LEFT JOIN vs. LEFT OUTER JOIN in SQL Server

Question

What is the difference between LEFT JOIN and LEFT OUTER JOIN?

Answer 1

To answer your question there is no difference between LEFT JOIN and LEFT OUTER JOIN, they are exactly same that said...

At the top level there are mainly 3 types of joins:

1. INNER
2. OUTER
3. CROSS

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1. INNER JOIN - fetches data if present in both the tables.

2. OUTER JOIN are of 3 types:

   1. LEFT OUTER JOIN - fetches data if present in the left table.
   2. RIGHT OUTER JOIN - fetches data if present in the right table.
   3. FULL OUTER JOIN - fetches data if present in either of the two tables.

3. CROSS JOIN, as the name suggests, does [n X m] that joins everything to everything.
   Similar to scenario where we simply lists the tables for joining (in the FROM clause of the SELECT statement), using commas to separate them.

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Points to be noted:

• If you just mention JOIN then by default it is a INNER JOIN.
• An OUTER join has to be LEFT | RIGHT | FULL you can not simply say OUTER JOIN.
• You can drop OUTER keyword and just say LEFT JOIN or RIGHT JOIN or FULL JOIN.

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For those who want to visualise these in a better way, please go to this link: A Visual Explanation of SQL Joins

Answer 2

As per the documentation: FROM (Transact-SQL):

<join_type> ::=      [ { INNER | { { LEFT | RIGHT | FULL } [ OUTER ] } } [ <join_hint> ] ]     JOIN 

The keyword OUTER is marked as optional (enclosed in square brackets). In this specific case, whether you specify OUTER or not makes no difference. Note that while the other elements of the join clause is also marked as optional, leaving them out will make a difference.

For instance, the entire type-part of the JOIN clause is optional, in which case the default is INNER if you just specify JOIN. In other words, this is legal:

SELECT * FROM A JOIN B ON A.X = B.Y 

Here's a list of equivalent syntaxes:

A LEFT JOIN B            A LEFT OUTER JOIN B A RIGHT JOIN B           A RIGHT OUTER JOIN B A FULL JOIN B            A FULL OUTER JOIN B A INNER JOIN B           A JOIN B 

Also take a look at the answer I left on this other SO question: SQL left join vs multiple tables on FROM line?.