LEFT JOIN vs. LEFT OUTER JOIN in SQL Server

KG_Sosa_48581 0
Viewed 1926809

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

outer-join left-join tsql sql-server
12 Answers

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?.

user_2670

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

  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.

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.

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

user_4163690

What is the difference between left join and left outer join?

Nothing. LEFT JOIN and LEFT OUTER JOIN are equivalent.

user_160760

Syntactic sugar, makes it more obvious to the casual reader that the join isn't an inner one.

Unsliced_29020

Just in the context of this question, I want to post the 2 'APPLY' operators as well:

JOINS:

  1. INNER JOIN = JOIN

  2. OUTER JOIN

    • LEFT OUTER JOIN = LEFT JOIN

    • RIGHT OUTER JOIN = RIGHT JOIN

    • FULL OUTER JOIN = FULL JOIN

  3. CROSS JOIN

SELF-JOIN: This is not exactly a separate type of join. This is basically joining a table to itself using one of the above joins. But I felt it is worth mentioning in the context JOIN discussions as you will hear this term from many in the SQL Developer community.

APPLY:

  1. CROSS APPLY -- Similar to INNER JOIN (But has added advantage of being able to compute something in the Right table for each row of the Left table and would return only the matching rows)
  2. OUTER APPLY -- Similar to LEFT OUTER JOIN (But has added advantage of being able to compute something in the Right table for each row of the Left table and would return all the rows from the Left table irrespective of a match on the Right table)

https://www.mssqltips.com/sqlservertip/1958/sql-server-cross-apply-and-outer-apply/

https://sqlhints.com/2016/10/23/outer-apply-in-sql-server/

Real life example, when to use OUTER / CROSS APPLY in SQL

I find APPLY operator very beneficial as they give better performance than having to do the same computation in a subquery. They are also replacement of many Analytical functions in older versions of SQL Server. That is why I believe that after being comfortable with JOINS, one SQL developer should try to learn the APPLY operators next.

user_106560540
Related