diff --git a/row.cpp b/row.cpp
index 66001e5..71561c2 100644
--- a/row.cpp
+++ b/row.cpp
@@ -3,6 +3,26 @@
 
 namespace usql {
 
+    int ColNullValue::compare(ColValue * other) {
+    	return other->isNull() ? 0 : -1;	// null goes to end
+    }
+
+    int ColIntegerValue::compare(ColValue * other) {
+	return other->isNull() ? 1 : m_integer - other->getIntValue();	// null goes to end
+    }
+
+    int ColDoubleValue::compare(ColValue * other) {
+    	if (other->isNull()) {	// null goes to end
+    		return 1;
+    	}
+    	double c = m_double - other->getDoubleValue();
+    	return c < 0 ? -1 : c == 0.0 ? 0 : 1;
+    }
+
+    int ColStringValue::compare(ColValue * other) {
+    	return other->isNull() ? 1 : m_string.compare(other->getStringValue());	// null goes to end
+    }
+
     Row::Row(int cols_count) {
 	    m_columns.reserve(cols_count);
 	    for (int i = 0; i < cols_count; i++) {
diff --git a/row.h b/row.h
index 3bd92ae..edbc641 100644
--- a/row.h
+++ b/row.h
@@ -11,16 +11,24 @@ namespace usql {
     struct ColValue {
 
 	virtual bool isNull() { return false; };
-	virtual long getIntValue() { throw Exception("Not supported"); };
-	virtual double getDoubleValue() { throw Exception("Not supported"); };
-	virtual std::string getStringValue() { throw Exception("Not supported"); };
+	virtual long getIntValue() = 0;
+	virtual double getDoubleValue() = 0;
+	virtual std::string getStringValue() = 0;
+
+	virtual int compare(ColValue * other) = 0;
+
+	virtual ~ColValue() = default;
     };
 
 
     struct ColNullValue : ColValue {
 
-	virtual bool isNull() { return true; };
-	virtual std::string getStringValue() { return "null"; };
+        bool isNull() override { return true; };
+	long getIntValue() override { throw Exception("Not supported"); };
+	double getDoubleValue() override { throw Exception("Not supported"); };
+	std::string getStringValue() override { return "null"; };
+
+	int compare(ColValue * other) override;
     };
 
 
@@ -29,11 +37,13 @@ namespace usql {
 	ColIntegerValue(long value) : m_integer(value) {};
 	ColIntegerValue(const ColIntegerValue &other) : m_integer(other.m_integer) {};
 
-	virtual long getIntValue() { return m_integer; };
-	virtual double getDoubleValue() { return (double) m_integer; };
-	virtual std::string getStringValue() { return std::to_string(m_integer); };
+	long getIntValue() override { return m_integer; };
+	double getDoubleValue() override { return (double) m_integer; };
+	std::string getStringValue() override { return std::to_string(m_integer); };
 
-	int m_integer;
+	int compare(ColValue * other) override;
+
+	long m_integer;
     };
 
 
@@ -42,9 +52,11 @@ namespace usql {
 	ColDoubleValue(double value) : m_double(value) {};
 	ColDoubleValue(const ColDoubleValue &other) : m_double(other.m_double) {}
 
-	virtual long getIntValue() { return (long) m_double; };
-	virtual double getDoubleValue() { return m_double; };
-	virtual std::string getStringValue() { return std::to_string(m_double); };
+	long getIntValue() override { return (long) m_double; };
+	double getDoubleValue() override { return m_double; };
+	std::string getStringValue() override { return std::to_string(m_double); };
+
+	int compare(ColValue * other) override;
 
 	double m_double;
     };
@@ -52,12 +64,14 @@ namespace usql {
 
     struct ColStringValue : ColValue {
 
-	ColStringValue(const std::string value) : m_string(value) {};
+	ColStringValue(const std::string &value) : m_string(value) {};
 	ColStringValue(const ColStringValue &other) : m_string(other.m_string) {};
 
-	virtual long getIntValue() { return std::stoi(m_string); };
-	virtual double getDoubleValue() { return std::stod(m_string); };
-	virtual std::string getStringValue() { return m_string; };
+	long getIntValue() override { return std::stoi(m_string); };
+	double getDoubleValue() override { return std::stod(m_string); };
+	std::string getStringValue() override { return m_string; };
+
+	int compare(ColValue * other) override;
 
 	std::string m_string;
     };
diff --git a/usql.cpp b/usql.cpp
index d54e447..c9b6ac3 100644
--- a/usql.cpp
+++ b/usql.cpp
@@ -208,11 +208,8 @@ void USql::execute_order_by(SelectFromTableNode &node, Table *table, Table *resu
 			ColValue *a_val = a.ith_column(col_def.order);
 			ColValue *b_val = b.ith_column(col_def.order);
 
-			if (a_val->isNull() && b_val->isNull()) return true;	// both is null so a goes to end
-			if (!a_val->isNull() && b_val->isNull()) return true;	// b is null so goes to end
-			if (a_val->isNull() && !b_val->isNull()) return false;	// a is null so goes to end
+			int compare = a_val->compare(b_val);
 
-			int compare = compare_col_values(col_def, a_val, b_val);
 			if (compare < 0) return order_by_col_def.ascending ? true : false;
 			if (compare > 0) return order_by_col_def.ascending ? false : true;
 		}
@@ -231,21 +228,6 @@ void USql::execute_offset_limit(OffsetLimitNode &node, Table *result) const {
 		result->m_rows.erase(result->m_rows.begin() + node.limit, result->m_rows.end());
 }
 
-int USql::compare_col_values(const ColDefNode &col_def, ColValue *a_val, ColValue *b_val) const {
-	double c;
-	switch (col_def.type) {
-		case (ColumnType::integer_type):
-			return a_val->getIntValue() - b_val->getIntValue();
-		case (ColumnType::float_type):
-			 c = a_val->getDoubleValue() - b_val->getDoubleValue();
-			 return c < 0 ? -1 : c==0.0 ? 0 : 1; 
-		case (ColumnType::varchar_type):
-			return a_val->getStringValue().compare(b_val->getStringValue());
-		default: 
-			throw Exception("Unsupported data type");
-	}
-}
-
 std::tuple<int, ColDefNode> USql::get_column_definition(Table *table, SelectColNode *select_col_node, int col_order ) {
 	std::string new_col_name = select_col_node->name;
 
diff --git a/usql.h b/usql.h
index 0178b79..2969532 100644
--- a/usql.h
+++ b/usql.h
@@ -55,8 +55,6 @@ private:
     Parser m_parser;
     std::list<Table> m_tables;
 
-    int compare_col_values(const ColDefNode &col_def, ColValue *a_val, ColValue *b_val) const;
-
     void execute_order_by(SelectFromTableNode &node, Table *table, Table *result) const;
 
     void execute_offset_limit(OffsetLimitNode &node, Table *result) const;