SQL - Pertanyaan Gabungan Dalaman Asas

Question

Saya mempunyai 3 MySQL jadual dan saya mahu menyertainya bersama-sama.

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Kursus:

CREATE TABLE `library_classes` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3;

INSERT INTO `library_classes` (`id`, `name`) VALUES
(1, 'Tolkien'),
(2, 'CS Lewis');

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Perbualan:

CREATE TABLE `library_sessions` (
  `id` int NOT NULL AUTO_INCREMENT,
  `class_id` int NOT NULL,
  `session_timestamp` timestamp NOT NULL,
  PRIMARY KEY (`id`),
  KEY `class_id` (`class_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3;

INSERT INTO `library_sessions` (`id`, `class_id`, `session_timestamp`) VALUES
(1, 1,  '2023-08-01 15:30:00'),
(2, 2,  '2023-08-02 10:15:00'),
(3, 1,  '2023-08-04 09:30:00');

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Tempahan:

CREATE TABLE `library_bookings` (
  `id` int NOT NULL AUTO_INCREMENT,
  `session_id` int NOT NULL,
  `email` varchar(255) NOT NULL,
  `datetime_submitted` timestamp NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3;

INSERT INTO `library_bookings` (`id`, `session_id`, `email`) VALUES
(1, 1,  'jose@gmail.com'),
(2, 2,  'jane@yahoo.com'),
(3, 2,  'john@hotmail.com');

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Apabila jose@gmail.com dilog masuk, saya mahu halaman saya kelihatan seperti ini:

Tolkien | 2023-08-01 15:30:00 | CANCEL
CS Lewis | 2023-08-02 10:15:00 | BOOK NOW
Tolkien | 2023-08-04 09:30:00 | BOOK NOW

Answer 1

Saya mengimport data anda ke dalam SQLFIDDLE;

Pertanyaan berikut akan memberi anda hasil yang diharapkan.

select lc.name, ls.session_timestamp,
if(lb.id is null, 'BOOK NOW', 'CANCEL') as booking_status
from library_classes lc
join library_sessions ls on lc.id = ls.class_id
left join library_bookings lb on ls.id = lb.session_id and lb.email = 'jose@gmail.com'
order by ls.session_timestamp;

Keluaran yang diharapkan ia berikan ialah:

name        session_timestamp       booking_status
Tolkien     2023-08-01T15:30:00Z    CANCEL
CS Lewis    2023-08-02T10:15:00Z    BOOK NOW
Tolkien     2023-08-04T09:30:00Z    BOOK NOW