---
title: Cardinality
description: Compute the number of distinct values in a field
canonical: https://docs.paradedb.com/documentation/aggregates/metrics/cardinality
---

The cardinality aggregation estimates the number of distinct values in a field.

```sql
SELECT pdb.agg('{"cardinality": {"field": "rating"}}') FROM mock_items
WHERE id @@@ pdb.all();
```

```ini Expected Response
      agg
----------------
 {"value": 5.0}
(1 row)
```

Unlike SQL's `DISTINCT` clause, which returns an exact value but is very computationally expensive, the cardinality aggregation uses the HyperLogLog++ algorithm to
closely approximate the number of distinct values.

See the [Tantivy documentation](https://docs.rs/tantivy/latest/tantivy/aggregation/metric/struct.CardinalityAggregationReq.html) for all available options.
