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

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

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

```
HLL_UNION_AGG ( expr [, allowDifferentLgConfigK ] )
```

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

 *expr*   
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_AGG-return-type"></a>

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

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

The following examples use the HyperLogLog (HLL) sketching algorithm to estimate the unique count of values across multiple HLL sketches.

The first example estimates the unique count of values in a dataset.

```
SELECT hll_sketch_estimate(hll_union_agg(sketch, true))
    FROM (SELECT hll_sketch_agg(col) as sketch
            FROM VALUES (1) AS tab(col)
          UNION ALL
          SELECT hll_sketch_agg(col, 20) as sketch
            FROM VALUES (1) AS tab(col));
  1
```

The inner query creates two HLL sketches:
+ The first SELECT statement creates a sketch from a single value of 1. 
+ The second SELECT statement creates a sketch from another single value of 1, but with a precision of 20. 

The outer query uses the HLL\$1UNION\$1AGG function to combine the two sketches into a single sketch. Then it applies the HLL\$1SKETCH\$1ESTIMATE function to this combined sketch to estimate the unique count of values.

The result of this query is the estimated unique count of the values in the `col` column, which is `1`. This means that the two input values of 1 are considered to be unique, even though they have the same value.

The second example includes a different precision parameter for the HLL\$1UNION\$1AGG function. In this case, both HLL sketches are created with a precision of 14 bits, which allows them to be successfully combined using `hll_union_agg` with the `true` parameter.

```
SELECT hll_sketch_estimate(hll_union_agg(sketch, true))
    FROM (SELECT hll_sketch_agg(col, 14) as sketch
            FROM VALUES (1) AS tab(col)
          UNION ALL
          SELECT hll_sketch_agg(col, 14) as sketch
            FROM VALUES (1) AS tab(col));
  1
```

The final result of the query is the estimated unique count, which in this case is also `1`. This means that the two input values of 1 are considered to be unique, even though they have the same value.