Written By Rick de Groot

Rick is the founder of BI Gorilla. He believes learning is one of life's greatest pleasures and shares his knowledge to help you improve your skills.

The Table.Group function in Power Query allows you to summarize your data based on specific columns. However, many users are unaware of the fifth argument that lets you customize how groups are formed. This article shows you how to use the Comparer Function in Table.Group. You’ll also learn how to create a custom comparer function for more precise data grouping.

Table of contents
Custom Comparer Function for Table.Group in Power Query M

Using Comparer Functions in Table.Group

The easiest way to use the Table.Group‘s Comparer argument is by using a comparer function from Power Query’s standard library. Here are the common ones:

We will work with the following dataset to demonstrate case sensitivity with the Table.Group function:

Dataset for Case Sensitivity by using Table.Group in Power Query

Default Case-Sensitive Grouping

The Table.Group function by default does a case-sensitive comparison using Comparer.Ordinal. Here’s an example of what this looks like:

  {"Location Id"}, 
  {{"Row Count", each Table.RowCount(_), Int64.Type}}, 

This code groups the data by the “Location Id” column in a case-sensitive manner, as illustrated below:

Table.Group Summarizes Values in a Case Sensitive Way in Power Query

Due to the bad quality of the data, the names that look similar can’t be grouped together.


While you could use the Table.FuzzyGroup function to address this, we will focus on how to solve it with Table.Group.

Case-Insensitive Grouping

In case you want to group items together, regardless of their capitalization, you can add another Comparer Function to the fifth argument of Table.Group. The function you need is Comparer.OrdinalIgnoreCase. Here’s how to use it:

    {"Location Id"}, 
    {{"Row Count", each Table.RowCount(_), Int64.Type},
     {"Rows", each _, Table.Type }}, 

Notice the additional line that creates a “Rows” column. The outcome of this expression is:

Table.Group Ignores Capitals when Summarizing Values in Power Query M

The grouping now includes a table object, allowing you to inspect the summarized rows. This makes it easier to inspect the summarized data. By using this method, the operation groups names without considering capitalization, which results in fewer grouped rows.

Now the standard library comparer functions are handy when you only need to test for case sensitivity or ignore cases. But what if you need to handle more complex conditions? That’s where custom comparers come in.

Working with Custom Comparers

Table.Group supports the use of custom comparers. This lets you to define your own logic for grouping values. Before we dive into custom comparers, let’s understand the basic requirements for a valid comparer.

In the M language, there are several functions for comparing values. These functions determine if one value comes before, after, or is identical to another value. They return -1 if the first value comes before the second, 0 if they are equal, and 1 if the first value comes after the second.

In other words, the comparison logic must be symmetric. If value1 is determined to come after value2, then when the values are switched, value1 should come before value2. This symmetry makes sure your grouping is consistent and accurate.

Let’s look at a few examples to understand how comparisons work:

// For case insensitive comparisons
Comparer.OrdinalIgnoreCase( "Ape", "Bear" ) // Output -1
Comparer.OrdinalIgnoreCase( "Duck", "duck" ) // Output 0
Comparer.OrdinalIgnoreCase( "Bear", "Ape" ) // Output 1

// For case sensitive comparisons
Comparer.Ordinal( "Duck", "duck" ) // Output -1
Comparer.Ordinal( "Duck", "duck" ) // Output 0
Comparer.Ordinal( "duck", "Duck" ) // Output 1

// Comparing Values
Value.Compare( 60, 10 ) // Output 1
Value.Compare( 60, 60 ) // Output 0
Value.Compare( 10, 60 ) // Output -1

This illustrates that a valid comparer function returns -1, 0, or 1. Now, how can we incorporate this into the grouping operation?

Single Column Logic

Case-Insensitive Comparison

Remember our previous example with Comparer.OrdinalIgnoreCase for case-insensitive comparisons? You can create a similar logic by building your own comparer function.

Here’s one way to do it:

Table.Group (
  { "Name" }, 
  { { "Row Count", each Table.RowCount ( _ ), Int64.Type }, 
    { "Rows", each _, Table.Type } }, 
  ( x, y ) => Value.Compare ( Text.Lower ( x[Name] ), Text.Lower ( y[Name] ) )

This code uses the Value.Compare function to compare values. It converts them to lowercase with Text.Lower first. This way, names are grouped together no matter how they are capitalized.

Group Data by Start of Quarter

LLet’s look at another example where we build custom comparers based on a single column and combine it with GroupKind.Global.

Imagine we have a dataset with the start dates of each month from January 1, 2000, to March 2001:

Dataset for grouping using GroupKind.Global in Power Query

Our goal is to group these dates by each quarter of the year. This means we want to group January, February, and March for Q1 2000, April, May, and June for Q2 2000, and so on. To achieve this using a custom comparer function, we will use the Value.Compare function as follows:

  {{"Details", each _, type table [Date = date]},
    {"NumOfRows", each Table.RowCount(_)}}, 
  (x, y) => 
    Value.Compare( Date.StartOfQuarter( x[Date] ), Date.StartOfQuarter( y[Date] ) )

This expression uses GroupKind.Global. This settings groups all values in the dataset without considering their order. Since the sorting order isn’t guaranteed, the custom comparer function should compare both the current value (x) and the next value (y) to ensure symmetry. By comparing the Date.StartOfQuarter value for all dates in the table and then grouping the data, you get the following result:

Table.Group using GroupKind.Global and a custom comparer in Power Query M

In this result, the data is grouped by the “Date” column, showing the first start of the quarter value within each group. Each group contains three values representing the months of that specific quarter of the year. Since the order of evaluation doesn’t matter with this logic, the result is accurate.

