IF WE WANT TO GET THE DATA FROM SOME TABLES WHICH HAS SOME 
RELATIONS BETWEEN THOSE THEN WE USE JOINS TO GET THE DATA 
BY USING THOSE PRIMARY KEYS.

THERE ARE 2 TYPES OF JOINS, INNER AND OUTER JOINS.

INNER IS USED TO GET THE DATA WHERE THE TABLES ARE IN SAME 
DATABASE AND OUTER JOIN IF THEY ARE IN OTHER.

Joins are 2 types. INNER JOIN, OUTER JOIN.

Inner Join:
In case of inner join there should be an entry in all the 
tables use in the view. 

Outer Join:
With the use of outer join you can join the tables even 
there is no entry in all the tables used in the view.

Joins are used to extract data from more than one DB table
which are logically related to each other.
So in order to join DB Tables they should have a common
field or attribute.
In ABAP, We have two types of Joins.
1) Inner Joins
2) Outer Joins ( Left Outer and Right Outer)

Inner Joins retrieve the records which matched with join
condition only.
While outer joins Retrieve the records which are matched
with Join Condition and Remaining records from the Parent Table.