code: private function fetchPrepareUnion()
      {
          foreach ($this->request[ 'union' ] as $request) {
              if (count($this->getSelect()) != count($request[ 'request' ]->getSelect())) {
                  throw new ColumnsNotFoundException('The number of fields in the selections are different : '
                  . implode(',', $this->getSelect())
                  . ' != '
                  . implode(',', $request[ 'request' ]->getSelect()));
              }
          }
      }
docs: Vérifie la cohérence des champs dans chaque requêtes entre les UNIONS.
  
  @throws ColumnsNotFoundException