# `APPROX_COUNT_DISTINCT`

**Applies to:** ✅ Data 360 SQL ✅ Tableau Hyper API

Computes an approximation of the count of distinct values with a specified expected relative error.

## Syntax

```sql
approx_count_distinct(<expression> [, <error>])
```

## Arguments

### Required

- `<expression>`: An expression of any type to count distinct values for.

### Optional

- `<error>`: A `double precision` value specifying the expected relative error. Must be in the range (0.002, 1]. Defaults to 0.023 (2.3% expected relative error).

## Returns

Returns a `bigint` representing the approximate count of distinct values.

## Considerations

- Faster than exact `count(distinct expression)` for large datasets.
- The error parameter controls the trade-off between accuracy and performance.
- Lower error values provide more accurate results but may require more computation.

## Examples

### Approximate Distinct Count

Get an approximate count of distinct values.

```sql
SELECT approx_count_distinct(customer_id) AS approx_customers
FROM orders;
```

### With Custom Error Rate

Specify a custom error rate for higher accuracy.

```sql
SELECT approx_count_distinct(product_id, 0.01) AS approx_products
FROM sales;
```

Returns an approximate count with 1% expected relative error.

## Related Documentation

- [COUNT](/docs/data/data-cloud-query-guide/references/dc-sql-reference/count.md)
- [Aggregate Functions](/docs/data/data-cloud-query-guide/references/dc-sql-reference/aggregate.md)