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  • Multiple Joins Work just like Single Joins

    Before reading this article, you should have a good understanding of single joins between two tables and be clear on the differences between inner and outer joins. Check out my previous post A Primer on Joins to help you accomplish this.

    Have you ever looked at a query like the one below and wondered how to read it, how the different joins work together, and what aliens on what planet wrote such a thing?

    001-Multiple-Joins-Work-just-like-Single-Joins

    Queries with multiple joins like this one often lead to confusion, such as the one behind this question that I have often heard from students: “There seems to be three tables joined to the Employee table in this query—two are inner joins and the other is an outer join. How can the same table have its non-matching rows eliminated and preserved at the same time in the same query?”

    In this article, I will show you that confusions like this one arise from a syntax that encourages us to misunderstand joins, and I’ll offer another way of looking at multiple-join queries that makes questions like the one above melt away.

    So, what leads to the confusion? Initially, it might seem that every table that is joined to this query is joined to a previous table; the ON clause suggests this with its references to columns in previous tables.

    002-ON-clause-Multiple-Joins-Work-just-like-Single-Joins

    But this is not what actually happens in a multi-join query, and so looking at things in this way will lead to head-scratching.

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    So, what does a multi-join query actually do? It actually does something very simple. It performs a series of incremental, single joins between two tables at a time (while this article refers only to tables for simplicity sake, joins can be between tables, views, table valued functions, CTEs, and derived table subqueries). Each single join produces a single derived table (DT) that is then joined to the next table and so on. Like this:

    multiple-joins-work-like-single-joins

    JOIN 1: Inner join between Employee and Contact resulting in a derived table, DT1. Because this is an inner join, rows in Employee are excluded if they don’t match any rows in Contact, and vice-versa.

    JOIN 2: Outer join between DT1 and JobCandidate resulting in a derived table, DT2. Because this is a left outer join, all rows in DT1 are preserved.

    JOIN 3: Inner join between DT2 and SalesPerson resulting in a derived table, DT3. Because this is an inner join, rows in DT2 are excluded if they don’t match any rows in SalesPerson, and vice-versa.

    JOIN 4: Outer join between DT3 and SalesOrderHeader resulting in a derived table, DT4. Because this is a left outer join, all rows in DT3 are preserved.

    JOIN 5: Outer join between DT4 and SalesTerritory resulting in a derived table, DT5. Because this is a left outer join, all rows in DT4 are preserved. DT5 is the final result of the query.

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    So, what about that confusion arising from the ON clause? With this new way of looking at multiple-join queries, we can now see that the proper way to read an ON clause is not that it joins the new table to a single table that came before it in the query! The only join that does that is the first one; all subsequent ones join a new table to the derived table that is a result of all the joins before it. If an ON clause includes a table alias, that is only to identify the column properly to the query. Table aliases are only required when there is ambiguity—when two or more columns have the same name in the derived table that precedes the current join because they came from different tables.

    004-derived-table-DT-Multiple-Joins-Work-just-like-Single-Joins

    I hope this new way of looking at multiple-join queries helps make it easier and more productive for you to work with joins.

    Have fun!

    Peter Avila
    SQL Server Instructor – Interface Technical Training
    Phoenix, AZ

    See what people are saying...

    1. Nipun Kumar

      Excellent article. Saved me from lots of confusion

    2. Petro

      Thanks. Excellent explanation!

    3. John

      Thanks from Warner Robins, Georgia. This really cleared some things up for me.

    4. vadivel murthy

      execellent explanation… thank you so much

    5. Karthikeyan N

      Perfect explanation.. I kept scratching my head about multiple joins until I came across this…

    6. Saumya Ranjan

      Thanks for the to-the-point explanation! Precise and Perfect, especially the pictures. Great work!!

    7. Akhilesh

      Thank you !!!!

    8. Mahesh khatai

      Thanx for the article . Helped me a lot in understanding the multiple table join .

    9. pradip

      Thanks for the good article.

      But I think there is confusion in Derived table image,
      DT1 should be in blue arrow instead of orange color.

      For reference, I am attaching image.

      https://drive.google.com/file/d/0BwvNpXimvpg3M1Z5TnhLdnB1UkE/view?usp=drivesdk

    10. Mbuku Ditutala

      I love people that make others’ lives easy and that’s the case. Thanks for clearing our doubts.

    11. Neil

      As someone new to SQL and just starting to need to join multiple tables, this is exactly what I needed. Especially the picture. Thank you very much for the clarity!

    12. David Aspden

      Awesome guide, thank you. It really makes it clear what happens when joining multiple tables. In regards to a “where” clause, this clause I think just limits the final result set by eliminating those records that don’t match?

    13. Debra

      Thank you very much! The illustration really helped 🙂

    14. Alex

      great!!! It’s easier to understand now how joins works!!!

    15. Pete Dunham

      Very good explanation! Many thanks.

    16. Neeraj Sehtya

      Thank you very much for sharing such a brief yet descriptive paper on joins. I had always wanted to know the modus-operandi of joins and this is the best resource that I have seen to explain it. Going to share it further with people who are struggling to understand this.

    17. Abebe

      Thank you very much. I had a big confusion how multiple join works. Now, it is very clear to me.

    18. Peter Avila

      Hi, Alexis. The result of a join is always a single virtual table. But so is the result of a query with no joins and only a single table! It doesn’t matter how many tables you have in your query—1 or many joined together—the result will always be a single virtual table with all of the columns from all of the tables. The filter you create in your WHERE clause can reference any one of those columns to filter out data from the virtual table. So, the affect that a WHERE clause has on a join is the same as it has on a single table: it filters out rows of the resulting virtual table—whether that virtual table is derived from a single table or a multitude of joins—that don’t pass the condition in the WHERE clause.

      I hope this answers your question. Please let me know if it doesn’t.

      Regards,

      Peter Avila

    19. Alexis

      what happens when we add at the end of that query a where clause ! How the where affect the join operations between tables!? Thanks!

    20. Sridhar

      Many thanks!This topic helped me very much in clearing the confusion in multiple type joins in one query.Thanks agiain.
      Sridhar.M(India)

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