the rows (an array, also filtered or sorted ones)
property path ("total", "address.zip") or function
"sum", "count", "avg", "min", "max", "countDistinct", "first",
"last" or an own AggregatorIf
the result (sum of no numbers is 0, avg/min/max of no numbers undefined)
aggregate(orders, "total", "sum"); // all orders
aggregate(orders.filter(o => o.city === "Berlin"), "total", "avg");
aggregate(orders, o => o.quantity * o.price, "max"); // a computed value
1.1.94
Aggregates one value of all rows.