OQL Aggregate Functions

The aggregate functions

```MAX```,
```AVG```,
```AVG``` over a DISTINCT expression,
```SUM``` over a DISTINCT expression,
```COUNT```, and
```COUNT``` over a DISTINCT expression
are supported.
The ```GROUP BY``` extension is also supported where appropriate.

The ```MIN``` function returns the smallest of the selected
expression.
The type of the expression must evaluate to a 
```java.lang.Comparable```.

The ```MAX``` function returns the largest of the selected
expression.
The type of the expression must evaluate to a 
```java.lang.Comparable```.

The ```AVG``` function returns the arithmetic mean of the set
formed by the selected expression.
The type of the expression must evaluate to a 
```java.lang.Number```.
For partitioned regions,
each node's buckets provide both a sum and the number of elements
to the node executing the query,
such that a correct average may be computed.

The ```AVG``` function where the DISTINCT modifier is applied
to the expression returns the arithmetic mean of the set
of unique (distinct) values.
The type of the expression must evaluate to a 
```java.lang.Number```.
For partitioned regions,
the distinct values in a node's buckets are returned
to the node executing the query.
The query node can then calculate the avarage over
the values that are unique across nodes,
after eliminating duplicate values that come from separate nodes.

The ```SUM``` function returns the sum over the set
formed by the selected expression.
The type of the expression must evaluate to a 
```java.lang.Number```.
For partitioned regions,
each node's buckets compute a sum over that node,
returning that sum
to the node executing the query,
when then sums across all nodes.

The ```SUM``` function where the DISTINCT modifier is applied
to the expression returns the sum over the set
of unique (distinct) values.
The type of the expression must evaluate to a 
```java.lang.Number```.
For partitioned regions,
the distinct values in a node's buckets are returned
to the node executing the query.
The query node can then calculate the sum over
the values that are unique across nodes,
after eliminating duplicate values that come from separate nodes.

The ```COUNT``` function returns the quantity of values in the set
formed by the selected expression.
For example, to return the quantity of employees who have a
positive sales amount:

``` pre
SELECT count(e.sales) FROM /employees e WHERE e.sales > 0.0

The COUNT function where the DISTINCT modifier is applied returns the quantity of unique (distinct) values in the set formed by the selected expression.

GROUP BY Extension for Aggregate Functions

GROUP BY is required when aggregate functions are used in combination with other selected items. It permits ordering. For example,

SELECT ID, MAX(e.sales) FROM /employees e GROUP BY ID