How to debug and extract a scalar value from a DAX table result (e.g., LINESTX)?

[FedericoPorcu]

Hi all,

I’m struggling with debugging a DAX expression where the function returns a table, but I need to extract a single scalar value from it.

A concrete example is using LINESTX, which returns a table with regression statistics. My goal is to retrieve a specific value (e.g., the slope), but I’m not fully clear on the correct approach and, more importantly, how to debug intermediate table results in DAX.

What I’d like to understand:

• What is the best way to inspect the output of a table-returning function during development?

• How do you typically extract a single value from that table inside a measure?

• Are there recommended patterns (e.g., SELECTCOLUMNS, ADDCOLUMNS, MAXX, etc.) for this use case?

• Where should I test these intermediate steps? (DAX Studio? Power BI “New Table”?)

I’m not looking for just a one-off solution, but rather a mental model / workflow for debugging and handling these situations in a clean way.

Thanks a lot!

[v-priyankata]

Hi @FedericoPorcu 

Thank you for reaching out to the Microsoft Fabric Forum Community.

@FBergamaschi Thanks for the inputs

I hope the information provided by user was helpful. If you still have questions, please don't hesitate to reach out to the community.

[FBergamaschi · Accepted solution]

Hi @FedericoPorcu 

my answers below

What I’d like to understand:

• What is the best way to inspect the output of a table-returning function during development?

• FB you can inspect it through an aggregation like using a COUNTROWS ( Table ) or CONCATENATEX (table, table[column] ). It is highy recommended to use variables to be able to inpect different steps on different tables and understand where a bug happens

• How do you typically extract a single value from that table inside a measure?
  FB you can aggregate as I said but there you ask for a signle value: you can use SELECTEDVALUE ( table[column] ), this value return the single value if it is really a single value, or blank if there are multiple - I mean different - values

• Are there recommended patterns (e.g., SELECTCOLUMNS, ADDCOLUMNS, MAXX, etc.) for this use case?

• FB see above, I discourage MAXX or SUMX because they do not react should there be more than one value (supposing you need to make sure you want to see the ONLY value visible, otherwise of course you can use COUNTROWS, SUMX, any aggregation. Not ADDCOLUMNS as that creates a table and a table is by definition a set of values in different columns (or you can use SUMX, COUNTROWS on top of the ADDCOLUMNS call)

• Where should I test these intermediate steps? (DAX Studio? Power BI “New Table”?)

• FB possibly DAX Studio to avoid polluting your pbix

Best

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Francesco Bergamaschi

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[FedericoPorcu]

Thanks for the answer - yes, this is exactly what I meant.