Altering a table primary key which has foreign key constrains in mysql

Question

I have a mysql database which have set of tables One table have a composite key as the primary key and and a single foreign key. Following are the table definitions.

CREATE TABLE IF NOT EXISTS `ohrm_emp_education` (
  `emp_number` int(11) NOT NULL,
  `education_id` int(11) NOT NULL,
  `institute` varchar(100) DEFAULT NULL,
  `major` varchar(100) DEFAULT NULL,
  `year` decimal(4,0) DEFAULT NULL,
  `score` varchar(25) DEFAULT NULL,
  `start_date` date DEFAULT NULL,
  `end_date` date DEFAULT NULL,
  PRIMARY KEY (`emp_number`,`education_id`),
  KEY `education_id` (`education_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;



ALTER TABLE `ohrm_emp_education`
ADD CONSTRAINT `ohrm_emp_education_ibfk_1` FOREIGN KEY (`emp_number`) REFERENCES `hs_hr_employee` (`emp_number`) ON DELETE CASCADE,
ADD CONSTRAINT `ohrm_emp_education_ibfk_2` FOREIGN KEY (`education_id`) REFERENCES `ohrm_education` (`id`) ON DELETE CASCADE;

But now I need to add a new column to this existing table and make it as the primary key. I tried it with the following query.

ALTER TABLE ohrm_emp_education
ADD column id int not null AUTO_INCREMENT,
DROP PRIMARY KEY,
ADD primary key (id) 

But it shows following error

#1025 - Error on rename of './test/#sql-4f6_19b' to './test/ohrm_emp_education' (errno: 150)

I tried with several answers which are found on the internet but couldn't solve it properly. Can someone help me on this. Thanks in advance.

Answer 1

Try to delete foreign keys first, something like this:

ALTER TABLE `ohrm_emp_education` DROP FOREIGN KEY `emp_number`;
ALTER TABLE `ohrm_emp_education` DROP FOREIGN KEY `education_id`;

And then alter table.