feat:bug fix

@@ -210,7 +210,7 @@ class MysqlCli
                 }
                 $value[1] = $this->transferValue($value[1]);
                 if ($value[1] === 'NULL') {
-                    $tmp = $value[0] . ' is NULL';
+                    $tmp .= $value[0] . ' is NULL';
                 } else {
                     $symbol = $value[2] ?? '=';
                     $tmp    .= $value[0] . ' ' . $symbol . ' ' . $value[1];