diff --git a/include/sqlpp11/core/aggregate_function/avg.h b/include/sqlpp11/core/aggregate_function/avg.h
index f1a8893a..aff05077 100644
--- a/include/sqlpp11/core/aggregate_function/avg.h
+++ b/include/sqlpp11/core/aggregate_function/avg.h
@@ -1,7 +1,7 @@
 #pragma once
 
 /*
- * Copyright (c) 2013-2020, Roland Bock, MacDue
+ * Copyright (c) 2013-2020, Roland Bock
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without modification,
@@ -27,16 +27,20 @@
  */
 
 #include <sqlpp11/core/operator/enable_as.h>
+#include <sqlpp11/core/operator/enable_comparison.h>
 #include <sqlpp11/core/aggregate_function/enable_over.h>
 #include <sqlpp11/core/type_traits.h>
 
 namespace sqlpp
 {
   template <typename Flag, typename Expr>
-  struct avg_t : public enable_as<avg_t<Flag, Expr>>, public enable_over<avg_t<Flag, Expr>>
+  struct avg_t : public enable_as<avg_t<Flag, Expr>>,
+                 public enable_comparison<avg_t<Flag, Expr>>,
+                 public enable_over<avg_t<Flag, Expr>>
   {
-    using _nodes = detail::type_vector<Expr, aggregate_function>;
-    using _can_be_null = std::true_type;
+    using _traits = make_traits<integral, tag::is_expression /*, tag::is_selectable*/>;
+
+    using _can_be_null = std::false_type;
     using _is_aggregate_expression = std::true_type;
 
     constexpr avg_t(Expr expr) : _expr(std::move(expr))
@@ -52,26 +56,18 @@ namespace sqlpp
     Expr _expr;
   };
 
+  template <typename Flag, typename Expr>
+  struct nodes_of<avg_t<Flag, Expr>>
+  {
+    using type = sqlpp::detail::type_vector<Expr>;
+  };
+
   template <typename Flag, typename Expr>
   struct value_type_of<avg_t<Flag, Expr>>
   {
     using type = sqlpp::force_optional_t<floating_point>;
   };
 
-  template <typename Context, typename Flag, typename Expr>
-  Context& serialize(Context& context, const avg_t<Flag, Expr>& t)
-  {
-    context << "AVG(";
-    if (std::is_same<distinct_t, Flag>::value)
-    {
-      serialize(context, Flag());
-      context << ' ';
-    }
-    serialize_operand(context, t._expr);
-    context << ")";
-    return context;
-  }
-
   template <typename T>
   using check_avg_arg =
       ::sqlpp::enable_if_t<(is_numeric<T>::value or is_boolean<T>::value) and not contains_aggregate_function_t<T>::value>;
diff --git a/include/sqlpp11/core/aggregate_function/count.h b/include/sqlpp11/core/aggregate_function/count.h
index 40c22131..8bf49d52 100644
--- a/include/sqlpp11/core/aggregate_function/count.h
+++ b/include/sqlpp11/core/aggregate_function/count.h
@@ -27,6 +27,7 @@
  */
 
 #include <sqlpp11/core/operator/enable_as.h>
+#include <sqlpp11/core/operator/enable_comparison.h>
 #include <sqlpp11/core/aggregate_function/enable_over.h>
 #include <sqlpp11/core/clause/select_flags.h>
 #include <sqlpp11/core/type_traits.h>
@@ -34,15 +35,16 @@
 namespace sqlpp
 {
   template <typename Flag, typename Expr>
-  struct count_t : public enable_as<count_t<Flag, Expr>>, public enable_over<count_t<Flag, Expr>>
+  struct count_t : public enable_as<count_t<Flag, Expr>>,
+                   public enable_comparison<count_t<Flag, Expr>>,
+                   public enable_over<count_t<Flag, Expr>>
   {
     using _traits = make_traits<integral, tag::is_expression /*, tag::is_selectable*/>;
 
-    using _nodes = detail::type_vector<Expr, aggregate_function>;
     using _can_be_null = std::false_type;
     using _is_aggregate_expression = std::true_type;
 
