Which of the following statements is true about referential integrity?

A.
It checks that no component of a primary key can have a null value and no duplicate entries
can exist.

B.
It checks that the value of a primary key must be consistent throughout the life of an instance.

C.
It distinguishes between null values and zero entries.

D.
It checks that no record in a child table can exist if its corresponding record is not available in
the parent table.

Explanation:
Referential integrity is enforced through a foreign key. There must be a reference of the foreign key
in its originating table. For example, take
two tables, Employee and Department. The DeptID attribute of the Department table is the primary
key, and when it is exported to Employee
table, it becomes the foreign key in the Employee table.
The data value of DeptID in the Employee table cannot exist if it does not exist in the Department table.

Referential integrity constraint is a feature in RDBMS (Relational Database Management System)
that prevents a user from entering
inconsistent data. There are various referential integrity rules that are applied on tables sharing a
relationship. These are as follows:
If the Employee table has a foreign key that is a primary key of the Department table, then the
Employee table cannot have a value in
the foreign key column if there is no corresponding value in the Department table.
If a record is deleted from the Department table, the corresponding record must be deleted from
the Employee table. This is known as
cascading delete.
Answer C is incorrect. According to an integrity constraint, a table can have only one primary key
constraint. A primary key constraint imposes that the primary key attribute should be unique and
cannot have NULL values. This ensures that the primary key value uniquely identifies each and every
row in a table.