为什么视图表使用同一个表的两个不同名称？

Question

   CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `pg2bikz3eqxbwgl5mybtxnawhccpgi`.`recalls_after_reserv` AS 
    select 
    `a`.`office_id` AS `office_id`,
    `a`.`patient_id` AS `patient_id`,
    `a`.`id` AS `recalls_id`,
    `a`.`came_at` AS `came_at`,
    `a`.`memo` AS `memo`,
    `a`.`noticed_with` AS `noticed_with`,
    `a`.`recall_menu` AS `recall_menu`,
    `a`.`recall_type_id` AS `recall_type_id`,
    `a`.`recalled_at` AS `recalled_at`,
    `a`.`recalls_at` AS `recalls_at`,
    `a`.`required_time` AS `required_time`,
    `a`.`reservation_id` AS `bef_reservation_id`,
    (case when isnull(`c`.`id`) then `d`.`id` else `c`.`id` end) AS `reservation_id`,
    (case when isnull(`c`.`id`) then `d`.`starts_at` else `c`.`starts_at` end) AS **starts_at**,
    (case when isnull(`c`.`id`) then `d`.`checked_in_at` else `c`.`checked_in_at` end) AS `checked_in_at`,
    (case when isnull(`c`.`id`) then `d`.`created_at` else `c`.`created_at` end) AS `created_at`,
    (case when isnull(`c`.`id`) then `d`.`deleted_at` else `c`.`deleted_at` end) AS `deleted_at`,
    (case when isnull(`c`.`id`) then `d`.`menu_id` else `c`.`menu_id` end) AS `next_menu_id`,
    (case when isnull(`c`.`id`) then `d`.`cancel_type` else `c`.`cancel_type` end) AS `cancel_type`,
    `a`.`menu_id` AS `menu_id`,
    `a`.`menu2_id` AS `menu2_id`,
    `a`.`menu3_id` AS `menu3_id`,
    (case when isnull(`c`.`id`) then NULL else `a`.`created_reservation_id` end) AS `created_reservation_id`,
    `z`.`datetime_disp` AS `datetime_disp` 
    from (
    ((`pg2bikz3eqxbwgl5mybtxnawhccpgi`.`recalls` `a` 
    left join `pg2bikz3eqxbwgl5mybtxnawhccpgi`.**reservations** `c` on(((`a`.`created_reservation_id` = `c`.`id`) and (isnull(`c`.`deleted_at`)
 or ((`c`.`deleted_at` is not null) and (`c`.`cancel_type` = 'patient')))))) 
    left join `pg2bikz3eqxbwgl5mybtxnawhccpgi`.`recalls_after_start_at` `z` on((`a`.`id` = `z`.`recalls_id`))) 
    left join `pg2bikz3eqxbwgl5mybtxnawhccpgi`.**reservations** `d` on(((`z`.`office_id` = `d`.`office_id`) 
    and (`z`.`patient_id` = `d`.`patient_id`) 
    and (`z`.`starts_at` = `d`.`starts_at`) 
    and (isnull(`z`.`menu_id`) or (`z`.`menu_id` = `d`.`menu_id`)) 
    and (isnull(`d`.`deleted_at`) or ((`d`.`deleted_at` is not null) and (`d`.`cancel_type` = 'patient')))))
    ) 
    where isnull(`a`.`deleted_at`);

在此视图表中，预留表有两个名称：c和d。 这就是为什么 ？有什么不同 ？如果你认为它有两个广泛的答案，你能给我一些链接来学习这些SQL吗？