fixes and improvements

exceptions related code needs work

parser.h

@@ -1,7 +1,7 @@
 #pragma once
 
 #include "lexer.h"
-#include <stdexcept>
+#include "exception.h"
 #include <string>
 
 #include <vector>
@@ -16,6 +16,7 @@ namespace usql {
 
     enum class NodeType {
 	true_node,
+	null_value,
 	int_value,
 	float_value,
 	string_value,
@@ -65,7 +66,7 @@ namespace usql {
 	int length;
 	bool null;
 
-	ColDefNode(const std::string col_name, const ColumnType col_type, int col_order, int col_len, bool nullable) :
+	ColDefNode(const std::string col_name,  ColumnType col_type, int col_order, int col_len, bool nullable) :
 		    Node(NodeType::column_def), name(col_name), type(col_type), order(col_order), length(col_len),
 		    null(nullable) {}
     };
@@ -93,6 +94,17 @@ namespace usql {
 	virtual ~ValueNode() {};
     };
 
+    struct NullValueNode : ValueNode {
+
+        NullValueNode() : ValueNode(NodeType::null_value) {}
+
+        bool isNull() override { return true; }
+
+        long getIntValue() override { throw Exception("not supported on null value"); };
+        double getDoubleValue() override { throw Exception("not supported on null value"); };
+        std::string getStringValue() override { throw Exception("not supported on null value"); };
+    };
+
     struct IntValueNode : ValueNode {
 	long value;