In full join all the records of both the table will come in resultant table , it doesn't depend on matching the fields . It is the combination of both left and right join . If matches not found then it will return null .Sometime it is also called Full Outer Join .
Syntax :

select * from table1 full outer join table2 on table1.column_name = table2.column_name ;

Example :
First Table name : Employee_Info

Employee_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

    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

Now to join these above two tables using Full join see below :

select Employee_Id , Employee_name , Student_name from Employee_Info full outer join student on Employee_Info.Employee_Id = student.Student_Id ;
 

Employee _Id       Employee_name                 Student_name 
        1            Mukesh                          Mukesh
        2            Ayush                           Null
        3            Ishan                           Ayush
        3            Null                            Ayush 
        4            Pranav                          Ishan
        4            Null                            Ishan    
        5            Abhishek                        Null
        6            Ravi                            Null
        7            David                           Null
        16           Null                            Pranav
        15           Null                            Abhishek 
        12           Null                            Ravi