mysql 添加外鍵約束報錯 1452

今天爲一張表的字段添加一個外鍵約束： ALTER TABLE `gjp`.`gjp$contacts` ADD CONSTRAINT `FK_city_contact` FOREIGN KEY (`city_id` ) REFERENCES `gjp`.`gjp$cities` (`city_id` ) ON DELETE NO ACTION ON UPDATE NO ACTION , ADD I

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