Function avg 

pub fn avg<ST: Foldable, expr>(
    expr: expr,
) -> avg<ST, <expr as AsExpression<ST>>::Expression>
where
    expr: AsExpression<ST>,

Represents a SQL AVG function. This function can only take types which are Foldable.

Window Function Usage

This function can be used as window function. See WindowExpressionMethods for details

Aggregate Function Expression

This function can be used as aggregate expression. See AggregateExpressionMethods for details.

Examples

Normal function usage

diesel::insert_into(numbers)
    .values(&vec![number.eq(1), number.eq(2)])
    .execute(conn)?;
let average = numbers.select(avg(number)).get_result(conn)?;
let expected = "1.5".parse::<BigDecimal>().unwrap();
assert_eq!(Some(expected), average);

Window function

fn run_test() -> QueryResult<()> {
let res = animals.select((name, avg(legs).partition_by(id))).load::<(Option<String>, Option<BigDecimal>)>(connection)?;

assert_eq!(vec![
        (Some("Jack".into()), "4".parse::<BigDecimal>().ok()),
        (None, "8".parse::<BigDecimal>().ok()),
    ], res);

Aggregate function expression

diesel::insert_into(numbers)
    .values(&vec![number.eq(1), number.eq(2), number.eq(3)])
    .execute(conn)?;

let average = numbers.select(avg(number).aggregate_filter(number.lt(3))).get_result(conn)?;
let expected = "1.5".parse::<BigDecimal>().unwrap();
assert_eq!(Some(expected), average);