Visualizing Merge Join Types in Power BI

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Over the last couple of years, I have been actively involved in doing Power BI trainings for my clients. Because of that, I am always looking for new and easier ways to explain Power BI concepts to my attendees. Yesterday, I saw a blog post from Reza Rad on Merge Types in Power BI and realized that this is one concept I always have to explain by drawing on a whiteboard during my training sessions. That is when I started thinking – maybe I could create a Power BI report to explain the join types while merging queries. Being able to click through the different join types and seeing the results would definitely make it more easier to understand than me drawing or just talking talking about it. Also, to make it more useful, I wanted to keep the ability to add / modify / delete records in the two tables so that my attendees could see in real time how it will affect the resultant merged tables.

For demonstrating the solution, I made use of 2 simple tables: Table A consists of Customer ID and Customer Name, Table B consists of Customer ID and Email. We will be merging both of these tables using the Customer ID column. In the report, you can see Table A is on the left side, Table B is on the right side and the resultant merged table is on the bottom. We also have a slicer on the top to choose the Join Type, and just under it, we have a description for the Join Type as well as a Venn Diagram. For both Table A and Table B, we have a IsJoined column which denotes whether the corresponding row is present in the Merged table for the selected Join Type. I have embedded the report below, feel free to click and see for yourself (or click on this link to see the full page view).

* The initial version of the report had only the 6 join types available out of the box in Power BI. After sharing this version of the report on Twitter, Imke Feldmann (t | b) said that it would be nice to display the Full Anti Join also, which is not available by default but can be easily added with the help of simple M code (something along the lines of Table.Combine({LeftAntiJoin, RightAntiJoin})). So I added that to the latest version of the report, along with a message that it requires custom code when the Full Anti Join option is selected.‏

Now, if you are reading this, most probably you are interested in learning how this was done. To be honest, this report ended up being a little more tricky than I thought it would be and it has some hidden tips and tricks.

– Table A and Table B data is entered through Power BI. So you can add more records and see how the merged table would look like for the changed data. For eg, what happens when I have duplicate customer ids and how will it affect my merged table?

– How does the slicer have images under it? – Chiclet slicer

– How did I make the Venn Diagram to change based on the filter selection? – Use Synoptic Designer to create the Venn diagram, and then link it to your dataset using a DAX measure that will highlight the appropriate area.

– How does the IsJoined column show whether the source table’s record is present in the merged table? – DAX Measure

– How do I display the right results in the Merged table? – A combination of using Power Query and DAX measures.

I know I have answered the questions only on a high level, but if there is enough interest in knowing more about this, please let me know and I might end up writing a follow up for this post detailing all the techniques. Let me know if you have any further questions apart from what I have listed or feedback / bug reports on the same, and I will try to accommodate / fix it as much as I can. Meanwhile, feel free to download the report and play with it yourself. Also, you know where to point the next time someone asks you on the different join types in Merge Queries within Power BI.

I have also published this report to the Power BI Community Data Stories Gallery. Feel free to comment / like / simply interact with the other reports and users out there.

Update

The report has been updated with a second page explaining the join types using Join Diagrams. The join diagrams are inspired from this post – https://blog.jooq.org/2016/07/05/say-no-to-venn-diagrams-when-explaining-joins/ and was pointed out to me on twitter by @thesqlgrrrl.