在我的数据库创建脚本中,创建脚本如下所示:
CREATE TABLE IF NOT EXISTS `rabbits` ( `id` INT UNSIGNED NOT NULL AUTO_INCREMENT, `name` VARCHAR(255) NOT NULL, `main_page_id` INT UNSIGNED COMMENT 'What page is the main one', PRIMARY KEY (`id`), KEY `main_page_id` (`main_page_id`) ) ENGINE=InnoDB; CREATE TABLE IF NOT EXISTS `rabbit_pages` ( `id` INT UNSIGNED NOT NULL AUTO_INCREMENT, `rabbit_id` INT UNSIGNED NOT NULL, `title` VARCHAR(255) NOT NULL, `content` TEXT NOT NULL, PRIMARY KEY (`id`), KEY `rabbit_id` (`rabbit_id`), CONSTRAINT `fk_rabbits_pages` FOREIGN KEY (`rabbit_id`) REFERENCES `rabbits` (`id`) ) ENGINE=InnoDB; ALTER TABLE `rabbits` ADD CONSTRAINT `fk_rabbits_main_page` FOREIGN KEY (`main_page_id`) REFERENCES `rabbit_pages` (`id`);
第一次运行良好,但是如果我再次运行,它将在最后一行失败,并带有“写入或更新时复制密钥”。
有什么办法可以做类似的ADD CONSTRAINT IF NOT EXISTS事情吗?就像我可以使用CREATE TABLE查询吗?
ADD CONSTRAINT IF NOT EXISTS
CREATE TABLE
有趣的问题。您可能需要在调用CREATE TABLE语句之前禁用外键,然后再启用它们。这将允许您直接在CREATE TABLEDDL中定义外键:
例:
SET FOREIGN_KEY_CHECKS = 0; Query OK, 0 rows affected (0.00 sec) CREATE TABLE IF NOT EXISTS `rabbits` ( `id` INT UNSIGNED NOT NULL AUTO_INCREMENT, `name` VARCHAR(255) NOT NULL, `main_page_id` INT UNSIGNED COMMENT 'What page is the main one', PRIMARY KEY (`id`), KEY `main_page_id` (`main_page_id`), CONSTRAINT `fk_rabbits_main_page` FOREIGN KEY (`main_page_id`) REFERENCES `rabbit_pages` (`id`) ) ENGINE=InnoDB; Query OK, 0 rows affected (0.04 sec) CREATE TABLE IF NOT EXISTS `rabbit_pages` ( `id` INT UNSIGNED NOT NULL AUTO_INCREMENT, `rabbit_id` INT UNSIGNED NOT NULL, `title` VARCHAR(255) NOT NULL, `content` TEXT NOT NULL, PRIMARY KEY (`id`), KEY `rabbit_id` (`rabbit_id`), CONSTRAINT `fk_rabbits_pages` FOREIGN KEY (`rabbit_id`) REFERENCES `rabbits` (`id`) ) ENGINE=InnoDB; Query OK, 0 rows affected (0.16 sec) SET FOREIGN_KEY_CHECKS = 1; Query OK, 0 rows affected (0.00 sec)
测试用例:
INSERT INTO rabbits (name, main_page_id) VALUES ('bobby', NULL); Query OK, 1 row affected (0.02 sec) INSERT INTO rabbit_pages (rabbit_id, title, content) VALUES (1, 'My Main Page', 'Hello'); Query OK, 1 row affected (0.00 sec) SELECT * FROM rabbits; +----+-------+--------------+ | id | name | main_page_id | +----+-------+--------------+ | 1 | bobby | NULL | +----+-------+--------------+ 1 row in set (0.00 sec) SELECT * FROM rabbit_pages; +----+-----------+--------------+---------+ | id | rabbit_id | title | content | +----+-----------+--------------+---------+ | 1 | 1 | My Main Page | Hello | +----+-----------+--------------+---------+ 1 row in set (0.00 sec) UPDATE rabbits SET main_page_id = 2 WHERE id = 1; ERROR 1452 (23000): A foreign key constraint fails UPDATE rabbits SET main_page_id = 1 WHERE id = 1; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 UPDATE rabbit_pages SET rabbit_id = 2 WHERE id = 1; ERROR 1452 (23000): A foreign key constraint fails