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A Left Join in Power BI is a type of join that combines data from two tables based on a common field, keeping all rows from the first (left) table and matching rows from the second (right) table. To perform a Left Join in Power BI, you can use the “Merge Queries” function and select the desired tables and common field to join on. For example, if you have a table with customer information and a table with sales data, you can perform a Left Join to see all customers and their associated sales data, even if they haven’t made a purchase yet. This allows for a comprehensive analysis of data from multiple sources.
The easiest way to perform a left join between two tables in Power BI is to use the Merge Queries function in the Power Query Editor.
The following example shows how to do so in practice.
Example: How to Perform a Left Join in Power BI
Suppose we have the following table in Power BI named data1 that contains information about the team name and points scored for various basketball players:
And suppose that we have another table named data2 that contains information about the team name and rebounds for various basketball players:
Suppose that we would like to perform a left join in which we keep all rows from the data1 table and join them with matching values from the Team column in the data2 table.
To do so, click the Home tab along the top ribbon, then click the Transform data icon:
This will bring up the Power Query Editor.
Next, click the Merge Queries icon in the Combine group of the Home tab.
Then click Merge Queries as New from the dropdown menu:
In the new window that appears, choose data1 as the first table, choose data2 as the second table, and choose Left Outer as the Join Kind.
Then click the header for the Team column in each table so that Power BI knows to use those columns for the join:
Once you click OK, the left join will be performed:
Next, click the left and right arrows on the header of the data2 column. Then check the box next to Rebounds to indicate that this column should be the only one included from data2 in the final merged table:
Once you click OK, the Rebounds column will be shown from the data2 table:
Once you exit out of the Power Query Editor, a message box will appear that asks if you’d like to apply your changes.
Click Yes.
You will then be able to see the new table named Merge1 in the Table view:
Notice that all of the rows from the left table (data1) are kept in the merged table while only the rows with matching values in the Team column from the right table (data2) are included in the merged table.
Any row in the Team column from data1 that did not have a matching value in the Team column from data2 simply have a blank value in the data2.Rebounds column.
Note: If you’d like, you can right click on the header named data2.Rebounds and rename the column to just Rebounds.
Additional Resources
The following tutorials explain how to perform other common tasks in Power BI: