<fix>[TD-11459] fix bugs result is empty if stddev with group by nchar

src/client/src/tscSubquery.c

@@ -2281,6 +2281,7 @@ void tscFirstRoundRetrieveCallback(void* param, TAOS_RES* tres, int numOfRows) {
       char* p = calloc(1, pSup->tagLen);
 
       while ((row = taos_fetch_row(tres)) != NULL) {
+        int32_t* length = taos_fetch_lengths(tres);
         memset(p, 0, pSup->tagLen);
 
         int32_t offset = 0;
@@ -2294,7 +2295,7 @@ void tscFirstRoundRetrieveCallback(void* param, TAOS_RES* tres, int numOfRows) {
             } else {
               if(pExpr->base.resType == TSDB_DATA_TYPE_NCHAR){
                 int32_t output = 0;
-                bool ret = taosMbsToUcs4(varDataVal(row[i]), varDataLen(row[i]), varDataVal(p + offset), pExpr->base.resBytes, &output);
+                bool ret = taosMbsToUcs4(row[i], length[i], varDataVal(p + offset), pExpr->base.resBytes, &output);
                 if (!ret) {
                   tscError("stddev convert tag error:%d", ret);
                 }else{

tests/pytest/functions/function_stddev.py

@@ -145,7 +145,7 @@ class TDTestCase:
         tdSql.query("select stddev(col1) from stdtableint group by num")
         tdSql.checkData(0, 0, 0.5)
         tdSql.checkData(1, 0, 0.5)
-        
+
     def stop(self):
         tdSql.close()
         tdLog.success("%s successfully executed" % __file__)