TSQL Table-Valued Functions And Dynamic M Parameters In Power BI DirectQuery

My favourite – and it seems many other people’s favourite – new feature in the February 2022 Power BI Desktop release is support for more datasources (including SQL Server, Azure SQL DB and Synapse) with dynamic M parameters. In my opinion dynamic M parameters are extremely important for anyone planning to use DirectQuery: they give you a lot more control over the SQL that is generated by Power BI and therefore give you a lot more control over query performance.

Teo Lachev has already stolen my thunder and blogged about how the new functionality allows you to use a TSQL stored procedure as the source of a table in DirectQuery mode. In this post I’m going to show you something very similar – but instead of using a stored procedure, I’m going to show a simple example of how to use a TSQL table-valued function, which I think has a slight advantage in terms of ease-of-use.

To start off I created a table-valued function in the Adventure Works 2017 sample database on my local SQL Server which returns some filtered data from the DimDate table:

CREATE FUNCTION [dbo].[udfDates] (
    @day_name varchar(50),
	@month_name varchar(50)
)
RETURNS TABLE
AS
RETURN
    SELECT 
        FullDateAlternateKey, EnglishDayNameOfWeek, EnglishMonthName, CalendarYear
    FROM
        DimDate
    WHERE
        EnglishDayNameOfWeek=@day_name
		and
		EnglishMonthName=@month_name;

Here’s how it can be called in a SQL SELECT statement:

select 
	FullDateAlternateKey,
    EnglishDayNameOfWeek,
    EnglishMonthName,
    CalendarYear
from 
	udfDates('Thursday', 'February')
where
	CalendarYear=2005

Next, I created a Power Query query in my dataset that called this function and returned a table in DirectQuery mode. The great thing about table-valued functions is that they appear in the Navigator pane when you connect to a SQL Server database from the Power Query Editor:

You can only hard-code the values you pass to the function’s parameters at this point but even if you do nothing here you can just return an empty table. After having done this I selected DirectQuery mode:

After this I created two Power Query text parameters, called DayName and MonthName, to hold the month and day names to be passed to the function:

I also created two Import mode queries called DayNames and MonthNames to hold all the valid values for the DayName and MonthName parameters:

The last thing to do in the Power Query Editor was to edit the query calling the function to pass the M parameters to it. Here’s the M code for the query after the modification made to the “Invoked Functiondbo_udfDates1” step:

let
  Source = Sql.Database(
    "localhost", 
    "AdventureWorksDW2017"
  ), 
  dbo_udfDates = Source
    {
      [
        Schema = "dbo", 
        Item   = "udfDates"
      ]
    }
    [Data], 
  #"Invoked Functiondbo_udfDates1"
    = dbo_udfDates(DayName, MonthName)
in
  #"Invoked Functiondbo_udfDates1"

At this point I closed the Power Query Editor and loaded the three tables to my dataset:

Next I bound the EnglishDayNameOfWeek column on the DayNames table to the DayName M parameter and the EnglishMonthName column on the MonthNames table to the MonthName M parameter:

Finally, I built a report with two slicers bound to the two dynamic M parameter columns and a table showing the output of the table-valued function:

Here’s an example of the TSQL generated by Power BI to populate the table in this report:


SELECT 
TOP (501) 
[t0].[FullDateAlternateKey],
[t0].[EnglishDayNameOfWeek],
[t0].[EnglishMonthName]
FROM 
(
(
select [$Table].[FullDateAlternateKey],
    [$Table].[EnglishDayNameOfWeek],
    [$Table].[EnglishMonthName],
    [$Table].[CalendarYear]
from [dbo].[udfDates]('Thursday', 'February') as [$Table]
)
)
 AS [t0]
WHERE 
(
[t0].[CalendarYear] = 2010
)

GROUP BY 
[t0].[FullDateAlternateKey],[t0].[EnglishDayNameOfWeek],
[t0].[EnglishMonthName],[t0].[CalendarYear]
ORDER BY [t0].[FullDateAlternateKey]
ASC
,[t0].[EnglishMonthName]
ASC
,[t0].[CalendarYear]
ASC
,[t0].[EnglishDayNameOfWeek]
ASC
 

I am by no means an expert in writing efficient TSQL so I can’t comment on the pros and cons of table-valued functions, stored procedures or using native SQL queries in Power BI (although the last of these has obvious maintainability issues). Hopefully, though, you can see the possibilities – and if you do get round to using this approach on a project, please let me know how you get on!

Handling “Select All” For Slicers Bound To Dynamic M Parameters In Power BI

Last year I blogged about handling multi-select scenarios for dynamic M parameters. Since then support for “Select All” has been added (see here) but since a regular slicer with no items selected behaves the same as one with all items selected, some extra logic is needed to make a slicer bound to a dynamic M parameter behave like a regular slicer.

To illustrate this, I created a simple dataset with two tables in it:

  1. A DirectQuery table linked to the AdventureWorksDW DimDate table (a simple date dimension table), stored in Snowflake
  2. An Import mode table called Day Name that contains one column and seven rows containing the names of the days of the week

I then created an M parameter called DayParam and bound it to the DayName column of the Day Name table and enabled the Multi-select and Select all options:

Here’s the M code that shows how to use the DayParam parameter to filter the DimDate table on the EnglishDayNameOfWeek column, and handle the scenario where “Select All” has been selected as well as the scenario where no item has been selected:

let
  Source = Snowflake.Databases(
    "xyz.snowflakecomputing.com",
    "DEMO_WH"
  ),
  AWORKS_Database = Source
    {
      [
        Name = "AWORKS",
        Kind = "Database"
      ]
    }
    [Data],
  PUBLIC_Schema = AWORKS_Database
    {[Name = "PUBLIC", Kind = "Schema"]}
    [Data],
  DIMDATE_Table = PUBLIC_Schema
    {[Name = "DIMDATE", Kind = "Table"]}
    [Data],
  #"Filtered Rows" =
    if DayParam = null then
      DIMDATE_Table
    else if Type.Is(
      Value.Type(DayParam),
      List.Type
    )
    then
      if DayParam = {"__SelectAll__"} then
        DIMDATE_Table
      else
        Table.SelectRows(
          DIMDATE_Table,
          each List.Contains(
            DayParam,
            [ENGLISHDAYNAMEOFWEEK]
          )
        )
    else
      Table.SelectRows(
        DIMDATE_Table,
        each [ENGLISHDAYNAMEOFWEEK]
          = DayParam
      )
in
  #"Filtered Rows"

The #”Filtered Rows” step does the following:

  • If nothing is selected in the slicer then the DayParam parameter will contain a null value, and in this case no filter will be applied to the DimDate table
  • If the DayParam parameter is of type list (when there is a multi-select in the slicer) then
    • If it’s a list containing a single text value of “__SelectAll__”, the default value returned when the slicer has “Select all” selected, then no filter will be applied to the DimDate table
    • Otherwise List.Contains will be used to filter the DimDate table by all the selected values
  • Otherwise the DayParam parameter will contain a single scalar value (because a single value has been selected in the slicer) and the DimDate table will be filtered by that value

Here’s how it works:

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