Rdbms

DBMS CONCEPTS


1. Database
A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose
2. DBMS
It is a collection of programs that enables user to create and maintain a database. In other words it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications.

3. Database system
The database and DBMS software together is called as Database system.


4. Advantages of DBMS?

Redundancy is controlled.
Unauthorized access is restricted.
Ø providing multiple user interfaces.
Ø Enforcing integrity constraints.
Ø Providing backup and recovery.


5. Disadvantage in File Processing System

Ø Data redundancy & inconsistency.
Ø Difficult in accessing data.
Ø Data isolation.
Ø Data integrity.
Ø Concurrent access is not possible.
Ø Security Problems.


6.The three levels of data abstraction


Ø Physical level: The lowest level of abstraction describes how data are stored.

Ø Logical level: The next higher level of abstraction, describes what data are stored in database and what relationship among those data.

Ø View level: The highest level of abstraction describes only part of entire database.


11. Data Independence


Data independence means that “the application is independent of the storage structure and access strategy of data”. In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.

Two types of Data Independence:

Ø Physical Data Independence: Modification in physical level should not affect the logical level.
Ø Logical Data Independence: Modification in logical level should affect the view level.
NOTE: Logical Data Independence is more difficult to achieve


12. View & How is it related to data independence?

A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary.
Growth and restructuring of base tables is not reflected in views. Thus the view can insulate users from the effects of restructuring and growth in the database. Hence accounts for logical data independence.


13 Data Model
A collection of conceptual tools for describing data, data relationships data semantics and constraints.


14. E-R model
This data model is based on real world that consists of basic objects called entities and of relationship among these objects. Entities are described in a database by a set of attributes.

15. Object Oriented model
This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values and the same methods are grouped together into classes.

16 Entity
It is a 'thing' in the real world with an independent existence.


17. Entity type
It is a collection (set) of entities that have same attributes.


18. Entity set
It is a collection of all entities of particular entity type in the database.

19. Extension of entity type
The collections of entities of a particular entity type are grouped together into an entity set.

20.Weak Entity set
An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set.

21.Attribute
It is a particular property, which describes the entity.

22 Relation Schema & Relation

A relation Schema denoted by R(A1, A2, …, An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is an ordered list of n-values t=(v1,v2, ..., vn).

23. Degree of a Relation
It is the number of attribute of its relation schema.


24. Relationship
It is an association among two or more entities.


25 Relationship set
The collection (or set) of similar relationships.

26. Relationship type
Relationship type defines a set of associations or a relationship set among a given set of entity types.

27. Degree of Relationship type
It is the number of entity type participating.

25. DDL (Data Definition Language)
A data base schema is specifies by a set of definitions expressed by a special language called DDL.

26. VDL (View Definition Language)
It specifies user views and their mappings to the conceptual schema.

29. DML (Data Manipulation Language)
This language that enable user to access or manipulate data as organised by appropriate data model.
Ø Procedural DML or Low level: DML requires a user to specify what data are needed and how to get those data.
Ø Non-Procedural DML or High level: DML requires a user to specify what data are needed without specifying how to get those data


30 Relational Algebra
It is procedural query language. It consists of a set of operations that take one or two relations as input and produce a new relation.

37. Relational Calculus
It is an applied predicate calculus specifically tailored for relational databases proposed by E.F. Codd.
E.g. of languages based on it are DSL ALPHA, QUEL.

38. Difference between Tuple-oriented relational calculus & domain-oriented relational calculus
The tuple-oriented calculus uses a tuple variables i.e., variable whose only permitted values are tuples of that relation. E.g. QUEL
The domain-oriented calculus has domain variables i.e., variables that range over the underlying domains instead of over relation. E.g. ILL, DEDUCE.

39. Normalization
It is a process of analysing the given relation schemas based on their Functional Dependencies (FDs) and primary key to achieve the properties
Ø Minimizing redundancy
Ø Minimizing insertion, deletion and update anomalies.

40. Functional Dependency
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.

41. When is a functional dependency F said to be minimal?
Ø Every dependency in F has a single attribute for its right hand side.
Ø 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.
Ø We cannot remove any dependency from F and still have set of dependency that is equivalent to F.








42. Multivalued dependency

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
Ø t3

§ = t4[X] = t1[X] = t2[X]

Ø t3[Y] = t1[Y] and t4[Y] = t2[Y]
Ø t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]

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

44. 1 NF (Normal Form)
The domain of attribute must include only atomic (simple, indivisible) values.

45. Fully Functional dependency
It is based on 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.

46. 2NF
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.

47. 3NF

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

48. BCNF (Boyce-Codd Normal Form)
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.

49. 4NF
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
Ø X is subset or equal to (or) XY = R.
Ø X is a super key.

50. 5NF
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
Ø Ri = R for some i.
Ø The join dependency is implied by the set of FD, over R in which the left side is key of R.


51. Atomicity and Aggregation
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.

Logged



Concept of DBMS

A database management system (DBMS) is a program that lets one or more computer users create and access data in a database. A DBMS can be thought of as a file manager that manages data in databases rather than files in file systems. The DBMS provides integrity of the data, i.e., making sure data continues to be accessible and is consistently organized as intended and security making sure only those who have privileges can access the data. The DBMS manages user requests and requests from other programs so that users and other programs are free from having to understand where the data is physically located on storage media and, in a multi-user system, who else may also be accessing the data.