-    constexpr count_t(const Expr expr) : _expr(std::move(expr))
+    constexpr count_t(Expr expr) : _expr(std::move(expr))
     {
     }
 
@@ -55,6 +57,12 @@ namespace sqlpp
     Expr _expr;
   };
 
+  template <typename Flag, typename Expr>
+  struct nodes_of<count_t<Flag, Expr>>
+  {
+    using type = sqlpp::detail::type_vector<Expr>;
+  };
+
   template <typename Flag, typename Expr>
   struct value_type_of<count_t<Flag, Expr>>
   {
diff --git a/include/sqlpp11/core/aggregate_function/enable_over.h b/include/sqlpp11/core/aggregate_function/enable_over.h
index 2bad1aa8..37724d7f 100644
--- a/include/sqlpp11/core/aggregate_function/enable_over.h
+++ b/include/sqlpp11/core/aggregate_function/enable_over.h
@@ -47,4 +47,10 @@ namespace sqlpp
     }
   };
 
+#warning: Need to use in tests
+  template <typename T>
+  struct has_enabled_over : public std::is_base_of<enable_over<T>, T>
+  {
+  };
+
 }  // namespace sqlpp
diff --git a/include/sqlpp11/core/aggregate_function/max.h b/include/sqlpp11/core/aggregate_function/max.h
index dd43acc1..9267fa71 100644
--- a/include/sqlpp11/core/aggregate_function/max.h
+++ b/include/sqlpp11/core/aggregate_function/max.h
@@ -27,6 +27,7 @@
  */
 
 #include <sqlpp11/core/operator/enable_as.h>
+#include <sqlpp11/core/operator/enable_comparison.h>
 #include <sqlpp11/core/aggregate_function/enable_over.h>
 #include <sqlpp11/core/clause/select_flags.h>
 #include <sqlpp11/core/type_traits.h>
@@ -34,10 +35,11 @@
 namespace sqlpp
 {
   template <typename Flag, typename Expr>
-  struct max_t : public enable_as<max_t<Flag, Expr>>, public enable_over<max_t<Flag, Expr>>
+  struct max_t : public enable_as<max_t<Flag, Expr>>,
+                 public enable_comparison<max_t<Flag, Expr>>,
+                 public enable_over<max_t<Flag, Expr>>
   {
     using _traits = make_traits<value_type_of_t<Expr>, tag::is_expression, tag::is_selectable>;
-    using _nodes = detail::type_vector<Expr, aggregate_function>;
     using _can_be_null = std::true_type;
     using _is_aggregate_expression = std::true_type;
 
@@ -54,6 +56,12 @@ namespace sqlpp
     Expr _expr;
   };
 
+  template <typename Flag, typename Expr>
+  struct nodes_of<max_t<Flag, Expr>>
+  {
+    using type = sqlpp::detail::type_vector<Expr>;
+  };
+
   template <typename Flag, typename Expr>
   struct value_type_of<max_t<Flag, Expr>>
   {
diff --git a/include/sqlpp11/core/aggregate_function/min.h b/include/sqlpp11/core/aggregate_function/min.h
index a1f983e3..a36351b2 100644
--- a/include/sqlpp11/core/aggregate_function/min.h
+++ b/include/sqlpp11/core/aggregate_function/min.h
@@ -27,6 +27,7 @@
  */
 
 #include <sqlpp11/core/operator/enable_as.h>
+#include <sqlpp11/core/operator/enable_comparison.h>
 #include <sqlpp11/core/aggregate_function/enable_over.h>
 #include <sqlpp11/core/clause/select_flags.h>
 #include <sqlpp11/core/type_traits.h>
@@ -34,10 +35,11 @@
 namespace sqlpp
 {
   template <typename Flag, typename Expr>
-  struct min_t : public enable_as<min_t<Flag, Expr>>, enable_over<min_t<Flag, Expr>>
+  struct min_t : public enable_as<min_t<Flag, Expr>>,
+                 public enable_comparison<min_t<Flag, Expr>>,
+                 enable_over<min_t<Flag, Expr>>
   {
     using _traits = make_traits<value_type_of_t<Expr>, tag::is_expression, tag::is_selectable>;
-    using _nodes = detail::type_vector<Expr, aggregate_function>;
     using _can_be_null = std::true_type;
     using _is_aggregate_expression = std::true_type;
 
