Database Reference
In-Depth Information
contributes to defining a unique value.
Its values must be uniquely identify each record in the table. It protects us against
duplicate records.
Its value must exclusively identify the value of each field within a given record.
With this, the table's candidate keys give the only means of identifying each field
value within the record.
Only in rare cases its value can be modified.
Example 3:
In following figure we probably identified Registration No., NAME, Address, Branch and
City as potential candidate keys. But, we have to examine these fields more closely to de-
termine which ones are truly eligible to become candidate keys.
Upon close examination, we can draw the following conclusions:
Registration No. is eligible. Because, this field conforms to every element of a candidate
key.
Name is ineligible because it can contain duplicate values. The values of a candidate key
must be unique. But, in this case there can be more than one occurrence of a particular
name.
Address is ineligible because it can contain duplicate values. Many people live in the
same house.
