In SQL, what’s the difference between an inner and outer join?

 

Joins are used to combine the data from two tables, with the result being a new, temporary table. The temporary table is created based on column(s) that the two tables share, which represent meaningful column(s) of comparison. The goal is to extract meaningful data from the resulting temporary table. Joins are performed based on something called a predicate, which specifies the condition to use in order to perform a join. A join can be either an inner join or an outer join, depending on how one wants the resulting table to look.

It is best to illustrate the differences between inner and outer joins by use of an example. Here we have 2 tables that we will use for our example:

Employee Location
EmpID EmpName
13 Jason
8 Alex
3 Ram
17 Babu
25 Johnson
EmpID EmpLoc
13 San Jose
8 Los Angeles
3 Pune, India
17 Chennai, India
39 Bangalore, India

It’s important to note that the very last row in the Employee table does not exist in the Employee Location table. Also, the very last row in the Employee Location table does not exist in the Employee table. These facts will prove to be significant in the discussion that follows.

Outer Joins

Let’s start the explanation with outer joins. Outer joins can be be further divided into left outer joins, right outer joins, and full outer joins. Here is what the SQL for a left outer join would look like, using the tables above:

select * from employee left outer join location 
on employee.empID = location.empID;

Subscribe to our newsletter for more free interview questions.

In this SQL we are joining on the condition that the employee ID’s match in the rows tables. So, we will be essentially combining 2 tables into 1, based on the condition that the employee ID’s match. Note that we can get rid of the "outer" in left outer join, which will give us the SQL below. This is equivalent to what we have above.

select * from employee left join location 
on employee.empID = location.empID;

A left outer join retains all of the rows of the left table, regardless of whether there is a row that matches on the right table. The SQL above will give us the result set shown below.

Employee.EmpID Employee.EmpName Location.EmpID Location.EmpLoc
13 Jason 13 San Jose
8 Alex 8 Los Angeles
3 Ram 3 Pune, India
17 Babu 17 Chennai, India
25 Johnson NULL NULL

The Join Predicate – a geeky term you should know

Earlier we had mentioned something called a join predicate. In the SQL above, the join predicate is "on employee.empID = location.empID". This is the heart of any type of join, because it determines what common column between the 2 tables will be used to "join" the 2 tables. As you can see from the result set, all of the rows from the left table are returned when we do a left outer join. The last row of the Employee table (which contains the "Johson" entry) is displayed in the results even though there is no matching row in the Location table. As you can see, the non-matching columns in the last row are filled with a "NULL". So, we have "NULL" as the entry wherever there is no match.

A right outer join is pretty much the same thing as a left outer join, except that the rows that are retained are from the right table. This is what the SQL looks like:

select * from employee right outer join location 
on employee.empID = location.empID;

// taking out the "outer", this also works:

select * from employee right join location 
on employee.empID = location.empID;

Using the tables presented above, we can show what the result set of a right outer join would look like:

Employee.EmpID Employee.EmpName Location.EmpID Location.EmpLoc
13 Jason 13 San Jose
8 Alex 8 Los Angeles
3 Ram 3 Pune, India
17 Babu 17 Chennai, India
NULL NULL 39 Bangalore, India

We can see that the last row returned in the result set contains the row that was in the Location table, but not in the Employee table (the "Bangalore, India" entry). Because there is no matching row in the Employee table that has an employee ID of "39", we have NULL’s in the result set for the Employee columns.

Inner Joins

Now that we’ve gone over outer joins, we can contrast those with the inner join. The difference between an inner join and an outer join is that an inner join will return only the rows that actually match based on the join predicate. Once again, this is best illustrated via an example. Here’s what the SQL for an inner join will look like:

select * from employee inner join location on 
employee.empID = location.empID

This can also be written as:

select * from employee, location
where employee.empID = location.empID
              

Now, here is what the result of running that SQL would look like:

Employee.EmpID Employee.EmpName Location.EmpID Location.EmpLoc
13 Jason 13 San Jose
8 Alex 8 Los Angeles
3 Ram 3 Pune, India
17 Babu 17 Chennai, India

Inner vs Outer Joins

We can see that an inner join will only return rows in which there is a match based on the join predicate. In this case, what that means is anytime the Employee and Location table share an Employee ID, a row will be generated in the results to show the match. Looking at the original tables, one can see that those Employee ID’s that are shared by those tables are displayed in the results. But, with a left or right outer join, the result set will retain all of the rows from either the left or right table.

Hiring? Job Hunting? Post a JOB or your RESUME on our JOB BOARD >>

Subscribe to our newsletter for more free interview questions.



FOLLOW Varoon Sahgal, Author of ProgrammerInterviewon

  • dixitsingla

    Best explanation

  • Shivam Bansal

    This site is awesome…

  • Akash

    I love this explanation,,It seems the author got the crux of sql..

  • rd

    simple and precise explanation

  • anonymus

    really ur site s so awesome…as u give a simple and neat explanation..i wish u post more in networking and operating systems..

  • apurva

    osum work :)

  • Vuyani Billy Nyathikazi

    WOW…I LIKE IT

  • Anirudh

    Thanks

  • vicky

    what about full outer join ???

    • anas

      full outer join gives both common and uncommon result from the tables…

  • Hegel

    This is good. Thank you.

  • Gixx

    On the DOT… Dude

  • Aamir Sohail

    you are awesome bddy.. post more about hints, execution plan table's data(most confusing) also write some topics for performance tuning…. plzz

  • Fida Husain

    really very good Explanation about join

  • Richa

    Thorough Explanation :) Thank u so much for other concepts as well!

  • GLKC

    Finally, clear and simple conceptual explanations that are easy to understand! Thank You!

  • Vaibhav Tikoo

    The way the explanation is given is just awesome…Thanks for sharing

  • Anwesh

    I have gone through many refernce material to understand Outer join, but i never understood…This is by far the best and the simplest explanation possible….Thanks a ton DUDU

  • neethu

    awsome explanation

  • Ravivarman

    Very nice explanation.!! Thanks very much..!!

  • Bharat Reddy

    really am struggling with joins Concept..
    now, i will roar cause i clearly understood .

  • Prashant

    Really very good and useful explanation….Thanks

  • Nishant

    Cleared concepts in lucid way

  • Zelinskey

    Really amazing explanation. Better than all other sites (almost > 20) which I referred. Read all the comments below and found none that doesn't appreciate you.

  • Edwin Bajwahuka

    yes, it is amazing