FOREIGN KEY:

• Foreign Key is a column or a combination of columns whose values match a Primary Key in a different table.
• It acts as a cross-reference between tables because it references the primary key of another table, thereby establishing a link between them.
• For example, a table called Player has a primary key called Player_ID. Another table called PlayerDetails has a foreign key which references Player_ID in order to uniquely identify the relationship between both the tables.
• A FOREIGN KEY in one table points to a PRIMARY KEY in another table.
• FOREIGN KEY Constraint on CREATE TABLE

• The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is created. CREATE TABLE Orders

  (O_Id int PRIMARY KEY,

OrderNo int NOT NULL,

P_Id int REFERENCES Persons (P_Id));

The "Persons" table:

The "Orders" table:

Foreign Key:

• Primary key of one relation is used as an attribute in another relation is called as foreign key.

• It is a part of referential integrity where one relation is referenced in another relation.

• Consider two relations as:

emp(empNo, EN, FN, email)

Dept(DeptNo, Name, Location)

• To establish a relation between emp and Dept, one may have to refer the primary key of Dept in emp.

• So now new relation can be written as:

emp (empNo, FN, LN, email, DNo)

• Here 'DNo' is a foreign key.

• This referential integrity can be shown in a diagramatic way as:

• Foreign key also helps in retrieving related information from two related tables.

• For eg. one may retrieve the department of any employee using foreign ey 'Dno'