mysql - Alter table error code 1834 -

i trying alter table follows. receiving error code:

1834 cannot delete rows table parent in foreign key constraint 'downloads_ibfk' of table downloads.

alter table users modify first_name varchar(20) not null unique;

you have foreign key constraint blocking alter table.

you can deactivate fk check preceding query

set foreign_key_checks=0;

but remember set 1 after query.

set foreign_key_checks=1;

Bradly