Major functions of a database are:

1. Creating records of various data types such as integer, real, character, etc.
2. Query will be made by a standardized language such as SQL (Standard Query Language).
3. Operation such as sort, delete, modify, select, etc.
4. Manipulation such as input, output, analysis, reformatting, etc.
5. Documentation such as metadata or description of the contents of the database should be compiled.

There are four types of database models:

1. Hierarchical model.
2. Network model.
3. Relational model.
4. Object oriented model.

A DBMS is usually an inherent part of a database product. Microsoft’s SQL Server is an example of a DBMS that serves database requests from multiple clients (users). On PCs, Microsoft Access is a popular example of a single- or small-group user DBMS. Other popular DBMSs are IBM’s DB2, Oracle’s line of database management products and Sybase’s products.



DBMS Questions

1. What is DBMS?

It is a collection of programs that enables user to create & maintain a database.
In other words it is general-purpose software that provides the users with the processes of
defining, constructing & manipulating the database for various applications.

2. What is Database?

A database is a logically coherent collection of data with some inherent meaning,
representing some aspect of real world & which is designed, built & populated with
data for a specific purpose.

3. What is a Database system?

The database & DBMS software together is called as Database system.

4. Advantages of DBMS?

Redundancy is controlled. Unauthorised access is restricted. Providing multiple user interfaces. Enforcing integrity constraints. Providing backup & recovery.

5. Disadvantage in File Processing System?

Data redundancy & inconsistency. Difficult in accessing data. Data isolation. Data integrity. Concurrent access is not possible. Security Problems.

6. Describe the three levels of data abstraction?

The are three levels of abstraction: Physical level: The lowest level of abstraction describes how data are stored. Logical level: The next higher level of abstraction, describes what data are stored in
database & what relationship among those data. View level: The highest level of abstraction describes only part of entire database.

7. Define the "integrity rules"

135

There are two Integrity rules. Entity Integrity: States that "Primary key cannot have NULL value" Referential Integrity: States that "Foreign Key can be either a NULL value
or should be Primary Key value of other relation.


8. What is extension & intension in DBMS?

Extension - It is the number of tuples present in a table at any instance. This is time dependent.

Intension - It is a constant value that gives the name, structure of table & the constraints laid on it.


9. What is System R? What are its two major subsystems?

System R was designed & developed over a period of 1974-79 at IBM San Jose
Research Center. It is a prototype & its purpose was to demonstrate that it is possible to
build a Relational System that can be used in a real life environment to solve real life
problems, with performance at least comparable to that of existing system.
Its two subsystems are Research Storage System Relational Data System.


10. How is the data structure of System R different from the relational DBMS structure?

Unlike Relational systems in System R Domains are not supported Enforcement of candidate key uniqueness is optional Enforcement of entity integrity is optional Referential integrity is not enforced





DBMS aptitude Questions

1. What is Data Independence in DBMS?

Data independence means that "the application is independent of the storage structure & access strategy of data". In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.

Two types of Data Independence:

Physical Data Independence: Modification in physical level should not affect the logical level.

Logical Data Independence: Modification in logical level should affect the view level.

NOTE: Logical Data Independence is more difficult to achieve

2. What is a view in DBMS? How it is related to data independence?

A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary. Growth & restructuring of base tables is not reflected in views. Thus the view 136 can insulate users from the effects of restructuring & growth in the database. Hence accounts for logical data independence.

3. What is Data Model in DBMS?

A collection of conceptual tools for describing data, data relationships data semantics & constraints.


4. What is E-R model?

This data model is based on real world that consists of basic objects called entities & of relationship among these objects. Entities are described in a database by a set of attributes.


5. What is Object Oriented model?

This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values & the same methods are grouped together into classes.

6. What is an Entity?
It is a 'thing' in the real world with an independent existence.

7. What is an Entity type?
It is a collection (set) of entities that have same attributes.

8. What is an Entity set?
It is a collection of all entities of particular entity type in the database.

9. What is an Extension of entity type?
The collections of entities of a particular entity type are grouped together into an
entity set.

10. What is Weak Entity set?
An entity set may not have sufficient attributes to form a primary key, and its
primary key compromises of its partial key and primary key of its parent entity, then it is
said to be Weak Entity set.



Basic Difference between DBMS and RDBMS
« on: July 11, 2008, 01:51:57 PM »

Database has to be persistent, meaning that the information stored in a database has to continue to exist even after the application(s) that saved and manipulated the data have ceased to run. A database also has to provide some uniform methods that are not dependent on a specific application for accessing the information that is stored inside the database.

This is a pretty liberal definition of a database. Lotus Notes calls its message stores "databases", and by this definition they qualify. MUMPS calls its associative storage a database, and while it takes a bit of a stretch, even that meets this definition. There are a number of new database technologies that include object-oriented databases and associative databases, and they seem to qualify as databases under this definition too.

Basic Characteristics of DBMS

• Represents complex relationship between data
• Controls data redundancy.
• Enforces user defined rules.
• Ensures data sharing.
• It has automatic and intelligent backup and recovery procedures.
• It has central dictionary to store information.
• Pertaining to data and its manipulation.
• It has different interfaces via which user can manipulate the data.
• Enforces data access authorization.