Difference Between "INNER JOIN" and "OUTER JOIN"

What is the difference between "INNER JOIN" and "OUTER JOIN"?

Assuming you're joining on columns with no duplicates, which is a very common case:

• 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.

Examples

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

Right outer join

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

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

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

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

What is difference between INNER join and OUTER join

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.

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

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.
check this

SQL: JOIN vs LEFT OUTER JOIN?

As previously noted above:

JOIN is synonym of INNER JOIN. It's definitively different from all
types of OUTER JOIN

So the question is "When should I use an outer join?"

Here's a good article, with several great diagrams:

https://www.sqlshack.com/sql-outer-join-overview-and-examples/

The short answer your your question is:

• Prefer JOIN (aka "INNER JOIN") to link two related tables. In practice, you'll use INNER JOIN most of the time.
• INNER JOIN is the intersection of the two tables. It's represented by the "green" section in the middle of the Venn diagram above.
• Use an "Outer Join" when you want the left, right or both outer regions.
• In your example, the result set happens to be the same: the two expressions happen to be equivalent.
• ALSO: be sure to familiarize yourself with "Show Plan" (or equivalent) for your RDBMS: https://www.sqlshack.com/execution-plans-in-sql-server/

'Hope that helps...

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?

Reading this original article on The Code Project will help you a lot: Visual Representation of SQL Joins.

Also check this post: SQL SERVER – Better Performance – LEFT JOIN or NOT IN?.

Find original one at: Difference between JOIN and OUTER JOIN in MySQL.

What is the difference between "INNER JOIN" and "OUTER JOIN"?

Assuming you're joining on columns with no duplicates, which is a very common case:

• 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.

Examples

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

Right outer join

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

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

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

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

Why is inner join and outer join so called?

One more perspective:

One of the earliest simple implementations for joins used nested loops.

1. For inner join, the outer loop would iterate over any relation and the inner loop would iterate over the other relation and create composite rows whenever join columns matched. Thus the output rows get created and populated in the inner loop. Hence this is called INNER JOIN.

2. When we want all rows in left side relation\table to be retained, the outer loop will have to iterate on the left table and rows would be added not only in the inner loop for matching cases but also in the outer loop for non-matching cases(where left table doesn't have a matching row in right table based on join columns). In this case, the left table needs to go to the outer loop, so it is called LEFT OUTER JOIN.

3. When we want all rows in right side relation\table to be retained, right table will need to go into outer loop, so it is called RIGHT OUTER JOIN.

4. When we want non matching rows of both tables to be retained, in the simplest approach, we would have two nested loops. One nested loop would have left table in the outer loop and the other nested loop would have right table in the outer loop. So both tables go to outer loop, hence it is called FULL OUTER JOIN.

Adding the link to the paper that talks about nested loop implementation : http://www.cs.berkeley.edu/~brewer/cs262/3-selinger79.pdf

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