# Use aggregation functions with arrays
<a name="arrays-and-aggregation"></a>
+ To add values within an array, use `SUM`, as in the following example.
+ To aggregate multiple rows within an array, use `array_agg`. For information, see [Create arrays from subqueries](creating-arrays-from-subqueries.md).

**Note**  
`ORDER BY` is supported for aggregation functions starting in Athena engine version 2.

```
WITH
dataset AS (
  SELECT ARRAY
  [
    ARRAY[1,2,3,4],
    ARRAY[5,6,7,8],
    ARRAY[9,0]
  ] AS items
),
item AS (
  SELECT i AS array_items
  FROM dataset, UNNEST(items) AS t(i)
)
SELECT array_items, sum(val) AS total
FROM item, UNNEST(array_items) AS t(val)
GROUP BY array_items;
```

In the last `SELECT` statement, instead of using `sum()` and `UNNEST`, you can use `reduce()` to decrease processing time and data transfer, as in the following example.

```
WITH
dataset AS (
  SELECT ARRAY
  [
    ARRAY[1,2,3,4],
    ARRAY[5,6,7,8],
    ARRAY[9,0]
  ] AS items
),
item AS (
  SELECT i AS array_items
  FROM dataset, UNNEST(items) AS t(i)
)
SELECT array_items, reduce(array_items, 0 , (s, x) -> s + x, s -> s) AS total
FROM item;
```

Either query returns the following results. The order of returned results is not guaranteed.

```
+----------------------+
| array_items  | total |
+----------------------+
| [1, 2, 3, 4] | 10    |
| [5, 6, 7, 8] | 26    |
| [9, 0]       | 9     |
+----------------------+
```