Technical Interview Questions and Answers :: DBMS

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 update anomalies.

A Functional dependency is denoted by X->Y between two sets of attributes X and Y that are subsets of R specifies a constraint on the possible tuple that can form a relation state r of R. The constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This means the value of X component of a tuple uniquely determines the value of component Y.

> Every dependency in F has a single attribute for its right-hand side.
> We cannot replace any dependency X A in F-> with a dependency Y A where Y-> is a proper subset of X and still have a set of dependency that is equivalent to F.
> We cannot remove any dependency from F and still have set of dependency that is equivalent to F.

Multivalued dependency denoted by X -> Y specified on relation schema R, where X and Y are both subsets of R, specifies the following constraint on any relation r of R: if two tuples t1 and t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also exist in r with the following properties
> t3[x] = t4[X] = t1[X] = t2[X]
> t3[Y] = t1[Y] and t4[Y] = t2[Y]
> t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]

It guarantees that the spurious tuple generation does not occur with respect to relation schemas after decomposition.

The domain of an attribute must include only atomic (simple, indivisible) values.

It is based on the concept of full functional dependency. A functional dependency X -> Y is full functional dependency if removal of any attribute A from X means that the dependency does not hold any more.

A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R is fully functionally dependent on primary key.

A relation schema R is in 3NF if it is in 2NF and for every FD X -> A either of the following is true
> X is a Super-key of R.
> A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively dependent on primary key.

A relation schema R is in BCNF if it is in 3NF and satisfies an additional constraint that for every FD X -> A, X must be a candidate key.