@@ -54,6 +56,12 @@ namespace sqlpp
     Expr _expr;
   };
 
+  template <typename Flag, typename Expr>
+  struct nodes_of<min_t<Flag, Expr>>
+  {
+    using type = sqlpp::detail::type_vector<Expr>;
+  };
+
   template <typename Flag, typename Expr>
   struct value_type_of<min_t<Flag, Expr>>
   {
diff --git a/include/sqlpp11/core/aggregate_function/over.h b/include/sqlpp11/core/aggregate_function/over.h
index 0a6ab11d..b8123e05 100644
--- a/include/sqlpp11/core/aggregate_function/over.h
+++ b/include/sqlpp11/core/aggregate_function/over.h
@@ -33,10 +33,10 @@
 namespace sqlpp
 {
   template <typename Expr>
-  struct over_t : public enable_as<over_t<Expr>>, public enable_comparison<over_t<Expr>>
+  struct over_t : public enable_as<over_t<Expr>>,
+                  public enable_comparison<over_t<Expr>>
   {
     using _traits = make_traits<integral, tag::is_expression>;
-    using _nodes = detail::type_vector<Expr, aggregate_function>;
 
     over_t(Expr expr)
       : _expr(expr)
@@ -53,11 +53,15 @@ namespace sqlpp
   };
 
   template<typename Expr>
-  struct value_type_of<over_t<Expr>>: public value_type_of<Expr> {};
+  struct nodes_of<over_t<Expr>>
+  {
+    using type = sqlpp::detail::type_vector<Expr>;
+  };
 
   template<typename Expr>
-  struct nodes_of<over_t<Expr>>: public nodes_of<Expr> {};
+  struct value_type_of<over_t<Expr>>: public value_type_of<Expr> {};
 
+#warning: should this be "is_aggregate_function"?
   template<typename Expr>
   using check_over_args = ::sqlpp::enable_if_t<contains_aggregate_function_t<Expr>::value>;
 
diff --git a/include/sqlpp11/core/aggregate_function/sum.h b/include/sqlpp11/core/aggregate_function/sum.h
index 392f8c1b..7fd04ce9 100644
--- a/include/sqlpp11/core/aggregate_function/sum.h
+++ b/include/sqlpp11/core/aggregate_function/sum.h
@@ -27,6 +27,7 @@
  */
 
 #include <sqlpp11/core/operator/enable_as.h>
+#include <sqlpp11/core/operator/enable_comparison.h>
 #include <sqlpp11/core/aggregate_function/enable_over.h>
 #include <sqlpp11/core/clause/select_flags.h>
 #include <sqlpp11/core/type_traits.h>
@@ -34,10 +35,11 @@
 namespace sqlpp
 {
   template <typename Flag, typename Expr>
-  struct sum_t : public enable_as<sum_t<Flag, Expr>>, enable_over<sum_t<Flag, Expr>>
+  struct sum_t : public enable_as<sum_t<Flag, Expr>>,
+                 public enable_comparison<sum_t<Flag, Expr>>,
+                 enable_over<sum_t<Flag, Expr>>
   {
     using _traits = make_traits<value_type_of_t<Expr>, tag::is_expression, tag::is_selectable>;
-    using _nodes = detail::type_vector<Expr, aggregate_function>;
     using _can_be_null = std::true_type;
     using _is_aggregate_expression = std::true_type;
 
@@ -54,6 +56,12 @@ namespace sqlpp
     Expr _expr;
   };
 
