APPROX_COUNT_DISTINCT

The APPROX_COUNT_DISTINCT function counts the approximate number of unique items in a field.

Syntax

APPROX_COUNT_DISTINCT( X )

Parameters

field_expression - a field or expression that contains the items to be counted.

How the `APPROX_COUNT_DISTINCT` function works

The APPROX_COUNT_DISTINCT function takes one parameter, which can be the name of a metric, dimension, or expression of any type. APPROX_COUNT_DISTINCT returns the approximate number of unique items in that field or expression.

APPROX_COUNT_DISTINCT is more efficient in terms of query processing than COUNT_DISTINCT , but returns less exact results. If your dataset is very large, or if the performance of your report is more important than exact counts, consider using APPROX_COUNT_DISTINCT . Using APPROX_COUNT_DISTINCT instead of COUNT_DISTINCT can also help reduce query costs when using BigQuery data sources.

For an in-depth explanation of how approximate aggregation works, see the BigQuery documentation .

Example

APPROX_COUNT_DISTINCT( Page ) - counts the approximate number of unique values in the Pagedimension.

Limits of `APPROX_COUNT_DISTINCT`

The APPROX_COUNT_DISTINCT function is available only when used with BigQuery data sources.
For data sources which don't support APPROX_COUNT_DISTINCT , APPROX_COUNT_DISTINCT will act like COUNT_DISTINCT .
You can't apply this function to a pre-aggregated metric ( Aggregationtype of Auto), or to an expression that is the result of another aggregation function. For example, a formula such as APPROX_COUNT_DISTINCT(Sessions) in a Google Analytics data source will produce an error.