Left outer join

One of the join kinds available in the Merge dialog box in Power Query is a left outer join, which keeps all the rows from the left table and brings in any matching rows from the right table. More information: Merge operations overview

Left outer join example

Figure shows a table on the left with Date, CountryID, and Units columns. The emphasized CountryID column contains values of 1 in rows 1 and 2, 3 in row 3, and 4 in row 4. A table on the right contains ID and Country columns. The emphasized ID column contains values of 1 in row 1 (denoting USA), 2 in row 2 (denoting Canada), and 3 in row 3 (denoting Panama). A table below the first two tables contains Date, CountryID, Units, and Country columns. The table has four rows, with the top two rows containing the data for CountryID 1, one row for CountryID 3, and one row for Country ID 4. Since the right table didn't contain an ID of 4, the value of the fourth row in the Country column contains null.

This article uses sample data to show how to do a merge operation with the left outer join. The sample source tables for this example are:

  • Sales: This table includes the fields Date, CountryID, and Units. CountryID is a whole number value that represents the unique identifier from the Countries table.

    Sales table containing Date, CountryID, and Units columns, with CountryID set to 1 in rows 1 and 2, 3 in row 3, and 4 in row 4

  • Countries: This table is a reference table with the fields id and Country. The id field represents the unique identifier for each record.

    Sample Country table for left outer join

    Countries table with id set to 1 in row 1, 2 in row 2, and 3 in row 3, and Country set to USA in row 1, Canada in row 2, and Panama in row 3.
    

In this example, you'll merge both tables, with the Sales table as the left table and the Countries table as the right one. The join will be made between the following columns.

Field from the Sales table Field from the Countries table
CountryID id

The goal is to create a table like the following, where the name of the country appears as a new Country column in the Sales table as long as the CountryID exists in the Countries table. If there are no matches between the left and right tables, a null value is the result of the merge for that row. In the following image, this is shown to be the case for CountryID 4, which was brought in from the Sales table.

Left outer join final table with the Country column added with the value of the fourth row of that column set to null

To do a left outer join

  1. Select the Sales query, and then select Merge queries.
  2. In the Merge dialog box, under Right table for merge, select Countries.
  3. In the Sales table, select the CountryID column.
  4. In the Countries table, select the id column.
  5. In the Join kind section, select Left outer.
  6. Select OK.

Merge dialog box showing the results of following the previous left outer join procedure

From the newly created Countries column, expand the Country field. Don't select the Use original column name as prefix check box.

Expand table column for Country

After performing this operation, you'll create a table that looks like the following image.

Final table with the Country column added with the value of the fourth row of that column set to null