public static function inSpecificationSQL($field, $value)
 {
     if ($value == null) {
         return "";
     }
     if (is_array($value)) {
         $values = '';
         foreach ($value as $row) {
             if ($values == '') {
                 $values = Specification::formatValueToSQL($row);
             } else {
                 $values = $values . ',' . Specification::formatValueToSQL($row);
             }
         }
         return "({$field} in ({$values}))";
     } else {
         return Specification::equalSpecificationSQL($field, $value);
     }
 }
 function getSQL()
 {
     return Specification::equalSpecificationSQL($this->field_name, $this->field_value);
 }