# approx\_count\_distinct

[Read this page in HTML](https://www.scopedb.io/docs/reference/functions/aggregate/approx_count_distinct)

Estimates the number of distinct non-NULL values.

## Syntax

```scopeql
approx_count_distinct(<expr>)
```

## Arguments

| Argument | Type     | Description  |
| -------- | -------- | ------------ |
| `<expr>` | Any type | Input value. |

## Returns

Returns `int`. Ignores NULL values. Returns 0 for empty input or when every value is NULL.

## Usage notes

Use [`count_distinct`](https://www.scopedb.io/docs/reference/functions/aggregate/count_distinct.md) when an exact count is required.

## Examples

Count checkout events and distinct buyers for each campaign:

```scopeql
VALUES
    ('spring', 'buyer-17'), ('spring', 'buyer-17'), ('spring', 'buyer-42'),
    ('loyalty', 'buyer-17'), ('loyalty', 'buyer-63'), ('loyalty', 'buyer-63'), ('loyalty', 'buyer-87')
SELECT $0 AS campaign, $1 AS buyer_id
GROUP BY campaign
AGGREGATE count() AS events, approx_count_distinct(buyer_id) AS buyers
ORDER BY campaign
```

```text
+----------+--------+--------+
| campaign | events | buyers |
+----------+--------+--------+
| loyalty  | 4      | 3      |
| spring   | 3      | 2      |
+----------+--------+--------+
```

The estimate matches the exact count in this small sample; larger inputs can produce different estimates. A buyer can appear in more than one campaign, so adding campaign counts can count the same buyer more than once.