error altering table, adding constraint foreign key getting error “Cannot add or update a child row”

Question

mysql> DESCRIBE questions;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+--         


        

Answer 1

You have at least one data value in answers.questions_id that does not occur in questions.id.

Here's an example of what I mean:

mysql> create table a ( id int primary key);

mysql> create table b ( aid int );

mysql> insert into a values (123);

mysql> insert into b values (123), (456);

mysql> alter table b add foreign key (aid) references a(id);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint 
fails (`test`.`#sql-3dab_e5c`, CONSTRAINT `#sql-3dab_e5c_ibfk_1` FOREIGN KEY
(`aid`) REFERENCES `a` (`id`))

You can use this to confirm that there are unmatched values:

SELECT COUNT(*)
FROM answers AS a
LEFT OUTER JOIN questions AS q ON a.questions_id = q.id
WHERE q.id IS NULL