Multiple Columns Logic

In another scenario, we have a dataset with two text columns. For the summarizing of the columns we have the following requirements:

  • Period: Group this column in a case-sensitive manner.
  • Product: Ignore capitalization when grouping this column.

Here’s what the dataset looks like:

Custom Comparer with Multiple Columns for Table.Group in Power Query M

It’s important to remember that our custom comparer function should be symmetric. This means that the comparison should work correctly regardless of the order of the values. To achieve this, we can use multiple comparer functions as follows:

Table.Group (
  { "Period", "Product" }, 
  { { "Details", each _, type table }, 
    { "NumOfRows", each Table.RowCount ( _ ) } }, 
  ( x, y ) =>
      IgnCase  = Comparer.OrdinalIgnoreCase ( x[Product], y[Product] ), 
      RespCase = Comparer.Ordinal ( x[Period], y[Period] ),
      Result = if IgnCase = 0 then RespCase else IgnCase

The result is the following table:

Custom Comparer Grouping Result in Power Query M

The custom comparer in the fifth argument performs two main operations to group values effectively.

  • Step 1: Ignoring Case Differences: The first operation, stored in the IgnCase variable, checks for similarities in the Product column while ignoring capitalization differences. This means entries like “Shirt” and “SHIRT” are considered equal.
  • Step 2: Case-Sensitive Comparison: If the values are similar, the comparer then uses the RespCase variable to compare the ‘Period’ column in a case-sensitive way, using Comparer.Ordinal. If this comparison also finds the values equal, they are grouped together.

You’ve now seen a few examples that use custom comparer functions. These custom comparers need strict symmetry in their logic. However, sometimes you might use of GroupKind.Local instead of GroupKind.Global, which respects the order of your data. In those cases, you can use a simpler custom comparer setup. Let’s find out how.

Simplified Custom Comparers with GroupKind.Local

When you want your grouping operation to respect the order of your data, you can use the GroupKind.Local enumeration. Let’s explore how our logic changes in this scenario.

Suppose you’re working with a table where some data is missing. The input comes from a pivot table and when importing it into Power Query, the missing rows are filled with null values:

Dataset to Group data using GroupKind.Local in Power Query M

We want Power Query to create groups based on the Location Id column.


You can also fill down the values in the Location Id column using the Table.FillDown function. After that, you can perform a simple Group By operation.

To make the Table.Group function start a new group each time there is a number value in the “Location Id” column, you can use a custom comparer function. Here’s how:

  {"Location Id"}, 
  {{"Details", each _, type table }, 
   {"Names", each Text.Combine(_[Name], ", ") }}, 
  (x, y) => Number.From( y[Location Id] is number ) 

In the custom comparer function’s arguments, x is the current value, and y is the next value to compare. Since we are using GroupKind.Local, the function respects the order of the values. In this case, it’s enough to build a custom comparer that only tests y, the next value.

So what is happening in the above code? For each value the function encounters, the function tests if the expression y[Location Id] is number returns true. Each time this test evaluates to true, meaning y is a number, the function starts a new group.


Whenever you define a custom comparer function, the outcome should be -1, 0 or 1. In the above example, we return 0 or 1 by applying the Number.From function on the outcomes of a condition, which are either true or false.

The outcome of this is:

Table.Group using GroupKind.Local and a custom comparer in Power Query M

The custom comparer function made sure that even if there are null values in the “Location Id” column, they will be grouped with the preceding non-null “Location Id”.

However, this isn’t the only way. Instead of checking whether the data type is a number, you can also test whether the value itself is not null. This starts a new group whenever the value is not null:

  {"Location Id"}, 
  {{"Details", each _, type table }, 
   {"Names", each Text.Combine(_[Name], ", ") }}, 
  (x, y) => Number.From( y[Location Id] <> null ) 

The outcome of this query is identical to the one above.


The takeaway from this is that when you’re working with GroupKind.Local, you don’t have to define a custom function that is symmetric. It is enough to define when a new group should start by evaluating y. However, here it is also valid to provide a symmetric comparer function.


To play with this code yourself, you can download the file below:


As this article showed, the fifth argument of Table.Group provides additional flexibility. You can group your values without creating any additional columns. However, it’s important to note that any custom comparer function you create will not fold. Therefore, if performance is of importance, consider adding separate columns and group your data by these instead. Your operations may actually fold.

Share on:
    • That’s great Tan. Just remember, it breaks query folding. So if you work with a database, you may want to simply add separate columns. Enjoy!


Leave a comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.