Generating A Date Dimension Table In Power Query

There are hundreds of different methods for generating a Date dimension table for use in Power Pivot: you can use TSQL, Excel formulas, import from the DateStream table in the Windows Azure Marketplace, and there’s even an app for it. Nevertheless I thought it would be a good idea to solve this problem in Power Query because:

• It seems like the natural tool to use, and a date table is one of the first tables a data steward will want to share with his/her users.
• It’s a very good way to learn about and demonstrate all the date functionality in M and Power Query. The more M code examples that there are out there, the better, right?
• Having just signed the contract to write a whole book on Power Query I need to master M very quickly!

Here’s how I did it. First of all, I decided to implement a function to create the table rather than a regular query; this function takes two parameters: the start date and the end date of the range of dates to appear in the table. Here’s the function I came up with:

let

    CreateDateTable = (StartDate, EndDate) =>

let

    /*StartDate=#date(2012,1,1),

    EndDate=#date(2013,12,31),*/

    //Create lists of month and day names for use later on

    MonthList = {"January", "February", "March", "April", "May", "June"

                 , "July", "August", "September", "October", "November", "December"},

    DayList = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"},

    //Find the number of days between the end date and the start date

    NumberOfDates = Duration.Days(EndDate-StartDate),

    //Generate a continuous list of dates from the start date to the end date

    DateList = List.Dates(StartDate, NumberOfDates, #duration(1, 0, 0, 0)),

    //Turn this list into a table

    TableFromList = Table.FromList(DateList, Splitter.SplitByNothing(), {"Date"}

                     , null, ExtraValues.Error),

    //Caste the single column in the table to type date

    ChangedType = Table.TransformColumnTypes(TableFromList,{{"Date", type date}}),

    //Add custom columns for day of month, month number, year

    DayOfMonth = Table.AddColumn(ChangedType, "DayOfMonth", each Date.Day([Date])),

    MonthNumber = Table.AddColumn(DayOfMonth, "MonthNumberOfYear", each Date.Month([Date])),

    Year = Table.AddColumn(MonthNumber, "Year", each Date.Year([Date])),

    DayOfWeekNumber = Table.AddColumn(Year, "DayOfWeekNumber", each Date.DayOfWeek([Date])+1),

    //Since Power Query doesn't have functions to return day or month names, 

    //use the lists created earlier for this

    MonthName = Table.AddColumn(DayOfWeekNumber, "MonthName", each MonthList{[MonthNumberOfYear]-1}),

    DayName = Table.AddColumn(MonthName, "DayName", each DayList{[DayOfWeekNumber]-1}),

    //Add a column that returns true if the date on rows is the current date

    IsToday = Table.AddColumn(DayName, "IsToday", each Date.IsInCurrentDay([Date]))

in

    IsToday

in

    CreateDateTable

Some things to point out:

• Look – comments! Both multi-line /* */ and single line //. Adding comments to complex Power Query queries and functions is of course a Good Thing and something we should all be doing.
• The way I created the table from the start and end date is quite interesting: the List.Dates() function allows you to generate a list of dates from one date to another; I was then able to take this list and turn it into a table with Table.FromList().
• Adding extra columns to the table for the year and month number and so on is fairly straightforward. However, as far as I can see, Power Query does not have the ability to return month or day names (as opposed to numbers) from a date, so to get round this I created two lists called MonthList and DayList containing the months of the year and the days of the week, in order, took the day and month numbers and used these to find the name at the nth position in the list (see the MonthName and DayName steps).
• Date.IsInCurrentDay() compares a date with the current system date and allows you to see if a date is today’s date. There are a whole bunch of other interesting functions like this such as Date.IsInPreviousWeek(), Date.IsInCurrentYear() and so on, useful for creating relative date dimension tables like the one I describe here.

With the function created it’s very easy to generate a table using it. For example, I created a parameter table with start date and end date columns like so:

I then used this table as the starting point for a new query that called my function:

let

    Source = Excel.CurrentWorkbook(){[Name="Table3"]}[Content],

    ChangedType = Table.TransformColumnTypes(Source,{{"Start Date", type date}

                  , {"End Date", type date}}),

    CallCreateDateTable = CreateDateTable(ChangedType[Start Date]{0}

                          ,ChangedType[End Date]{0})

in

    CallCreateDateTable

Here’s the output:

You can download my demo workbook with all the code in from here.

PS If you haven’t downloaded the November update for Power Query already, I recommend you do so – the UI has been improved a lot.