The database staging query for a DirectQuery dataset is slightly different than an import mode dataset. For this query, an additional variable is added to the let expression, as shown in the following example:

let
    Source = Sql.Database(ProdServer, ProdDB),
    DummyVariable = null
in
    Source

The additional variable (DummyVariable) is ignored by the query and the same Sql.Database() function that references the server and database parameters for the import mode dataset can also be used for the DirectQuery dataset.