添加外键但出现约束冲突错误

Question

我有四张桌子：

1)Country:Two columns
  a) country_id
  b) country_name

2)State: Three columns
  a) state_id
  b) state_name
  c) country_id(foreign key)

 3)City: Three Columns
   a) city_id
   b) city_name
   c) state_id(foreign key)

 4) Doctor: 20 Columns
   a) doc_id (primary key)
   b) doc_name
   c) email,gender age and so on 

我已经用 dara 填充了这些表。

我在 doctor 表中添加三个新列：country_id、state_id 和 city_id 作为外键。

当我将 Country_id、state_id 和 city_id 作为外键插入到该表中时，出现此错误

1452 - Cannot add or update a child row: a foreign key constraint fails (`medical_network`.`#sql-4174_45`, CONSTRAINT `#sql-4174_45_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON DELETE CASCADE ON UPDATE CASCADE)

我知道这是一个基本错误，但找不到错误。

医生表：

CREATE TABLE `doctor` (
 `doc_id` int(9) NOT NULL AUTO_INCREMENT,
 `doc_fname` varchar(15) NOT NULL,
 `doc_lname` varchar(15) NOT NULL,
 `doc_province` varchar(20) NOT NULL,
 `doc_city` varchar(25) NOT NULL,
 `doc_gender` varchar(10) NOT NULL,
 `doc_fathername` varchar(32) NOT NULL,
 `doc_age` varchar(20) NOT NULL,
 `doc_cnic` varchar(16) NOT NULL,
 `doc_medclgname` varchar(100) NOT NULL,
 `doc_specilization` varchar(100) NOT NULL,
 `doc_phoneno` varchar(20) NOT NULL,
 `doc_officeno` varchar(20) NOT NULL,
 `doc_email` varchar(25) NOT NULL,
 `doc_gradyear` int(10) NOT NULL,
 `doc_licenseno` varchar(30) NOT NULL,
 `doc_hosp` varchar(200) NOT NULL,
 `doc_pass` varchar(35) NOT NULL,
 `doc_address` varchar(100) NOT NULL,
 `doc_country` varchar(100) NOT NULL,
 `doc_img` varchar(200) NOT NULL,
 `totime` time NOT NULL,
 `fromtime` time NOT NULL,
 `todate` date NOT NULL,
 `fromdate` date NOT NULL,
 `country_id` int(20) NOT NULL,
 PRIMARY KEY (`doc_id`),
 KEY `country_id` (`country_id`)
) ENGINE=InnoDB AUTO_INCREMENT=30 DEFAULT CHARSET=latin

国家表：

CREATE TABLE `country` (
 `country_id` int(20) NOT NULL AUTO_INCREMENT,
 `country_name` varchar(200) NOT NULL,
 PRIMARY KEY (`country_id`)
) ENGINE=InnoDB AUTO_INCREMENT=252 DEFAULT CHARSET=latin1

状态表查询：

CREATE TABLE `state` (
 `state_id` int(20) NOT NULL AUTO_INCREMENT,
 `state_name` varchar(200) NOT NULL,
 `country_id` int(20) NOT NULL,
 PRIMARY KEY (`state_id`),
 KEY `country_id` (`country_id`),
 CONSTRAINT `state_ibfk_1` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=latin1

城市表：

CREATE TABLE `city` (
 `city_id` int(20) NOT NULL AUTO_INCREMENT,
 `city_name` varchar(200) NOT NULL,
 `state_id` int(20) NOT NULL,
 PRIMARY KEY (`city_id`),
 KEY `state_id` (`state_id`),
 CONSTRAINT `city_ibfk_1` FOREIGN KEY (`state_id`) REFERENCES `state` (`state_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=latin1

Answer 1

1. 首先添加要在表中创建外键的所有列。

2. 添加一些相关数据。

3. 现在更改表并使这些新添加的列成为外键。

   即 ALTER TABLE TblName AD FOREIGN KEY (P_Id) REFERENCES ParentTable(P_Id)