diff --git a/parser/model/flags.go b/parser/model/flags.go
index 33b040edfe42c..f622ac4a621e2 100644
--- a/parser/model/flags.go
+++ b/parser/model/flags.go
@@ -40,4 +40,6 @@ const (
 	FlagIgnoreZeroInDate = 1 << 7
 	// FlagDividedByZeroAsWarning indicates if DividedByZero should be returned as warning.
 	FlagDividedByZeroAsWarning = 1 << 8
+	// FlagInUnionStmt indicates if this is a UNION statement.
+	FlagInUnionStmt = 1 << 9
 )