You can create relationships in Power BI between tables. Relationships are useful for some functions to work across multiple tables and produce the result. Relationship between tables also makes visualization and report elements more efficient, because result of selection in one chart can affect another chart from different table. However there is a limitation in Power BI relationship that you can’t create relationship based on more than one column. In other words if you want to create relationship (or join two tables) with two or more columns, you cannot! Fortunately there is a work around that I’ll explain in this post. For this post you need to be familiar with Power BI and Power Query, if you are not, read them through Power BI online book.
Defining the Problem
Assume that we have a budget table with fiscal year, fiscal period, and budget amount. Here is an screenshot of this table:
If I want to do date analysis and date based calculations it is best to create a relationship between the budget table and a date dimension. Here is a view of my date dimension: (Here is an example of creating date dimension with Power Query, and the script for creating date dimension in SQL Server)
To join these two tables I can add a day column to the budget table, and then join them based on three columns: fiscal year, fiscal period, and day (day of month). So here is the budget table with the new day column added and month value a bit polished to remove “Mth” from the month label;
Now if I want to create relationship between date dimension and budget table based on these three columns I cannot! The create relationship dialog doesn’t allow me to select multiple columns, and because with a single column a Key field won’t be identified so the relationship can’t be created.
The workaround for this problem is easy. Power BI doesn’t allow relationship in model based on multiple columns, but Power Query can join tables with as many as columns we want. So what I can do as a workaround is to join budget table to date dimension in Power Query and fetch the date key. then I’ll use the date key as a single field relationship in Power BI modelling section.
First I open Merge Queries from the Combine section of Home tab;
Here is how I join two tables based on multiple columns: I can hold CTRL key and select columns one by one (in the right order of joining)
Then I’ll have the new table embedded as a result of the join;
So I’ll just pick the key field from embedded table;
And after save and closing query editor window I can create relationship in Power BI model based on a single column;