Inner join Vs Outer join

This is most common interview question for many developers ... I would like to give a easy understanding of this to remember for life ...

•     An inner join of A and B gives the result of A intersect B, i.e. the inner part of a venn diagram intersection.
•     An outer join of A and B gives the results of A union B, i.e. the outer parts of a venn diagram union.

Suppose you have two Tables, with a single column each, and data as follows:

A    B
-    -
1    3
2    4
3    5
4    6

Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.

Inner join

An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.

select * from a INNER JOIN b on a.a = b.b;
select a.*,b.*  from a,b where a.a = b.b;

a | b
--+--
3 | 3
4 | 4

Left outer join

A left outer join will give all rows in A, plus any common rows in B.

select * from a LEFT OUTER JOIN b on a.a = b.b;
select a.*,b.*  from a,b where a.a = b.b(+);

a |  b 
--+-----
1 | null
2 | null
3 |    3
4 |    4

Full outer join

A full outer join will give you the union of A and B, i.e. All the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.

select * from a FULL OUTER JOIN b on a.a = b.b;

 a   |  b 
-----+-----
   1 | null
   2 | null
   3 |    3
   4 |    4
null |    6
null |    5

Inner join

Full Outer join

Reference : [1]

Do you like this article? Share It!