You probably know that, when you are importing data from multiple tables in SQL Server into the Excel Data Model in Excel 2013 using Power Query, Power Query will automatically create relationships between those tables in the Data Model. But did you know that you can get Power Query to do this for other data sources too?
Now wait – don’t get excited. I’ve known about this for a while but not blogged about it because I don’t think it works all that well. You have to follow some very precise steps to make it happen and even then there are some problems. That said, I think we’re stuck with the current behaviour (at least for the time being) so I thought I might as well document it.
Consider the following Excel worksheet with two tables in it, called Dimension and Fact:
If you were to load these two tables into the Excel Data Model, you would probably want to create a relationship between the two tables based on the FruitID column. Here are the steps to use Power Query to create the relationship automatically:
- Click inside the Dimension table and then, on the Power Query tab in the Excel ribbon, click the From Table button to create a new query.
- When the Query Editor window opens, right click on the FruitID column and select Remove Duplicates.
Why are we doing this when there clearly aren’t any duplicate values in this column? The new step contains the expression
…and one of the side-effects of using Table.Distinct() is that it adds a primary key to the table. Yes, tables in Power Query can have primary keys – the Table.AddKey() function is another way of doing this. There’s a bit more information on this subject in my Power Query book, which I hope you have all bought!
- Click the Close & Load to.. button to close the Query Editor, and then choose the Only Create Connection option to make sure the output of the query is not loaded anywhere and the query is disabled, then click the Load button. (Am I the only person that doesn’t like this new dialog? I thought the old checkboxes were much simpler, although I do appreciate the new flexibility on where to put your Excel table output)
- Click inside the Fact table in the worksheet, click the From Table button again and this time do load it into the Data Model.
- Next, in the Power Query tab in the Excel ribbon, click the Merge button. In the Merge dialog select Dimension as the first table, Fact as the second, and in both select the FruitID column to join on.
- Click OK and the Query Editor window opens again. Click the Close & Load to.. button again, and load this new table into the Data Model.
- Open the Power Pivot window and you will see that not only have your two tables been loaded into the Data Model, but a relationship has been created between the two:
What are the problems I talked about then? Well, for a start, if you don’t follow these instructions exactly then you won’t get the relationship created – it is much harder than I would like. There may be other ways to make sure the relationships are created but I haven’t found them yet (if you do know of an easier way, please leave a comment!). Secondly if you delete the two tables from the Data Model and delete the two Power Query queries, and then follow these steps again, you will find the relationship is not created. That can’t be right. Thirdly, I don’t like having to create a third query with the Merge, and would prefer it if I could just create two queries and define the relationship somewhere separately. With all of these issues I don’t think there’s any practical use for this functionality right now.
I guess the reason I think the ability to create relationships automatically is so important is because the one thing that the Excel Data Model/Power Pivot/SSAS Tabular sorely lacks is a simple way to script the structure of a model. Could Power Query and M one day be the modelling language that Marco asks for here? To be fair to the Power Query team this is not and should not be their core focus right now: Power Query is all about data acquisition, and this is data modelling. If this problem was solved properly it would take a lot of thought and a lot of effort. I would love to see it solved one day though.
You can download the sample workbook for this post here.