使用Mysql工作台添加外键时出错

Question

Im adding foreign key to my table 'empcompensation' referring from another table 'employees'. It shows error.

ALTER TABLE `simhrmdb`.`empcompensation` 
ADD CONSTRAINT `Fk_employee`
  FOREIGN KEY (`employee`)
  REFERENCES `simhrmdb`.`employees` (`id`)
  ON DELETE CASCADE
  ON UPDATE CASCADE;

Operation failed: There was an error while applying the SQL script to the database.
ERROR 1452: Cannot add or update a child row: a foreign key constraint fails
(`simhrmdb`.`#sql-c5c_6`, CONSTRAINT `Fk_employee` FOREIGN KEY (`employee`) REFERENCES `employees` (`id`) ON DELETE CASCADE ON UPDATE CASCADE) SQL Statement: ALTER TABLE `simhrmdb`.`empcompensation` ADD CONSTRAINT `Fk_employee` FOREIGN KEY (`employee`) REFERENCES `simhrmdb`.`employees` (`id`) ON DELETE CASCADE ON UPDATE CASCADE

i added the same 'id' from 'employees' table to few others table before and it worked perfectly. But now it creates error. Empcompensation table is the new table that i added.

In that error log, i can see some letters #sql-c5c_6 . I dont understand this .

Answer 1

This says that some of the values in the employee column cannot be found in the employees table. Find out what they are by using a LEFT JOIN

SELECT employee FROM empcompensation 
LEFT JOIN employees on employee = employees.id WHERE employees.id IS NULL

You will then have to figure out what to do with them (delete them, create corresponding entries in the employees table etc)