Structuring Database for Accounting

is that of a key which has not redundancy. This can be shown diagrammatically as shown in figure . . Therefore, minimal super-key (also called Key ) is defined as that part of super-key from which any attribute cannot be removed without sacrificing the uniqueness constraint.

The value of key attribute can be used to identify each tuple in a relation. A key is determined from the meaning of the attributes. The uniqueness feature of key must continue to hold when new tuple in a relation is added. Sometimes a relation may have more than one key in which case each of such keys is called a candidate key .

One such key is termed as primary key of relation. The choice of which candidate key to be primary is generally subjective and may depend on circumstances of mini-world. For Example: Both PAN(Permanent Account Number) and EMPID are candidate keys in EMPLOYEES relation because of being unique. But EMPID should be selected in an organisation being native to the organisational environment.

Fig. . : Flow chart to reach a minimal super-key (c) Entity integrity constraint : States that no primary key value can be null because it is used to identify individual tuple in a relation. Null value implies