Ans:                     Weak Entity:

An entity type whose instances cannot exist without being linked with instances of some

other entity type, i.e., they cannot exist independently. For example, in an organization

we want to maintain data about the vehicles owned by the employees. Now a particular

vehicle can exist in this organization only if the owner already exists there as employee.

Similarly, if employee leaves the job and the organization decides to delete the record of

the employee then the record of the vehicle will also be deleted since it cannot exist

without being linked to an instance of employee.

       
                                  Strong Entity Type:

An entity type whose instances can exist independently, that is, without being linked to the instances of any other entity type is called strong entity type. A major property of the strong entity types is that they have their own identification, which is not always the case with weak entity types. For example, employee in the previous example is an independent or strong entity type, since its instances can exist independently.

        
                             Associative entity:

An entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship between those entity instances.

                                     Locking:

A process in which any data that are retrieved by a user for updating must be locked, or denied to other users, until the update is completed or aborted. Locking data is much like checking a book out of the library; it is unavailable to others until the borrower returns it.

                                        Concurrency:

Concurrency in terms of databases means allowing multiple users to access the data contained within a database at the same time. If concurrent access is not managed by the Database Management System (DBMS) so that simultaneous operations don't interfere with one another problems can occur when various transactions interleave, resulting in an inconsistent database.

Logical schema:

The representation of a database for a particular data management technology is called logical schema.

                                Conceptual schema:

A detailed, technology independent specification of the overall structure of organizational

data.

Data definition language (DDL):

 Commands used to define a database, including those for creating, altering, and dropping tables and establishing constraints.

Data manipulation language (DML):

Commands used to maintain and query a database, including those for updating, inserting, modifying, and querying data.

Q.No.2: What is normalization? With the help of any example normalize data up to 3NF.

 Normalization:

The process of decomposing relations with anomalies to produce smaller, well-       structured relations.

Un-Normalized Form (UNF):

If a table contains non-atomic values at each row, it is said to be in UNF. An atomic value is

Something that cannot be further decomposed. A non-atomic value, as the name suggests, can be further decomposed and simplified. Consider the following table:

In the sample table above, there are multiple occurrences of rows under each key Emp-Id. Although considered to be the primary key, Emp-Id cannot give us the unique identification facility for any single row. Further, each primary key points to a variable length record (3 for E01, 2 for E02 and 4 for E03).

First Normal Form (1NF)

A relation is said to be in 1NF if it contains no non-atomic values and each row can provide a unique combination of values. The above table in UNF can be processed to create the following table in 1NF.

As you can see now, each row contains unique combination of values. Unlike in UNF, this relation contains only atomic values, i.e. the rows cannot be further decomposed, so the relation is now in 1NF.

Second Normal Form (2NF):

A relation is said to be in 2NF f if it is already in 1NF and each and every attribute fully depends on the primary key of the relation. Speaking inversely, if a table has some attributes which is not dependent on the primary key of that table, then it is not in 2NF. Let us explain. Emp-Id is the primary key of the above relation. Emp-Name, Month, Sales and Bank-Name all depend upon Emp-Id. But the attribute Bank-Name depends on Bank-Id, which is not the primary key of the table. So the table is in 1NF, but not in 2NF. If this position can be removed into another related relation, it would come to 2NF.

After removing the portion into another relation we store lesser amount of data in two relations without any loss information. There is also a significant reduction in redundancy.

Third Normal Form (3NF)

A relation is said to be in 3NF, if it is already in 2NF and there exists no transitive dependency in that relation. Speaking inversely, if a table contains transitive dependency, then it is not in 3NF, and the table must be split to bring it into 3NF.

What is a transitive dependency? Within a relation if we see

A => B [B depends on A]

And

B => C [C depends on B]

Then we may derive

A => C[C depends on A]

Such derived dependencies hold well in most of the situations. For example if we have

Roll => Marks

And

Marks =>  Grade

Then we may safely derive

Roll => Grade.

This third dependency was not originally specified but we have derived it.

The derived dependency is called a transitive dependency when such dependency becomes

improbable. For example we have been given

Roll => City

And

City => STDCode

If we try to derive Roll => STDCode it becomes a transitive dependency, because obviously the

STDCode of a city cannot depend on the roll number issued by a school or college. In such a case the relation should be broken into two, each containing one of these two dependencies:

Roll =>City

And

City => TD code

Q.No.3 Explain ER model and its element. Also give examples.

The History of Entity Relationship Model

Entity

Entity Types	Properties	Instances
EMPLOYEE	Human being, has name, has father name, has are registration number, has qualification, designation	Mr. Burhan , Mr. Kashif and many others
FURNITURE	Used to sit or work on, different material, having legs, cost, purchased	Chair, table etc.
ELECTRIC APPLIANCES	Used for office work, consumable or non-consumable,	Papers, pencil, paper weight etc.

STUDENT                      Student ID, Student Name, Home Address, Phone Number, Major AUTOMOBILE              Vehicle ID, Color, Weight, Horsepower EMPLOYEE                   Employee ID, Employee Name, Payroll Address, Skill

R1 ⋈θ R2

Full Outer Join: (R⋈S):

Conventional file processing system	Database management system
1.      A computerized record-keeping system	1.      Set of individual programs and data handled by programmers.
2.      Data and Programs were interconnected and interdependent, change in one often required change in the other which obviously meant extra costs, time and resource wastage.	2.      Program and data are independent of each      other in Database management system. So the change one not required to change in other which meant no extra cost, time and resource wastage is required.
3.      As data about the same entity could be stored in different files / locations, it caused potential data inconsistency because often the data was not updated in all the different locations it was stored.	3.      Data is stored at one place so it does not                                   Caused data inconsistency.
4.      It does not provide multiple user interfaces	4.      It provide multiple user interfaces
5.      Data is not access easily because special programs needed to access data	5.      Data is easily accessed due to standard query procedures
6.      File system does not provide high security	6.      DBMS provide high security features
7.      It does not provide concurrent access of data to multiple users	7.      It provide concurrent access of data to multiple users
8.      It does not provide the ability to apply integrity constrains on data.	8.      It provide the ability to apply integrity constrains on data from field level to table
9.      It does not provide the facility to share data.	9.      It provide the facility to share data.

The Internal Level (Physical Representation of Data):