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Some thoughts of your typical data shepard / data groomer / data dance teacher sort of person.


Dynamic date variables using M (Power Query Formula Language)

Whilst working on developing some reports for our Service desk l was asked if we could just return a dataset containing the last 6 and 12 months of data.  This is the type of request l would normally use a TSQL script to resolve.  In this case one of the purposes of this project was to allow us to dog food both PowerBI and also self service BI more generally.

The challenge was quite simple find a way to filter data in a dataset based on specific datetime. to this with a specific date-time using the UI is very simple.  In Figure 1 below l will filter the column titled Issue_Created, which contains date-time values.


Figure 1 - The field 'Issue_created' selected showing the filter icon

From the list l can use this to filter by certain dates as shown below in Figure 2.  This lead me to wonder if it was possible to filter by date ranges.


Figure 2 - Filter by specific date

Below is a video showing how to filter records that either equal or occur after specific date, see Figure 3.

Figure 3 - Creating a filter to show record on or after a specific date.

Once l had set up the filter l then opened the 'Advanced Editor' to look at the M Code.   The code that does the filtering looks like this 

#"Filtered Rows1" = Table.SelectRows(#"Filtered Rows", each [Created] >= #datetime(2015, 7, 7, 5, 16, 52))

The syntax looked relatively simple and straightforward, one of the main challenges is that in the snippet above the datetime is static not dynamic.  At this point there was a tweak to the specification, my internal clients l have every faith in me :-).  What they required was based on today's date, get the first day of the month and then return first day of the month 6 months and 12 month before.  This is a pattern which l have done many times in TSQL, this was going to be a bit more of an adventure.

Long story short, after some experimenting l arrived at the following formulas

= Date.AddMonths(Date.StartOfMonth(DateTime.LocalNow()),-6)
First day of the month 6 months ago based on current date

= Date.AddMonths(Date.StartOfMonth(DateTime.LocalNow()),-12)
First day of the month 12 months ago based on current date.

Whats the next step?  Create a variable which will contain the DAX formulas that have created above.  The first step to open PowerBI desktop and go to 'Edit Queries', this will open the 'Query Editor' window.  In the 'Query Editor' window click on the 'New Source' button and select 'Blank Query' from the pull down list see Figure 4.


Figure 4 - Creating a new Blank Query to act as a parameter

Now the blank query has been created the next step is to name the parameter, the name that l use in the example is Previous_FDM_6Months.  Next l type in the DAX formula -> = Date.AddMonths(Date.StartOfMonth(DateTime.LocalNow()),-6).  Once thats done, then click the green tick to update and evaluate the parameter.


Figure 5 - Setting the parameter name and inserting the DAX formula

So now l have parameter which returns a datetime value based on current date and time.  The last step was to amend the part of the M code l had grabbed earlier to filter the dataset.  Below is the line from the M code l use in production

let
    Source = Sql.Database("A_Cloud_Database","Some_Data"),
    dbo_Some_Data = Source{[Schema="dbo",Item="SomeRandomData"]}[Data],
    #"Filtered_Rows_Last12_Months" = Table.SelectRows(dbo_Some_Data, each [Issue_Created] >= Previous_FDM_12Months),
    .........

The data is extracted from the database and filter only return data where the Issue_Created is equal to or greater than the datetime value returned by the parameter Previous_FDM_12Months.

A complete reference of all the datetime functions in power query lanague aka M can be found here https://msdn.microsoft.com/en-us/library/mt296608.aspx.  If you want to download a PowerBI desktop file with the functions you can download it here.