Calculating Previous Year Sales using DAX in Power BI

Surely! Here, I will create a function that uses the calculations to determine the sales of the previous year using DAX (Data Analysis Expressions). In DAX, to calculate the previous year's value, we usually use the SAMEPERIODLASTYEAR function. But in the case of using offset, you can use DATEADD function.

However, keep in mind that DAX doesn't use offsets like some other programming languages. Instead, it relies heavily on in-built time intelligence functions.

Here is the sample DAX code:

Previous Year Sales = 
CALCULATE(
    SUM( Sales[Sales Amount] ),
    DATEADD( Dates[Date], -1, YEAR )
)

This code calculates the previous year's sales by summing up the 'Sales Amount' for the same period of previous year through DATEADD function.

In the DAX formula:

• CALCULATE is a DAX function-helper to modify the context in a calculation.
• SUM aggregates the Sales amount.
• DATEADD is used to subtract one YEAR from the current date, getting the previous year sales.

Remember to replace Sales[Sales Amount] and Dates[Date] with your actual table names and column names.

This code should be part of a Measure in your data model so that it can be used throughout your Power BI report for analytics, visualizations and further data modelling.

As a best practice, ensure to create a well-structured data table and correctly relate it to other tables in the data model. You may find related courses about data modeling and DAX on 'Enterprise DNA Platform' for further learning.