If you have 2 tables like below:
Table1 : A1 B1 Table2 : B2 C2
- - - -
1 2 1 1
2 4 2 4
3 5 5 2
If you use Inner Join you get:
A1 B1 B2 C2
- - - -
1 2 2 4
3 5 5 2
If you use Full Outer Join you get:
A1 B1 B2 C2
- - - -
1 2 2 4
3 5 5 2
2 4 NULL NULL
NULL NULL 1 1
If you use Left Outer Join you get:
A1 B1 B2 C2
- - - -
1 2 2 4
3 5 5 2
2 4 NULL NULL