Query DB to fetch more data for each cell value

How do I query database to fetch some value for each of the cell value in a table ?

I have a use case where I want to query a database for each of the cell value and display it in a calculated column or replace it on the same column.

Not sure how to utilise mapper here, how can I query database using Mapper or Transformer ?

@alex-w Can you share how can I implement something like this ?