# Aggregation Function

Aggregation functions are essential for summarizing data in meaningful ways when creating a measure. These functions operate on a set of values and return a single summarized result.

### Supported Aggregation Functions

ReOrc supports the following aggregation functions:

#### 1. `SUM`

Calculates the total sum of a numeric field.

**Example:**

```
SUM(revenue)
```

*Returns the total revenue across all records.*

#### 2. `COUNT`

Counts the number of records.

**Example:**

```
COUNT(order_id)
```

*Returns the total number of orders.*

#### 3. `COUNT DISTINCT`

Counts the number of unique values in a field.

**Example:**

```
COUNT_DISTINCT(customer_id)
```

*Returns the number of unique customers.*

#### 4. `AVG`

Computes the average value of a numeric field.

**Example:**

```
AVG(order_amount)
```

*Returns the average order amount.*

#### 5. `MIN`

Finds the smallest value in a field.

**Example:**

```
MIN(price)
```

*Returns the lowest product price.*

#### 6. `MAX`

Finds the largest value in a field.

**Example:**

```
MAX(price)
```

*Returns the highest product price.*

#### 7. `CUSTOM`

Allows users to define their own aggregation logic using formulas.

**Example:**

```
SUM(revenue) / COUNT(order_id)
```

*Calculates the average revenue per order.*

### Usage in Measures

When defining a measure, an aggregation function must be specified to determine how the data should be summarized. Below is an example of how an aggregation function is used to define a measure:

```
measures:
  - name: total_revenue
    label: "Total Revenue"
    type: number
    aggregation: SUM
    expression: "SUM(revenue)"
```

This measure calculates the total revenue by summing up all revenue values.