Alter table error code 1834 - mysql

Question

I am trying to alter a table as follows. But I am receiving an Error Code:
1834 Cannot delete rows from table which is parent in a foreign key constraint 'downloads_ibfk' of table downloads.
ALTER TABLE users
MODIFY first_name VARCHAR(20) NOT NULL UNIQUE;

Answer 1

You obviously have a foreign key constraint blocking the ALTER TABLE.
You can deactivate the FK check by preceding your query with
SET FOREIGN_KEY_CHECKS=0;
But remember to set it back to 1 after the query.
SET FOREIGN_KEY_CHECKS=1;

Answer 2

By deactivating FOREIGN_KEY_CHECKS, as suggested by #zonzon, you may get some unexpected results.
The error could also be due to the initial column schema and an incomplete ALTER TABLE query.
If you have the same issue, here is how I fixed it:
My initial column schema was:
# users.login | varchar(128) | collate utf8_bin | not null;
ALTER TABLE users ADD login varchar(128) COLLATE 'utf8_bin' NOT NULL;
What I was trying to execute, without success:
ALTER TABLE users CHANGE COLUMN login login varchar(128) NULL DEFAULT NULL;
# Cannot delete rows from table which is parent in a foreign key constraint '%s' of table '%s'
What I have executed with success:
ALTER TABLE Internaute CHANGE COLUMN login login varchar(128) COLLATE 'utf8_bin' NULL DEFAULT NULL;
# This query is more complete, I indicated the column's `COLLATE`. Otherwise the default collation is applied.
Why my first query didn't work?
In this query, I was also changing the collation (to the default one), so I think, my ALTER TABLE operation used the COPY algorithm to process my request.
Using this algorithm, a deletion is performed at some point. This is why you could get the error Cannot delete rows from table ... in an ALTER TABLE operation.
By default, the ALTER TABLE operations use other algorithms (INSTANT or INPLACE) that prevent the delete step. But, an ALTER TABLE operation that changes a column's collation is not supported by these other algorithms.
You can get more information here about the ALTER TABLE algorithms and their supported operations.