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Inner and outer joins SQL examples and the Join block

join-block-venn-diagram-datamartist
In this post I'll show you how to do all the main types of Joins with clear SQL examples. The examples are written for Microsoft SQL Server, but very similar syntax is used in Oracle, MySQL and other databases.

Joins can be said to be INNER or OUTER joins, and the two tables involved are referred to as LEFT and RIGHT. By combining these two concepts you get all the various types of joins in join land: Inner, left outer, right outer, and the full outer join.

Tables used for SQL Examples

Join-Example-Students-And-Advisors-Tables

In the screen shots I've configured Datamartist to only show the name columns to save space. The SQL code shown is "Select *" so it will return all the columns. You can see that in the Datamartist tool the type of join is selected by just checking the parts of the venn diagram that contain the rows you want.

1) Inner Join SQL Example

select * from dbo.Students S INNER JOIN dbo.Advisors A ON S.Advisor_ID=A.Advisor_ID

Join-Example-Students-And-Advisors-Inner-Join

2) Left Outer Join SQL Example

select * from dbo.Students S LEFT OUTER JOIN dbo.Advisors A ON S.Advisor_ID=A.Advisor_ID

Join-Example-Students-And-Advisors-Left-Outer-Join

4) Full Outer Join SQL Example

select * from dbo.Students S FULL OUTER JOIN dbo.Advisors A ON S.Advisor_ID=A.Advisor_ID
Join-Example-Students-And-Advisors-Full-Outer-Join

5) SQL example for just getting the rows that don't join

select * from dbo.Students S FULL OUTER JOIN dbo.Advisors A ON S.Advisor_ID=A.Advisor_ID where A.Advisor_ID is null or S.Student_ID is null
Join-Example-Students-And-Advisors-non-joining-Join

6) SQL example for just rows from one table that don't join

select * from dbo.Students S FULL OUTER JOIN dbo.Advisors A ON S.Advisor_ID=A.Advisor_ID where A.Advisor_ID is null
Join-Example-Students-And-Advisors-left-exlusive-Join

But what about the duplicate row thing?

Now, since in this case we had a simple one to one relationship, the number of rows that were returned made the venn diagrams make sense, and add up pretty normally with table one and two.

What happens if the data in the tables are not a simple one to one relationship? What happens if we add one duplicate advisor with the same ID, but a different name?
Join-Example-Students-And-Advisors-duplicate-advisors

A join will create a row for every combination of rows that join together. So if there are two advisors with the same key, for every student record that has that key, you will have two rows in the inner part of the join. The advisor duplicate makes duplicate student records for every student with that advisor.

You can see how this could add up to a lot of extra rows. The number of rows is the product of the two sets of joining rows. If the tables get big, just a few duplicates will cause the results of a join to be much larger than the total number of rows in the input tables- this is something you have to watch very carefully when joining- check your row counts.

So there you have it. If you want to try joining tables with the Datamartist tool- give it a try. It's a super fast install, and you'll be joining like a pro in no time.

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35 Comments

  1. thanks a lot
    very nice examples with clarity data

  2. this is the very cleared description of joins...
    thanks

  3. thanks a lot
    very nice examples with clarity data

  4. Crystal clear explanations with diagrams & ex's, really helped a lot. Thank You.

  5. Thanks... understandable article .

  6. Very nice and clear tutorial.

  7. Great article. Thanks a lot for all these clear explanations that made it easy to understand how to join multiple table with SQL.

  8. looks very nice and clear tutorial !!! many thanks for making me easier understanding on "Join" !!!! Keep up great works..............

  9. Awesome! Thanks a lot!

  10. hi,

    What a great example. My doubts are cleared after long time..!!!!!!!

    Thanks

  11. Good Example.

  12. Great article! Very clear and concise!

    Thank you very much!

  13. The concept has been very neatly explained with diagrams and examples which makes it all the more easier to understand. Nice Job!

    Thanks a lot!

  14. This is a very good way to clarity the Join concept.
    Anybody can easily understand it.
    Thanks a lot.

  15. This is the best website I had seen that clearly pin points the difference between the Joins.
    Thanks a Lot for your effort to create this

  16. * Sql Introduction
    * Using Stored Procedures
    * sql queries
    * groupby functions etc
    * Joins

    Complete explanation

    Link here for Sql Tutorial
    http://geeksprogrammings.blogspot.in/search/label/StructuredQueryLanguage%28SQL%29

    Link here for joins explanation
    geeksprogrammings.blogspot.in/2013/06/joins-in-sql.html

  17. thanks to guide me...............................

  18. Good Example...
    Thanks

  19. Thanks for Your Higher Education

  20. good examples when compare to other websites Keep it up!!!!!

  21. Excellent explantion with examples which helped me a lot..

  22. Very nice example of join

  23. Missing right outer join example. point 3)

  24. Thanks , Nice examples

  25. Thanks you very much, very nice tutorial

  26. This also the best tutorial on Join that I have seen. The best explanation is the concept on types of join, and using the two tables, left and right.

    Thanks

  27. This piece of writing is truly a nice one it helps new internet people,
    who are wishing in favor of blogging.

  28. This article is one of the best examples i've seen on this topic.

    Thank you !

  29. good explanation,

  30. Nice explanation

  31. What will be the output in case we try Right Outer join? (right table being Advisors)

  32. 6) SQL example for just rows from one table that don't join

    select * from dbo.Students S FULL OUTER JOIN dbo.Advisors A ON S.Advisor_ID=A.Advisor_ID where A.Advisor_ID is null

    I think this wouid be
    select * from dbo.Students S FULL OUTER JOIN dbo.Advisors A ON S.Advisor_ID=A.Advisor_ID where A.Advisor_Name is null

  33. thanks for the wonderful implimentation of the join...

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