### Database/DBMS Interview Questions And Answers - Part XVI.

68) What Are The Lossless Inference Rules For Functional Dependencies (FD's) Or State The Armstrong's Axioms?

Answer: Inference rules also known as Armstrong's Axioms are published by Armstrong. These properties are as given below:

1. Reflexivity Property: X -> Y is true if Y is subset of X.

2. Augmentation Property: If X -> Y is true, then  XZ -> YZ is also true.

3. Transitivity Property: If X -> Y and Y -> Z then X -> Z is implied.

4. Union Property: If X -> Y and X -> Z are true, then X -> YZ is also true. This property indicates that if right hand side of FD contains many attributes then FD exists for each of them.

5. Decomposition Property: If X -> Y is implied and Z is subset of Y, then X -> Z is implied. This property is the reverse of union property.

6. PseudoTransitivity Property: If X -> Y and WY -> Z are given, then XW -> Z is true.

69) What Is Domain-Key Normal Form (DKNF)?

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

The idea is to specify (theoretically, at least) the "Ultimate Normal Form" that takes into account all possible types of dependencies and constraints.

For a relation in DKNF, it becomes very straightforward to enforce all database constraints by simply checking that each attribute value in a tuple is of the appropriate domain and that every key constraint is enforced.  The practical utility of DKNF is limited

70) Define Transaction? Explain Different States Of Transaction?

Answer: A transaction is an action, or series of actions, carried out by a single user or application program, which reads or updates the contents of the database.

Basically it's a discrete unit of work that must be completely processed or not processed at all. Example: Transferring funds from a one account to another. Because failures occur, transactions are broken up into states to handle various situations.

Active (The Initial State), the transaction stays in this state while it is executing. A transaction is terminated only if it has either been committed or aborted.

Partially committed, after the final statement has been executed. At this point failure is still possible since changes may have been only done in main memory, a hardware failure could still occur.

Failed, after the discovery that normal execution can no longer proceed. Once a transaction cannot be completed, any changes that it made must be undone by rolling it back.

Aborted, after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. The DBMS could either
• Restart the transaction – only if no internal logical error.
• Kill the transaction.
A transaction may only be restarted as a result of some hardware or software error, and a restarted transaction is considered a new transaction.

Committed, after successful completion. Once committed, the transaction can no longer be undone by aborting it. Its effect could be undone only by a compensating transaction.

ALSO CHECK THE BELOW FAQ's:

71) Describe The ACID Properties Of A Transaction?
72) What Is Schedule & What Is The Importance Of Schedule?
73) What Do You Mean By Serializability In Transaction Processing?
74) What Are The Two Methods That Guarantee Serializability?
75) What Is Locking? What Are The Types Of Locking?

76) Discuss Two Phase Locking (2PL) Protocol?
77) What Is Meant By Deadlock In DBMS?
78) What Are The Three Techniques For Handling Deadlocks?
79) How You Can Identify, Prevent & Resolve A Deadlock?
80) What Is Timestamping? Between Timestamping & Locking Which Is A Better Concurrency Control Technique?

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The more detail you give the stronger your answers will be. Try to summarize all your accomplishments and relate what makes you unique.

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