How can I create a same time last year column in Query Editor?

[shds]

In the Power BI Desktop Query Editor I created an Unpivoted table based on certain attributes (columns to rows) which gives me correct values in a separate column. 

What I would like to do is create another column called "LastYear" which takes the row's Datetime and looks up the Value of the same date last year.

The PARALLELPERIOD expression doesn't seem to be giving the right results and I cannot get it to work in a Custom Column.

Here's what I tried for a calculated column in Power BI Desktop:

LastYear = CALCULATE(SUM('Table1'[Value]); PARALLELPERIOD('Table1'[Datetime]; -1; year))

Putting this formula in a Custom Column in Query Editor it first tells me there are no syntax errors found, but once I try to insert the column then it keeps saying "Expressions.Error: The name 'CALCULATE' wasn't recognized. Make sure it's spelled correctly.'

What am I doing wrong?

[mike_honey · Accepted solution]

The Query Editor does not use DAX syntax.  Instead it uses the "Power Query Formula Language", also called "M" or "PQL".

Here's the link to it's formula doco:

https://msdn.microsoft.com/library/mt253322?ui=en-US&rs=en-US&ad=US

For your scenario, I would try something like this in your Custom Column:

= Date.AddYears ( [Datetime] , -1 )

Then to use that to "Lookup" the previous value, I would use a Merge to the same query - it should show as Table1 (Current). In the Merge definition, if your Custom Column is callend Datetime -1Y, then I would choose that column in the top table and Datetime in the bottom Table1 (Current).

Then I would expand the Value column from the resulting NewColumn.  The Expand button will be in the table header row, just to the right of NewColumn.

[shds]

Thanks Mike, that pointed me in the right direction!