diff --git a/tests/system-test/2-query/union.py b/tests/system-test/2-query/union.py
index a03a114ced..5b3e27affa 100644
--- a/tests/system-test/2-query/union.py
+++ b/tests/system-test/2-query/union.py
@@ -46,6 +46,8 @@ class TDTestCase:
                     f"rtrim( {tbname}.{char_col} )",
                     f"substr( {tbname}.{char_col}, 1 )",
                     f"count( {tbname}.{char_col} )",
+                    f"cast( {tbname}.{char_col} as nchar(3) )",
+                    f"cast( {tbname}.{char_col} as nchar(8) )",
                 )
             )
             query_condition.extend( f"cast( {tbname}.{un_char_col} as binary(16) ) " for un_char_col in NUM_COL)
@@ -128,6 +130,8 @@ class TDTestCase:
         return f" group by {col} having {having}" if having else f" group by {col} "
 
     def __single_sql(self, select_clause, from_clause, where_condition="", group_condition=""):
+        if "on" not in from_clause and select_clause.split(".")[0] != from_clause.split(".")[0]:
+            return
         return f"select {select_clause} from {from_clause} {where_condition} {group_condition}"