# APPROX PERCENTILE function
<a name="approx-percentile"></a>

APPROX PERCENTILE is used to estimate the percentile value of a given expression or column without having to sort the entire dataset. This function is useful in scenarios where you need to quickly understand the distribution of a large dataset or track percentile-based metrics, without the computational overhead of performing an exact percentile calculation. However, it's important to understand the trade-offs between speed and accuracy, and to choose the appropriate error tolerance based on the specific requirements of your use case.

## Syntax
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```
APPROX_PERCENTILE(expr, percentile [, accuracy])
```

## Arguments
<a name="approx-percentile-syntax.arguments"></a>

 *expr*   
The expression or column for which you want to estimate the percentile value.  
It can be a single column, a complex expression, or a combination of columns. 

*percentile*  
The percentile value you want to estimate, expressed as a value between 0 and 1.   
For example, 0.5 would correspond to the 50th percentile (median).

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

## Returns
<a name="approx-percentile-syntax.returns"></a>

Returns the approximate percentile of the numeric or ANSI interval column col which is the smallest value in the ordered col values (sorted from least to greatest) such that no more than percentage of col values is less than the value or equal to that value. 

The value of percentage must be between 0.0 and 1.0. The accuracy parameter (default: 10000) is a positive numeric literal which controls approximation accuracy at the cost of memory. 

Higher value of accuracy yields better accuracy, `1.0/accuracy` is the relative error of the approximation. 

When percentage is an array, each value of the percentage array must be between 0.0 and 1.0. In this case, returns the approximate percentile array of column col at the given percentage array.

## Examples
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The following query estimates the 95th percentile of the `response_time` column, with a maximum relative error of 1% (0.01).

```
SELECT APPROX_PERCENTILE(response_time, 0.95, 0.01) AS p95_response_time
FROM my_table;
```

The following query estimates the 50th, 40th, and 10th percentile values of the `col` column in the `tab` table.

```
SELECT approx_percentile(col, array(0.5, 0.4, 0.1), 100) FROM VALUES (0), (1), (2), (10) AS tab(col)
```

The following query estimates the 50th percentile (median) of the values in the col column.

```
SELECT approx_percentile(col, 0.5, 100) FROM VALUES (0), (6), (7), (9), (10) AS tab(col)
```