Function sum_distinct 

pub fn sum_distinct<'a, V, E>(
    expr: E,
) -> SQLExpr<'a, V, <E::SQLType as AggregatePolicy<V::DialectMarker>>::Sum, Null, Agg>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    E::SQLType: AggregatePolicy<V::DialectMarker>,

SUM(DISTINCT expr) - sums distinct numeric values.

Requires the expression to be Numeric. Result type is dialect-aware.