> ## Documentation Index
> Fetch the complete documentation index at: https://docs.firebolt.io/llms.txt
> Use this file to discover all available pages before exploring further.

<AgentInstructions>

## Submitting Feedback

If you encounter incorrect, outdated, or confusing documentation on this page, submit feedback:

POST https://docs.firebolt.io/feedback

```json
{
  "path": "/reference-sql/functions-reference/aggregation/hll-count-build",
  "feedback": "Description of the issue"
}
```

Only submit feedback when you have something specific and actionable to report.

</AgentInstructions>

> Reference material for HLL_COUNT_BUILD

# HLL_COUNT_BUILD

Counts the approximate number of unique not NULL values, aggregating the values to HLL++ sketches represented as
the [BYTEA data type](/reference-sql/data-types/bytea).
Multiple sketches can be merged to a single sketch using the aggregate function [HLL\_COUNT\_MERGE](/reference-sql/functions-reference/aggregation/hll-count-merge).
To estimate the final distinct count value, the scalar function [HLL\_COUNT\_ESTIMATE](/reference-sql/functions-reference/numeric/hll-count-estimate) can be
used.
`HLL_COUNT_BUILD` uses the HLL++ algorithm and allows you to control the set sketch size precision, similar
to [HLL\_COUNT\_DISTINCT](/reference-sql/functions-reference/aggregation/hll-count-distinct).

`HLL_COUNT_BUILD` requires less memory than exact count distinct aggregation, but also introduces statistical uncertainty.
The default precision is 12, with a maximum of 20 set optionally.

<Note>
  Higher precision comes at a memory and performance cost.
</Note>

## Syntax

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
HLL_COUNT_BUILD(<expression> [, <precision> ]) [FILTER ([WHERE] <condition>)]
```

## Parameters

| Parameter      | Description                                                                                                            | Supported input types |
| :------------- | :--------------------------------------------------------------------------------------------------------------------- | :-------------------- |
| `<expression>` | Any column name or function that return a column name.                                                                 | Any type              |
| `<precision>`  | Optional literal integer value to set precision. If not included, the default precision is 12. Precision range: 12-20. | `INTEGER`, `BIGINT `  |
| `<condition>`  | An optional boolean expression to filter rows used in aggregation                                                      | `BOOL`                |

## Return Type

`BYTEA`

## Example

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
CREATE TABLE data_to_count AS
SELECT *
FROM generate_series(0, 10000000, 3) a;

SELECT count(distinct a) AS accurate_count
FROM data_to_count;
```

| accurate\_count (BIGINT) |
| :----------------------- |
| 3333334                  |

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
CREATE TABLE data_to_count2 AS
SELECT *
FROM generate_series(0, 10000000, 2) a;

SELECT count(distinct a) AS accurate_count
FROM data_to_count2;
```

| accurate\_count (BIGINT) |
| :----------------------- |
| 5000001                  |

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
CREATE TABLE sketch_of_data_to_count AS
SELECT hll_count_build(a) a
FROM data_to_count;

INSERT INTO sketch_of_data_to_count
SELECT hll_count_build(a)
FROM data_to_count2;

SELECT hll_count_estimate(a) AS hll_estimate, a AS sketch
FROM sketch_of_data_to_count
ORDER BY 1;
```

| hll\_estimate (BIGINT) | sketch (BYTEA)               |
| :--------------------- | :--------------------------- |
| 3291008                | \x2f41676772656761746546.... |
| 4948957                | \x2f41676772656761746546.... |

```sql theme={"theme":{"light":"github-light","dark":"github-dark"}}
SELECT hll_count_estimate(hll_count_merge(a)) AS hll_estimate
FROM sketch_of_data_to_count;
```

| hll\_estimate (BIGINT) |
| :--------------------- |
| 6606880                |