[SQL] REFERENCES 外來鍵的用法(含範例與mysql)

Tue, 01 Mar 2022 07:20:41 +0000

下方演示: MySQL
GUI: tabPlus

在學習SQL時一定會學到外來鍵，外來鍵的目的是確定資料的參考完整性(referential integrity)，簡單來說，就是只有被准許的資料值才會被存入資料庫內

下方以一個生活例子來舉例:
1個頻道(電視台)會依據不同時間有不同節目，當然沒有頻道就不會有節目撥出，也就是一對多。如下圖:

頻道:

CREATE TABLE `channel` (
 `id` int PRIMARY KEY,
 `channel_name` varchar(255) NOT NULL
);

節目:

CREATE TABLE `programme` (
 `id` int PRIMARY KEY AUTO_INCREMENT,
 `channel_id` int,
 `programme_name` varchar(255),
 `start_time` datetime DEFAULT (now()),
 `end_time` datetime DEFAULT (now()),
 FOREIGN KEY(channel_id) REFERENCES channel(id)
);

加入資料1:

INSERT INTO `channel` (`id`, `channel_name`) VALUES
(7, '恩恩新聞'),
(8, '探索世界'),
(9, '東西購物');

加入資料2:

INSERT INTO `programme` (`channel_id`, `programme_name`, `start_time`, `end_time`) VALUES
(7, '3點新聞', '2022-03-01 15:00:00', '2022-03-01 16:00:00'),
(7, '4點新聞', '2022-03-01 16:00:00', '2022-03-01 17:00:00'),
(7, '5點新聞', '2022-03-01 17:00:00', '2022-03-01 18:00:00'),
(8, '說書人', '2022-03-01 15:00:00', '2022-03-01 16:00:00'),
(8, '科學時間', '2022-03-01 16:00:00', '2022-03-01 17:00:00'),
(8, '海底裡', '2022-03-01 17:00:00', '2022-03-01 18:00:00'),
(9, '3點購物時間', '2022-03-01 15:00:00', '2022-03-01 16:00:00'),
(9, '4點購物時間', '2022-03-01 16:00:00', '2022-03-01 17:00:00'),
(9, '5點購物時間', '2022-03-01 17:00:00', '2022-03-01 18:00:00'),

嘗試當加入一筆沒有在channel內的id

INSERT INTO `programme` (`channel_id`, `programme_name`, `start_time`, `end_time`) VALUES
(10, '一起出去玩', '2022-03-01 17:00:00', '2022-03-01 18:00:00');

錯誤訊息如下:

Cannot add or update a child row: a foreign key constraint fails (`general`.`programme`, CONSTRAINT `programme_ibfk_1` FOREIGN KEY (`channel_id`) REFERENCES `channel` (`id`))

補充

當表已經建立完畢後，設置外來鍵:

ALTER TABLE `programme` ADD FOREIGN KEY (`channel_id`) REFERENCES `channel` (`id`);

可參考:fooish

[MySQL] MySQL InternalError: 1251, u’Client does not support authentication protocol requested by server; consider upgrading MySQL client

Tue, 11 May 2021 11:13:00 +0000

某次在local測試時，因為要新增package而重新build了一次docker。
後來更新get-pip出現錯誤訊息如下:

InternalError: (1251, u'Client does not support authentication protocol requested by server; consider upgrading MySQL client')

客戶端不支持服務器請求的身份驗證協議；請考慮升級MySQL客戶端

目前是使用python2.7，MySQL 8.0.23，
不推薦將MYSQL降級，可以通過在創建數據庫時告訴MySQL服務器使用舊版身份驗證插件來實現此目的。

ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'password';
flush privileges;

如果不行的話，將@’localhost’拿掉，再重新執行一次，下方還有其他指令，可嘗試以下指令擇一試看看。

ALTER USER 'root'@'%' IDENTIFIED WITH mysql_native_password BY '123456';
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY '123456';
ALTER USER 'root'@'localhost' IDENTIFIED BY '123456' PASSWORD EXPIRE NEVER;
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY '123456';
FLUSH PRIVILEGES; # 最後一定要加這段

我是將@’localhost’拿掉後，才成功的。若失敗的話，請參考下方連結，內有許多解決方式。
參考連結:stackoverflow、Mysql 解決1251

Mysql on Quietbo

[SQL] REFERENCES 外來鍵的用法(含範例與mysql)

[MySQL] MySQL InternalError: 1251, u’Client does not support authentication protocol requested by server; consider upgrading MySQL client