Left anti join
One of the join kinds available in the Merge dialog box in Power Query is a left anti join, which brings in only rows from the left table that don't have any matching rows from the right table. More information: Merge operations overview
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 2 in row 4. A table on the right contains ID and Country columns. The emphasized ID column contains values of 3 in row 1 (denoting Panama) and 4 in row 2 (denoting Spain). A table below the first two tables contains Date, CountryID, Units, and Country columns. The table has three rows, with two rows containing the data for CountryID 1, and one row for CountryID 2. Since none of the remaining CountryIDs match any of the countries in the right table, the rows in the Country column in the merged table all contain null.
This article uses sample data to show how to do a merge operation with the left anti 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.
Countries: This table is a reference table with the fields id and Country. The id field represents the unique identifier for each record.
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|
The goal is to create a table like the following, where only the rows from the left table that don't match any from the right table are kept.
Left anti join final table with Date, CountryID, Units, and Country column headers, and three rows of data of which the values for the Country column are all null.
To do a left anti join
- Select the Sales query, and then select Merge queries.
- In the Merge dialog box, under Right table for merge, select Countries.
- In the Sales table, select the CountryID column.
- In the Countries table, select the id column.
- In the Join kind section, select Left anti.
- Select OK.
Take a closer look at the message at the bottom of the dialog box that reads "The selection excludes 1 of 4 rows from the first table." This message is crucial to understanding the result that you get from this operation.
In the Sales table, you have a CountryID of 1 and 2, but neither of them are found in the Countries table. That's why the match only found one of four rows in the left (first) table.
In the Countries table, you have the Country Spain with an id of 4, but there are no records for CountryID 4 in the Sales table. That's why only one of two rows from the right (second) table found a match.
From the newly created Countries column, expand the Country field. Don't select the Use original column name as prefix check box.
After doing this operation, you'll create a table that looks like the following image. The newly expanded Country field doesn't have any values. That's because the left anti join doesn't bring any values from the right table—it only keeps rows from the left table.
Final table with Date, CountryID, Units, and Country column headers, and three rows of data of which the values for the Country column are all null.
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