sql-server - inner - left outer join vs right outer join




LEFT JOIN vs. LEFT OUTER JOIN in SQL Server (11)

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


LEFT JOIN performs a join starting with the first (left-most) table and then any matching second (right-most) table records.

LEFT JOIN and LEFT OUTER JOIN are the same.

According to DoFactory


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


There are only 3 joins:

  • A) Cross Join = Cartesian (E.g: Table A, Table B)
  • B) Inner Join = JOIN (E.g: Table A Join/Inner Join Table B)
  • C) Outer join:

       There are three type of outer join
       1)  Left Outer Join     = Left Join
       2)  Right Outer Join    = Right Join
       3)  Full Outer Join     = Full Join    
    

Hope it'd help.


Left Join and Left Outer Join are one and the same. The former is the shorthand for the latter. The same can be said about the Right Join and Right Outer Join relationship. The demonstration will illustrate the equality. Working examples of each query have been provided via SQL Fiddle. This tool will allow for hands on manipulation of the query.

Given

Left Join and Left Outer Join

Results


Right Join and Right Outer Join

Results


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), and what this means in this case is that whether you specify it or not makes no difference. Note that while the other elements of the join clause is also marked as optional, leaving them out will of course 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?.


I find it easier to think of Joins in the following order:

  • CROSS JOIN - a Cartesian product of both tables. ALL joins begin here
  • INNER JOIN - a CROSS JOIN with a filter added.
  • OUTER JOIN - an INNER JOIN with missing elements (from either LEFT or RIGHT table) added afterward.

Until I figured out this (relatively) simple model, JOINS were always a bit more of a black art. Now they make perfect sense.

Hope this helps more than it confuses.


I'm a PostgreSQL DBA, as far as I could understand the difference between outer or not outer joins difference is a topic that has considerable discussion all around the internet. Until today I never saw a difference between those two; So I went further and I try to find the difference between those. At the end I read the whole documentation about it and I found the answer for this,

So if you look on documentation (at least in PostgreSQL) you can find this phrase:

"The words INNER and OUTER are optional in all forms. INNER is the default; LEFT, RIGHT, and FULL imply an outer join."

In another words,

LEFT JOIN and LEFT OUTER JOIN ARE THE SAME

RIGHT JOIN and RIGHT OUTER JOIN ARE THE SAME

I hope it can be a contribute for those who are still trying to find the answer.


Nothing to say in words beside this:


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


There are mainly three types of JOIN

  1. Inner: fetches data, that are present in both tables
    • Only JOIN means INNER JOIN
  2. Outer: are of three types

    • LEFT OUTER - - fetches data present only in left table & matching condition
    • RIGHT OUTER - - fetches data present only in right table & matching condition
    • FULL OUTER - - fetches data present any or both table
    • (LEFT or RIGHT or FULL) OUTER JOIN can be written w/o writing "OUTER"
  3. Cross Join: joins everything to everything


Why are LEFT/RIGHT and LEFT OUTER/RIGHT OUTER the same? Let's explain why this vocabulary. Understand that LEFT and RIGHT joins are specific cases of the OUTER join, and therefore couldn't be anything else than OUTER LEFT/OUTER RIGHT. The OUTER join is also called FULL OUTER as opposed to LEFT and RIGHT joins that are PARTIAL results of the OUTER join. Indeed:

Table A | Table B     Table A | Table B      Table A | Table B      Table A | Table B
   1    |   5            1    |   1             1    |   1             1    |   1
   2    |   1            2    |   2             2    |   2             2    |   2
   3    |   6            3    |  null           3    |  null           -    |   -
   4    |   2            4    |  null           4    |  null           -    |   -
                        null  |   5             -    |   -            null  |   5
                        null  |   6             -    |   -            null  |   6

                      OUTER JOIN (FULL)     LEFT OUTER (partial)   RIGHT OUTER (partial)

It is now clear why those operations have aliases, as well as it is clear only 3 cases exist: INNER, OUTER, CROSS. With two sub-cases for the OUTER. The vocabulary, the way teachers explain this, as well as some answers above, often make it looks like there are lots of different types of join. But it's actually very simple.





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