DBMS Short Question And Answer Set-2

1. Define the terms i) Entity set ii) Relationship set
Entity set: The set of all entities of the same type is termed as an entity set.
Relationship set : The set of all relationships of the same type is termed as a relationship set.

2. Define single valued and multivalued attributes.
Single valued attributes: attributes with a single value for a particular entity are called single valued attributes.
Multivalued attributes : Attributes with a set of value for a particular entity are called multivalued attributes.

3. What are stored and derived attributes?
Stored attributes: The attributes stored in a data base are called stored attributes.
Derived attributes: The attributes that are derived from the stored attributes are called derived attributes.

4. What are composite attributes?
Composite attributes can be divided in to sub parts.

5. Define null values
In some cases a particular entity may not have an applicable value for an attribute or if we do not know the value of an attribute for a particular entity. In these cases null value is used.

6. Define the terms i) Entity type ii) Entity set
Entity type: An entity type defines a collection of entities that have the same attributes.
Entity set: The set of all entities of the same type is termed as an entity set.

7. What is meant by the degree of relationship set?
The degree of relationship type is the number of participating entity types.

8. Define the terms
i) Key attribute
ii) Value set
Key attribute : An entity type usually has an attribute whose values are distinct from each individual entity in the collection. Such an attribute is called a key attribute.
Value set: Each simple attribute of an entity type is associated with a value set that specifies the set of values that may be assigned to that attribute for each individual entity.

9. Define weak and strong entity sets?
Weak entity set: entity set that do not have key attribute of their own are called weak entity sets.
Strong entity set: Entity set that has a primary key is termed a strong entity set.

10. What does the cardinality ratio specify?
Mapping cardinalities or cardinality ratios express the number of entities to which another entity can be associated. Mapping cardinalities must be one of the following:

  • One to one
  • One to many
  • Many to one
  • Many to many

11. Explain the two types of participation constraint.

  •  Total: The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in at least one relationship in R.
  •  Partial: if only some entities in E participate in relationships in R, the participation of entity set E in relationship R is said to be partial.

12. What is meant by lossless-join decomposition? 
We claim the above decomposition is lossless. How can we decide whether decomposition is lossless?
1. Let R be a relation schema.
2. Let F be a set of functional dependencies on R.
3. Let R1and R2 form a decomposition of R.
4. The decomposition is a lossless-join decomposition of R if at least one of the following functional dependencies are in :
a. R1∩ R2→ R1
b. R1∩ R2→ R2

13. List the disadvantages of relational database system
Repetition of data
Inability to represent certain information.

14. What is first normal form?
The domain of attribute must include only atomic (simple, indivisible) values.

15. What is meant by functional dependencies?
Consider a relation schema R and a C R and ß C R. The functional dependency a ß holds on relational schema R if in any legal relation r(R), for all pairs of tuples t1 and t2 in r such that t1 [a] =t1 [a], and also t1 [ß] =t2 [ß].

16. What are the uses of functional dependencies?
To test relations to see whether they are legal under a given set of functional dependencies.
To specify constraints on the set of legal relations.

17. Explain trivial dependency?
Functional dependency of the form a ß is trivial if ß C a. Trivial functional dependencies are satisfied by all the relations.

18. What are axioms?
Axioms or rules of inference provide a simpler technique for reasoning about functional dependencies.

19. What is meant by computing the closure of a set of functional dependency?
+ The closure of F denoted b y F is the set of functional dependencies logically implied by F.

20. What is meant by normalization of data?
It is a process of analyzing the given relation schemas based on their Functional Dependencies (FDs) and primary key to achieve the properties

  • Minimizing redundancy
  • Minimizing insertion, deletion and updating anomalies