# `distinct_count`

## Description

Returns the number of distinct, non-null values for a given expression.

- Each unique value is counted once per group.

Note

`distinct_count` is an aggregation function and must be used with a grouping keyword such as `groupby`.

## Syntax

```dataprime
distinct_count(expression: any): number
```

## Arguments

| Name       | Type | Required | Description                                          |
| ---------- | ---- | -------- | ---------------------------------------------------- |
| expression | any  | **true** | Expression whose unique, non-null values are counted |

## Example

**Use case: Count active users per application**

Suppose you want to know how many unique users have been active in a given timeframe.

### Example data

```json
{ "applicationname": "checkout-service", "username": "alice" },
{ "applicationname": "checkout-service", "username": "bob" },
{ "applicationname": "checkout-service", "username": "alice" }
```

### Example query

```dataprime
groupby $l.applicationname aggregate distinct_count($d.username) as active_users
```

### Example output

| applicationname  | active_users |
| ---------------- | ------------ |
| checkout-service | 2            |