Aggregation

Aggregation is collecting many individual things into one summary. A ledger aggregates accounts — it gathers all the individual account records into a single organized whole. A sum aggregates numbers. A census aggregates individuals into a population count.

Aggregation loses detail in exchange for overview. The individual items still exist, but the aggregate shows only their combined shape. A good aggregation preserves the structure that matters and discards the structure that does not. A bad aggregation hides important differences behind a single number.

When this library uses aggregated-by as a predicate, the subject is the individual items and the object is what collects them together.