add a Select::distinct flag

src/select.h

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2022, Tomasz Sowa
+ * Copyright (c) 2022-2023, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -50,6 +50,7 @@ public:
 		default_type = 0,
 		no_auto_generated_columns = 1,
 		with_rows_counter = 2,
+		distinct = 4,
 	};
 
 
@@ -117,6 +118,11 @@ public:
 	}
 
 
+	bool is_distinct() const
+	{
+		return is_flag_set(distinct);
+	}
+
 };
 
 }