# APPROX COUNT\$1DISTINCT function
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APPROX COUNT\$1DISTINCT provides an efficient way to estimate the number of unique values in a column or dataset.

## Syntax
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```
approx_count_distinct(expr[, relativeSD])
```

## Arguments
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 *expr*   
The expression or column for which you want to estimate the number of unique values.  
It can be a single column, a complex expression, or a combination of columns. 

*relativeSD*  
An optional parameter that specifies the desired relative standard deviation of the estimate.  
It is a value between 0 and 1, representing the maximum acceptable relative error of the estimate. A smaller relativeSD value will result in a more accurate but slower estimation.   
If this parameter isn't provided, a default value (usually around 0.05 or 5%) is used.

## Returns
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Returns the estimated cardinality by HyperLogLog\$1\$1. relativeSD defines the maximum relative standard deviation allowed.

## Example
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The following query estimates the number of unique values in the `col1` column, with a relative standard deviation of 1% (0.01).

```
SELECT approx_count_distinct(col1, 0.01)
```

The following query estimates that there are 3 unique values in the `col1` column (the values 1, 2, and 3).

```
SELECT approx_count_distinct(col1) FROM VALUES (1), (1), (2), (2), (3) tab(col1)
```