Database Keys :
Keys are very important part of Relational database.
They are used to establish and identify relation between tables.

They also ensure that each record within a table can be uniquely identified by combination of one or more fields within a table.
  1. Super Key :Super Key is defined as a set of attributes within a table that uniquely identifies each record within a table. Super Key is a superset of Candidate key.
  2. Candidate keysCandidate Keys  are defined as the set of fields from which primary key can be selected. It is an attribute or set of attribute that can act as a primary key for a table to uniquely identify each record in that table.
  3. Primary key: Primary Key is a candidate key that is most appropriate to become main key of the table. It is a key that uniquely identify each record in a table.
  4. Composite key : Key that consist of two or more attributes that uniquely identify an entity occurance is called Composite key. But any attribute that makes up the Composite key is not a simple key in its own.
  5. Secondary keys or alternative keys : The candidate key which are not selected for primary key are known as secondary keys or alternative keys.
  6. Foreign key: An attribute (or combination of attributes) in one table whose values must either match the primary key in another table or be null.
Primary Key:

A primary key is a field in a table which uniquely identifies each row in a table. 
Primary keys must contain unique values. 
A primary key column cannot have NULL values. 
A table can have only one primary key, which may consist of single or multiple fields.

How to create a primary key?

Use attribute primary key to make key primary key.
For example:
CREATE TABLE Student( RollNumber INT, NAME VARCHAR (20), 
PRIMARY KEY (RollNumber) );

How to delete a primary key?


Foreign Key:

A foreign key is a key used to link two tables together. This is sometimes also called as a referencing key.

Foreign key matches a primary key in another table.

How to create a foreign key?

Use reference keyword to make a key foreign key.
For example:
Student_ID INT references Student(RollNumber),
PRIMARY KEY (CollegeCode) );

How to delete a foreign key?


More topics from DBMS to read: covered following topics in these notes.
Python Programming ↓ 👆
Java Programming ↓ 👆
JAVA covered following topics in these notes.
JAVA Programs
Principles of Programming Languages ↓ 👆
Principles of Programming Languages covered following topics in these notes.

Previous years solved papers:
A list of Video lectures References:
  1. Sebesta,”Concept of programming Language”, Pearson Edu 
  2. Louden, “Programming Languages: Principles & Practices” , Cengage Learning 
  3. Tucker, “Programming Languages: Principles and paradigms “, Tata McGraw –Hill. 
  4. E Horowitz, "Programming Languages", 2nd Edition, Addison Wesley

    Computer Organization and Architecture ↓ 👆

    Computer Organization and Architecture covered following topics in these notes.

    1. Structure of desktop computers
    2. Logic gates
    3. Register organization
    4. Bus structure
    5. Addressing modes
    6. Register transfer language
    7. Direct mapping numericals
    8. Register in Assembly Language Programming
    9. Arrays in Assembly Language Programming


    1. William stalling ,“Computer Architecture and Organization” PHI
    2. Morris Mano , “Computer System Organization ”PHI

    Computer Network ↓ 👆
    Computer Network covered following topics in these notes.
    1. Data Link Layer
    2. Framing
    3. Byte count framing method
    4. Flag bytes with byte stuffing framing method
    5. Flag bits with bit stuffing framing method
    6. Physical layer coding violations framing method
    7. Error control in data link layer
    8. Stop and Wait scheme
    9. Sliding Window Protocol
    10. One bit sliding window protocol
    11. A protocol Using Go-Back-N
    12. Selective repeat protocol
    13. Application layer
    1. Andrew S. Tanenbaum, David J. Wetherall, “Computer Networks” Pearson Education.
    2. Douglas E Comer, “Internetworking with TCP/IP Principles, Protocols, And Architecture",Pearson Education
    3. KavehPahlavan, Prashant Krishnamurthy, “Networking Fundamentals”, Wiley Publication.
    4. Ying-Dar Lin, Ren-Hung Hwang, Fred Baker, “Computer Networks: An Open Source Approach”, McGraw Hill.