Foreign Key is used to establish a link between two tables . It always define in reference of Primary key ( another table ) . It act as a Primary key in its own table and foreign key for another table .

Difference between Primary key and Foreign key in SQL :
1) Primary key can't be null while Foreign key can be null .
2) Primary key is always unique on other hand Foreign can be duplicate in a table .
3) Primary key can onlye be one in a table while foreign key can be more than one or more .

Example :
First Table name : Employee_Info

  Id        Employee_name          Employee_Age               Employee _Salary
  1            Mukesh                  23                           100000
  2            Ayush                   24                           200000
  3            Ishan                   20                           400000
  4            Pranav                  35                           700000
  5            Abhishek                26                           800000
  6            Ravi                    25                           300000
  7            David                   40                           800000

Second Table name : student

       Id    Student_name            Student_Age          Student_Id
       1        Mukesh                  23                    1
       3        Ayush                   24                    2
       4        Ishan                   20                    4
       16        Pranav                  35                   7
       15        Abhishek                26                   8
       12        Ravi                    25                   3

Note : In Employee_Info table " Id " is a primary key and in student table " Id " act as a Foreign key for Employee_Info table and for own table it act as a Primary key

Advantage :
It prevent from the action that destroy the link between the tables i.e we can make a link between the tables .
Syntax In MySQL for single column :

create table Employee_Info ( Id int not null , Employee_Name not null , Employee_Age , Employee_Salary , Primary key ( Employee_Id ) , Foreign key ( Id ) references student ( Id ) ) ;

Syntax In SQL/Oracle/MS Access for single column :

create table Employee_Info ( Id int not null Primary key , Employee_Name not null , Employee_Age , Employee_Salary ,  Foreign key references student ( Id ) ) ;

How to Alter Foreign key ?
Syntax In MySQL/SQL/Oracle/MS Access for single column :

Alter table Employee_Info add constraint fk_employeeId Foreign key ( Id ) references student ( Id ) ) ;

How to Drop Foreign key ?
Syntax In MySQL for single column :

Alter table Employee_Info ROP Foreign key fk_employeeId  ;

Syntax In SQL/Oracle/MS Access for single column :

Alter table Employee_Info drop constraint fk_employeeId  ;