Question

1. Define the following terms as they apply to the relational model of data: domain, attribute, n- tuple, relation schema, relation state, degree of a relation, relational database schema, and relational database state. 2. Why are tuples in a relation not ordered? 3. Why are duplicate tuples not allowed in a relation? 4. What is the difference between a key and a superkey? 5. Why do we designate one of the candidate keys of a relation to be the primary key? 6. Discuss the characteristics of relations that make them different from ordinary tables and files. 7. Discuss the various reasons that lead to the occurrence of NULL values in relations. 8. Discuss the entity integrity and referential integrity constraints. Why is each considered important? 9. Define foreign key. What is this concept used for? 10. What is a transaction? How does it differ from an Update operation?

Please answer the above questions as I wish to confirm my answers. Thanks.

Answer 1

2) Why are tuples in a relation not ordered?
Answer:
Tuples in a relation do not have a particular order similar to elements in a set. Tuples are only logical definitions attempting to represent facts defined at the abstract level. A relation is not sensitive to and dependent on the tuples ordering. Mathematically, elements of a set do not have any order among them, so tuples in a relation do not follow a particular order.

3) Why are duplicate tuples not allowed in a relation?
Answer:
Duplicate tuples, when allowed in a relation, would violate the specifications of the relational integrity constraints. The important constraint is two tuples cannot and should not contain the same values for their attributes at any relation state of a database, creating redundancy of database, in turn, making the data processing such as querying, inserting, deleting, updating, etc., slowing the database's speed or simply its performance.

4) What is the difference between a key and a superkey?
Answer:
Key (K):
* It is just an attribute.
* It is a minimal super key.
* It is either a single or a combination of multiple fields.
* It is the shortest super key.
* It is also called a candidate key.
* Candidate Keys are individual columns in a table.
* A candidate key qualifies for the uniqueness of each row or tuple.
* All keys or candidate keys are super keys.
* It is a super key without any redundant attributes.

SuperKey (SK) is an attribute or a set of attributes.
* It is used to uniquely identify the tuples of a relation.
* It is used to uniquely identify all attributes in a relation.
* Hence, it is any subset of attributes.
* It does not depend on any other key.
* It is a set of one or more attributes to uniquely identify a row in a table.
* All super keys cannot and are not candidate keys, but the vice versa is not true.

9) Define foreign key. What is this concept used for?
Answer:
It is basically a field (column). It could even be a collection of fields. A foreign key in one table refers to a primary key in another table. The table with a foreign key is called a child table. The table with a candidate key is called the referenced or the parent table.

It is used for linking two tables together. It basically links between data in two tables. Implementation of foreign keys is more complex than primary keys though. They are thus used as a cross-reference between tables.