Comment by mamcx

Because the poor formatting and example (making the code look too much as sql!), some nuance was lost. Consider instead:

    let keys = SELECT key FROM BTreeIndex_CustomerCode;

    let city = SELECT name;

    let groups = GROUP BY (whatever -- And I mean GROUP, not SUMMARIZE!);

    let filter = WHERE id = @id;
    let query = city + filter(id=1);

    for city in query GROUP BY keys
         ....