Function distinct 

pub fn distinct<'a, V, E>(
    expr: E,
) -> SQLExpr<'a, V, <E as Expr<'a, V>>::SQLType, <E as Expr<'a, V>>::Nullable>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,

DISTINCT - marks an expression as DISTINCT.

Typically used inside aggregate functions.