Keys

KEYS
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?

ALTER TABLE Student DROP 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:
CREATE TABLE College( CollegeCode INT NOT NULL,
Student_ID INT references Student(RollNumber),
PRIMARY KEY (CollegeCode) );

How to delete a foreign key?

ALTER TABLE Student DROP FOREIGN KEY;

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