# HLL\$1UNION function
<a name="HLL_UNION"></a>

The HLL\$1UNION function combines two HLL sketches into a single, unified sketch. It uses the HyperLogLog (HLL) algorithm to combine two sketches into a single sketch. Queries can use the resulting buffers to compute approximate unique counts as long integers with the `hll_sketch_estimate` function.

## Syntax
<a name="HLL_UNION-syntax"></a>

```
HLL_UNION (( expr1, expr2 [, allowDifferentLgConfigK ] ))
```

## Argument
<a name="HLL_UNION-argument"></a>

 *exprN*   
A `BINARY` expression holding a sketch generated by HLL\$1SKETCH\$1AGG.

*allowDifferentLgConfigK*  
A optional BOOLEAN expression controlling whether to allow merging two sketches with different lgConfigK values. The default value is `false`.

## Return type
<a name="HLL_UNION-return-type"></a>

The HLL\$1UNION function returns a BINARY buffer containing the HyperLogLog sketch computed as a result of combining the input expressions. When the `allowDifferentLgConfigK` parameter is `true`, the result sketch uses the smaller of the two provided `lgConfigK` values.

## Examples
<a name="HLL_UNION-examples"></a>

The following examples use the HyperLogLog (HLL) sketching algorithm to estimate the unique count of values across two columns, `col1` and `col2`, in a dataset.

 The `hll_sketch_agg(col1)` function creates an HLL sketch for the unique values in the `col1` column. 

The `hll_sketch_agg(col2)` function creates an HLL sketch for the unique values in the col2 column. 

The `hll_union(...)` function combines the two HLL sketches created in steps 1 and 2 into a single, unified HLL sketch.

The `hll_sketch_estimate(...)` function takes the combined HLL sketch and estimates the unique count of values across both `col1` and `col2`.

The `FROM VALUES` clause generates a test dataset with 5 rows, where `col1` contains the values 1, 1, 2, 2, and 3, and `col2` contains the values 4, 4, 5, 5, and 6. 

The result of this query is the estimated unique count of values across both `col1` and `col2`, which is 6. The HLL sketching algorithm provides an efficient way to estimate the number of unique elements, even for large datasets, without having to store the full set of unique values. In this example, the `hll_union` function is used to combine the HLL sketches from the two columns, which allows the unique count to be estimated across the entire dataset, rather than just for each column individually.

```
SELECT hll_sketch_estimate(
  hll_union(
    hll_sketch_agg(col1),
    hll_sketch_agg(col2)))
  FROM VALUES
    (1, 4),
    (1, 4),
    (2, 5),
    (2, 5),
    (3, 6) AS tab(col1, col2);
  6
```

The difference between the following example and the previous one is that the precision parameter (12 bits) isn't specified in the `hll_sketch_agg` function call. In this case, the default precision of 14 bits is used, which may provide a more accurate estimate for the unique count compared to the previous example that used 12 bits of precision.

```
SELECT hll_sketch_estimate(
  hll_union(
    hll_sketch_agg(col1, 14),
    hll_sketch_agg(col2, 14)))
  FROM VALUES
    (1, 4),
    (1, 4),
    (2, 5),
    (2, 5),
    (3, 6) AS tab(col1, col2);
```