How to add Foreign Key (MySQL) - mysql

Question

I'm extremely new to SQL and I'm trying to perform a simple ALTER TABLE to create a new column and use it as a foreign key to reference another simple table in my database. I've altered both tables to be InnoDB
However, when executing the ALTER TABLE code, I get the following error:
Error 1452 Cannot add or update a child row:
a foreign key constraint fails (`toys`.<result 2 when
explaining filename '#sql-6d4_6'>, CONSTRAINT
`#sql-6d4_6_ibfk_1` FOREIGN KEY (`toy_id`) REFERENCES `toys` (`toy_id`))
Below are the DESC of both tables:
Table 1:
FIELD TYPE NULL KEY EXTRA
toy_id int(11) NO PRI auto_increment
toy varchar(50) YES
Table 2:
FIELD TYPE NULL KEY EXTRA
boy_id int(11) NO PRI auto_increment
boy varchar(50) YES
And this is the ALTER query I was trying to perform:
ALTER TABLE boys
ADD COLUMN toy_id INT NOT NULL,
ADD CONSTRAINT toys_toy_id_fk
FOREIGN KEY(toy_id)
REFERENCES toys(toy_id);
I've looked all over trying to figure it out, but with no luck. Thanks in advance, and please be kind to this newbie :)
EDIT:
Here are the SHOW CREATE TABLE for both tables:
TABLE 1:
CREATE TABLE `toys` (
`toy_id` int(11) NOT NULL AUTO_INCREMENT,
`toy` varchar(50) DEFAULT NULL,
PRIMARY KEY (`toy_id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8
TABLE 2:
CREATE TABLE `boys` (
`boy_id` int(11) NOT NULL AUTO_INCREMENT,
`boy` varchar(50) DEFAULT NULL,
PRIMARY KEY (`boy_id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8

Answer 1

You can't add a NOT NULL column to a table that has more than zero rows, when the column is constrained to values that match those in the parent table, and yet has only NULL values because it's a new, unpopulated column with no DEFAULT.
The workaround is to do it in stages: add the column, but don't declare it NOT NULL, and don't declare the foreign key yet.
ALTER TABLE boys
ADD COLUMN toy_id INT;
Then populate it with valid data that matches some value(s) in your toys table.
UPDATE boys SET toy_id = ...;
Then alter the column to be NOT NULL, and create the constraint:
ALTER TABLE boys MODIFY COLUMN toy_id INT NOT NULL,
ADD CONSTRAINT toys_toy_id_fk
FOREIGN KEY(toy_id)
REFERENCES toys(toy_id);