Technical Interview Questions and Answers :: DBMS

A relation schema R is said to be in 4NF if for every Multivalued dependency X ->-> Y that holds over R, one of following is true
> X is subset or equal to (or) XY = R.
> X is a super key.

A Relation schema R is said to be 5NF if for every join dependency {R1, R2, ..., Rn} that holds R, one the following is true
> Ri = R for some i.
> The join dependency is implied by the set of FD, over R in which the left side is key of R.

A relation is said to be in DKNF if all constraints and dependencies that should hold on the constraint can be enforced by simply enforcing the domain constraint and key constraint on the relation.

Partial Key:
It is a set of attributes that can uniquely identify weak entities and that are related to same owner entity. It is sometimes called as Discriminator.

Alternate Key:
All Candidate Keys excluding the Primary Key are known as Alternate Keys.

Artificial Key:
If no obvious key, either stand alone or compound is available, then the last resort is to simply create a key, by assigning a unique number to each record or occurrence. Then this is known as developing an artificial key.

Compound Key:
If no single data element uniquely identifies occurrences within a construct, then combining multiple elements to create a unique identifier for the construct is known as creating a compound key.

Natural Key:
When one of the data elements stored within a construct is utilized as the primary key, then it is called the natural key.

Indexing is a technique for determining how quickly specific data can be found.
Types:
> Binary search style indexing
> B-Tree indexing
> Inverted list indexing
> Memory resident table
> Table indexing

A RDBMS maintains a description of all the data that it contains, information about every relation and index that it contains. This information is stored in a collection of relations maintained by the system called metadata. It is also called data dictionary.

The phase that identifies an efficient execution plan for evaluating a query that has the least estimated cost is referred to as query optimization.

Join Dependency:
A Join dependency is a generalization of multivalued dependency. A JD {R1, R2... Rn} is said to hold over a relation R if R1, R2, R3, ..., Rn is a lossless-join decomposition of R . There is no set of sound and complete inference rules for JD.

Inclusion Dependency:
An Inclusion Dependency is a statement of the form that some columns of a relation are contained in other columns. A foreign key constraint is an example of inclusion dependency.

Once the DBMS informs the user that a transaction has successfully completed, its effects should persist even if the system crashes before all its changes are reflected on disk. This property is called durability.

Atomicity:
Either all actions are carried out or none are. Users should not have to worry about the effect of incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.

Aggregation:
A concept which is used to model a relationship between a collection of entities and relationships. It is used when we need to express a relationship among relationships.