate elements, each tuple of a relation is unique with respect to that relation. Hence, at least the combination of all attributes has the unique identification property.

In practice it is not usually necessary to involve all the attributes-some lesser combination is normally sufficient. Thus, every relation does have a primary (possibly composite) key.

Tuples represent entities in the real world. Primary key serves as a unique identifier for those entities.

Candidate key

In a relation, there can be more than one attribute combination possessing the unique identification property. These combinations, which can act as primary key, are called candidate keys.

EmpNo	SocSecurityNo	Name	Age
1011	2364236	Harry	21
1012	1002365	Sympson	19
1013	1056300	Larry	24

Fig. 8.6 Table having “EmpNo” and “SocSecurityNo” as candidate keys

Alternate key

A candidate key that is not a primary key is called an alternate key. In fig. 8.6 if EmpNo is primary key then SocSecurityNo is the alternate key.

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