     }
 }
 
+// TODO: add support for ABS(aggregate)
+// TODO: add support for LENGTH(aggregate)
+// TODO: add support for IFNULL(aggregate, ...)
+// TODO: add support for IFNULL(..., aggregate)

    }
}

/// :nodoc:
public func !== <RowDecoder>(lhs: AssociationAggregate<RowDecoder>, rhs: SQLExpressible) -> AssociationAggregate<RowDecoder> {
    return AssociationAggregate { request in
        let (request, expression) = lhs.prepare(request)

    }
}

/// Returns an aggregate that sums an aggregate with the `+` SQL operator.
///
/// For example:
///
///     let request = Author.annotated(with: 1 + Author.books.count)
public func + <RowDecoder>(lhs: SQLExpressible, rhs: AssociationAggregate<RowDecoder>) -> AssociationAggregate<RowDecoder> {
    return AssociationAggregate { request in
        let (request, expression) = rhs.prepare(request)
