Directquery table limitations

Hi, Gav 

According to your requirement, you want to create a calculated column for a table whose connection mode is direct query.

However, direct query has great restrictions on calculated columns. For performance reasons, we do not recommend that you add calculated columns for direct query tables.

In order to add the staff number to the main table, I think you can try to achieve this in the original database (using stored procedure or ETL tools).

For more information about direct query, you can follow this link:

https://docs.microsoft.com/en-us/power-bi/connect-data/desktop-directquery-about

Best Regards,

Community Support Team _Robert Qin

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