block union for nested query

src/client/src/tscSQLParser.c

@@ -888,6 +888,7 @@ int32_t tscValidateSqlInfo(SSqlObj* pSql, struct SSqlInfo* pInfo) {
     }
 
     case TSDB_SQL_SELECT: {
+      const char * msg1 = "no nested query supported in union clause";
       code = loadAllTableMeta(pSql, pInfo);
       if (code != TSDB_CODE_SUCCESS) {
         return code;
@@ -901,6 +902,10 @@ int32_t tscValidateSqlInfo(SSqlObj* pSql, struct SSqlInfo* pInfo) {
 
         tscTrace("0x%"PRIx64" start to parse the %dth subclause, total:%"PRIzu, pSql->self, i, size);
 
+        if (size > 1 && pSqlNode->from->type == SQL_NODE_FROM_SUBQUERY) {
+          return invalidOperationMsg(tscGetErrorMsgPayload(pCmd), msg1);
+        }
+
 //        normalizeSqlNode(pSqlNode); // normalize the column name in each function
         if ((code = validateSqlNode(pSql, pSqlNode, pQueryInfo)) != TSDB_CODE_SUCCESS) {
           return code;