The unique count aggregation type counts only the unique values of a specified event property.

UNIQUE COUNT calculation method

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UNIQUE COUNT billable metric

Here is how you can create a unique count aggregation from the UI:
  1. Access the “Billable metrics” section via the side menu;
  2. Create a new billable metric;
  3. Define a name, a code and an optional description;
  4. Select “count unique” as the aggregation type;
  5. Define it this metric is metered or recurring;
  6. Define the property to aggregate;
  7. Apply dimension groups if any; and
  8. Click “Add billable metric”.
This billable metric can be both metered or recurring.

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Calculation example

Lago calculates the COUNT_DISTINCT(events.properties.property_name) for the two following events received. 
//Event received #1
{
    "transaction_id": "transaction_1",
    "external_customer_id": "1",
    "timestamp": "2022-03-16T00:00:00Z",
    "code": "api_requests",
    "properties": {
        "request_id": "id_1"
    }
}

//Event received #2
{
    "transaction_id": "transaction_2",
    "external_customer_id": "1",
    "timestamp": "2022-03-17T00:00:00Z",
    "code": "api_requests",
    "properties": {
        "request_id": "id_1"
    }
}
In that case, with this aggregation type, Lago only counts the unique values request_id property in the event payloads, resulting in a billable value of 1 unit.