diff --git a/parser.cpp b/parser.cpp
index d7334d7..c98b772 100644
--- a/parser.cpp
+++ b/parser.cpp
@@ -329,7 +329,7 @@ namespace usql {
 		m_lexer.skipToken(TokenType::keyword_by);
 
 		do {
-			int col_index  = -1;
+			int col_index  = FUNCTION_CALL;
 			bool asc = true;
 			
 			auto token_type = m_lexer.tokenType();
diff --git a/parser.h b/parser.h
index 763fdcd..a5f2d65 100644
--- a/parser.h
+++ b/parser.h
@@ -8,6 +8,8 @@
 #include <string>
 #include <vector>
 
+static const int FUNCTION_CALL = -1;
+
 namespace usql {
 
     enum class ColumnType {
diff --git a/usql.cpp b/usql.cpp
index d6d5c0c..84028b2 100644
--- a/usql.cpp
+++ b/usql.cpp
@@ -199,7 +199,7 @@ std::unique_ptr<Table> USql::execute_select(SelectFromTableNode &node) {
 			for (auto idx = 0; idx < result->columns_count(); idx++) {
 				auto row_col_index = source_table_col_index[idx];
 
-				if (row_col_index == -1) {	// TODO introduce constant here
+				if (row_col_index == FUNCTION_CALL) {
 					auto evaluated_value = eval_value_node(table, *row, node.cols_names->operator[](idx).value.get());
 					ValueNode *col_value = evaluated_value.get();