Question

5.5. Why do we designate one of the candidate keys of a relation to be the primary
key?
5.6. Discuss the characteristics of relations that make them different from ordinary
tables and files.
5.7. Discuss the various reasons that lead to the occurrence of NULL values in
relations.

Answer 1

5.5. A relation schema having more than one key are called candidate keys. One of the candidate key is called primary key of the relation since it becomes easier to handle databases when there is a single distinct key for a particular relation instead of having more than one keys. Primary key is generally a single attribute or a smaller number of attributes. Primary key is a candidate key which is used to identify tuples in a relation when a relation schema has several candidate keys. We designate one of the candidate key of a relation to be a primary key because that primary key is use for any foreign key references.Entity integrity constraint.

5.6. Tables and files have some particular order but relations don’t.Ordering of tuples in a Relation: The tuples are not considered to be ordered, even though they appear to be in the tabular form. - Ordering of attributes in a relation schema R and of values within each tuples: We consider the attributes in R  and the values to be ordered. Values in a tuple: All values are considered to be atomic or indivisible. A special null value is used to represent values that are unknown or inapplicable to certain tuples.

5.7. The desired reasons that lead to the occurrence of null values in relations are 1.When value of an attribute is not applicable for a tuple. it can be marked as NULL. 2. When value for an attribute unknown for a tuple, it can be marked as NULL.