+  template <typename Flag, typename Expr>
+  struct nodes_of<sum_t<Flag, Expr>>
+  {
+    using type = sqlpp::detail::type_vector<Expr>;
+  };
+
   template <typename Flag, typename Expr>
   struct value_type_of<sum_t<Flag, Expr>>
   {
diff --git a/include/sqlpp11/core/type_traits.h b/include/sqlpp11/core/type_traits.h
index 726bc815..9ebdc787 100644
--- a/include/sqlpp11/core/type_traits.h
+++ b/include/sqlpp11/core/type_traits.h
@@ -725,12 +725,6 @@ namespace sqlpp
     using _tags = detail::make_type_set_t<Tags...>;
   };
 
-  struct aggregate_function
-  {
-    using _nodes = detail::type_vector<>;
-    using _contains_aggregate_function = std::true_type;
-  };
-
   template <typename NameTagProvider, typename Member>
   using member_t = typename name_tag_of_t<NameTagProvider>::template _member_t<Member>;
 
diff --git a/tests/core/types/aggregate_function.cpp b/tests/core/types/aggregate_function.cpp
index bc21e3e9..804a1664 100644
--- a/tests/core/types/aggregate_function.cpp
+++ b/tests/core/types/aggregate_function.cpp
@@ -53,9 +53,26 @@ void test_aggregate_functions(Value v)
   static_assert(is_same_type<decltype(max(v_maybe_null)), OptValueType>::value, "");
   static_assert(is_same_type<decltype(min(v_maybe_null)), OptValueType>::value, "");
 
-#warning: test can be aliased
-#warning: test has comparison operators
-#warning: test nodes
+  // Aggregate functions enable the `as` member function.
+  static_assert(sqlpp::has_enabled_as<decltype(count(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_as<decltype(max(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_as<decltype(min(v_not_null))>::value, "");
+
+  // Aggregate functions enable comparison member functions.
+  static_assert(sqlpp::has_enabled_comparison<decltype(count(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_comparison<decltype(max(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_comparison<decltype(min(v_not_null))>::value, "");
+
+  // Aggregate functions enable OVER.
+  static_assert(sqlpp::has_enabled_over<decltype(count(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_over<decltype(max(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_over<decltype(min(v_not_null))>::value, "");
+
+  // Aggregate functions have their arguments as nodes
+  using L = typename std::decay<decltype(v_not_null)>::type;
+  static_assert(std::is_same<sqlpp::nodes_of_t<decltype(count(v_not_null))>, sqlpp::detail::type_vector<L>>::value, "");
+  static_assert(std::is_same<sqlpp::nodes_of_t<decltype(max(v_not_null))>, sqlpp::detail::type_vector<L>>::value, "");
+  static_assert(std::is_same<sqlpp::nodes_of_t<decltype(min(v_not_null))>, sqlpp::detail::type_vector<L>>::value, "");
 }
 
 template <typename Value>
@@ -75,6 +92,25 @@ void test_numeric_aggregate_functions(Value v)
   // Aggregate of nullable
   static_assert(is_same_type<decltype(sum(v_maybe_null)), OptValueType>::value, "");
   static_assert(is_same_type<decltype(avg(v_maybe_null)), OptFloat>::value, "");
+
+  // Aggregate functions enable the `as` member function.
+  static_assert(sqlpp::has_enabled_as<decltype(sum(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_as<decltype(avg(v_not_null))>::value, "");
+
+  // Aggregate functions enable OVER.
+  static_assert(sqlpp::has_enabled_over<decltype(sum(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_over<decltype(avg(v_not_null))>::value, "");
+
+  // Aggregate functions enable comparison member functions.
+  static_assert(sqlpp::has_enabled_comparison<decltype(sum(v_not_null))>::value, "");
+  static_assert(sqlpp::has_enabled_comparison<decltype(avg(v_not_null))>::value, "");
+
+  // Aggregate functions have their arguments as nodes
+  using L = typename std::decay<decltype(v_not_null)>::type;
+  static_assert(std::is_same<sqlpp::nodes_of_t<decltype(sum(v_not_null))>, sqlpp::detail::type_vector<L>>::value, "");
+  static_assert(std::is_same<sqlpp::nodes_of_t<decltype(avg(v_not_null))>, sqlpp::detail::type_vector<L>>::value, "");
+
+#warning: test enable_over
 }
 
 int main()