type
  TXSql=record
     _Open:TOpenSql;
     _Close:TCloseSql;
     _Query:TQuerySql;
    ..... думаем... :)
  end;
  TSQL = class
     private
      db: TXSql;
     ...... 
  end;

  TMySql = class(TSql)
     ....
  end;

  TSQLite = class(TSql)
     ....
  end;