Function query_scalar 

pub fn query_scalar<'q, DB, O>(
    sql: &'q str,
) -> QueryScalar<'q, DB, O, <DB as HasArguments<'q>>::Arguments>
where
    DB: Database,
    (O,): for<'r> FromRow<'r, DB::Row>,

Make a SQL query that is mapped to a single concrete type using FromRow.