Question

Can I write custom M functions to reuse transformations across queries?
Power BI supports modular query design using custom M functions, which helps maintain reusable and maintainable ETL logic. This question examines how to create parameterized M functions, their advantages in standardizing business logic across reports, and how to structure query dependencies efficiently without breaking query folding or increasing refresh latency.

Answer 1

Yes, Power BI allows you to create custom M functions in Power Query, enabling modular, reusable, and maintainable data transformation logic across multiple queries. This is especially useful when you have repetitive transformation steps that need to be applied to multiple datasets or columns.

 How to Create a Custom M Function

1. Write a Function Using the let...in Syntax
   Use the following structure to define your function in a new blank query:

let
    CleanData = (inputTable as table) as table =>
        let
            RemovedNulls = Table.SelectRows(inputTable, each [Column1] <> null),
            ChangedTypes = Table.TransformColumnTypes(RemovedNulls, {{"Column1", type number}})
        in
            ChangedTypes
in
    CleanData

This defines a function CleanData that removes nulls from Column1 and changes its type to a number.

• Invoke the Function in Other Queries
  Apply this transformation to other queries by calling the function:

CleanData(SourceTable)