Database Reference
In-Depth Information
An integrity constraint that determines the range of possible values for a domain.field spe-
cification represents all the elements of a field. This type of data integrity warrants the fol-
lowing: the identity and purpose of a field is clear and all of the tables in which it appears
are properly identified; field definitions are consistent throughout the database; the values
of a field are consistent and valid; and the types of modifications, comparisons, and opera-
tions that can be applied to the values in the field are clearly identified. Each field specific-
ation incorporates three types of elements: general, physical, and logical.
General elements constitute the most fundamental information about the field and include
items such as Field Name, Description, and Parent Table.
Physical elements determine how a field is built and how it is represented to the person
using it. This category includes items such as Data Type, Length, and Display Format.
Logical elements describe the values stored in a field and include items such as Required
Value, Range of Values, and Default Value.
4.12 ENTITY INTEGRITY
Definition
No prime attribute of a relation may hold a null value
Entity constraints ensure the integrity of the entities being modeled by the system. Entity
integrity is an integrity rule which states that every table must have a primary key and that
the column or columns chosen to be the primary key should be unique and not null. If each
value of a primary key must be distinct, no duplicate rows can logically appear in a table.
Example 8:
Let Account (AccountNo, Name, Type of Account, Amount) with a primary key Ac-
countNo be a relation in a bank database. Following figure shows its possible instance.
