The three-schema architecture divides the database into three-level used to create a separation between the physical database and the user application. In simple terms, this architecture hides the details of physical storage from the user.The database administrator (DBA) responsible is to change the structure of database storage without affecting the user’s view. It deals with the data, the relationship between them and the different access methods implemented on the database. The logical design of a database is called a schema.This architecture contains three layers of database management system (DBMS) −External levelConceptual levelInternal levelThe three layers of the DBMS are pictorially ... Read More

Attributes are the properties which describe an entity. Example The attributes of student entity are as follows − Roll number Name Branch Age Types of attributes The different types of attributes are as follows − Composite attribute It can be divided into smaller sub parts, each sub part can form an independent attribute. For example −     ... Read More

The network model was created to represent complex data relationships more effectively when compared to hierarchical models, to improve database performance and standards.It has entities which are organized in a graphical representation and some entities are accessed through several paths. A User perceives the network model as a collection of records in 1:M relationships.Given below is the pictorial representation of the network model in DBMS −FeaturesThe features of a Network Model are as follows −Ability to Merge Relationships − In this model, because of more relationships the data is more related. It has an ability to manage one-to-one relationships as ... Read More

It is a collection of concepts that can be used to describe the structure of a database. The structure of the database means data types, relationships and constraints that should hold the data nothing but it provides the necessary means to achieve the abstraction.A Model is basically a conceptualization between attributes and entities.There are basically three main data models in the Database Management System (DBMS), which are as follows −NetworkHierarchicalRelationalBut these days many new data models are being developed.Components of data modelThe data model can have three components which are as follows −Structural part − It consists of a set ... Read More

A set of functional dependencies (FD) F is said to cover another set of functional dependencies E if every FD in E is also in F closure; that is, if every dependency in E can be inferred from F.Alternatively, we can say E is covered by F. Two sets of functional dependencies E and F are equivalent if E+= F+. That is E is equivalent to F if E covers F and F covers E.To determine whether F covers E we calculate X+ with respect to F for each FD X->y in E and then check whether X+ includes the ... Read More

Decomposition means dividing a relation R into {R1, R2, ......Rn}. It is dependency preserving and lossless.Dependency preserving decompositionLet R is decomposed into {R1, R2, ...., Rn} with projected FD set {F1, F2, ......Fn}. This decomposition is dependency preserving if F+ ={F1 U F2 U.........Fn}+.ExampleLet the relation R{A, B, C, D, E} F:{AB->C, C->D, AB->D} R is decomposed to R1(A, B, C), R2(D, E). Prove decomposition is dependency preserving.SolutionF1={AB->C}F2={C->D}=> (F1 u F2) = {AB->C, C->D}AB+ under (F1 U F2) = {A, B, C, D} => AB->D is under (F1 U F2)F+ = (F1 U F2)+=> Decomposition is dependency preserving.Decomposition is not ... Read More

Decomposition is said to be lossy, if it is impossible to reconstruct the original table from the decomposed tables without loss of information.Decomposition is said to be lossless, if it is possible to reconstruct the original table by using a natural join without any loss of information.AlgorithmGiven below is an algorithm to check if decomposition is lossy or lossless −Step 1 − Create a table with M rows and N columnsM= number of decomposed relations.N= number of attributes of original relation.Step 2 − If a decomposed relation Ri has attribute A thenInsert a symbol (say ‘a’) at position (Ri, A)Step ... Read More

Canonical cover is called minimal cover which is called the minimum set of FDs. A set of FD FC is called canonical cover of F if each FD in FC is a Simple FD, Left reduced FD and Non-redundant FD.Simple FD − X->Y is a simple FD if Y is a single attribute.Left reduced FD: X->Y is a left reduced FD if there are no extraneous attributes in X.{extraneous attributes: let XA->Y then A is a extraneous attribute if X_>Y}Non-redundant FD − X->Y is a Non-redundant FD if it cannot be derived from F- {X->y}.ProblemFind the canonical cover of FD ... Read More

A minimal cover of a set of functional dependencies (FD) E is a minimal set of dependencies F that is equivalent to E.The formal definition is: A set of FD F to be minimal if it satisfies the following conditions −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 dependencies that is equivalent to F.We cannot remove any dependency from F and still have a set of dependencies that are equivalent to ... Read More

Closure of an attribute x is the set of all attributes that are functional dependencies on X with respect to F. It is denoted by X+ which means what X can determine.AlgorithmLet’s see the algorithm to compute X+Step 1 − X+ =XStep 2 − repeat until X+ does not changeFor each FD Y->Z in FIf Y ⊆ X+ then X+ = X+ U ZExample 1Consider a relation R(A, B, C, D, E, F)F: E->A, E->D, A->C, A->D, AE->F, AG->K.Find the closure of E or E+SolutionThe closure of E or E+ is as follows −  E+ = E     =EA ... Read More