I have a table already created, and one of the fields "member_id" was set to unique when I first created the table.
Then later I went back to also make another field "member_username" unique also, I get the following error:
#1062 - Duplicate entry '' for key 'member_username'
Here is the code the phpmyadmin used, but gave above error message:
ALTER TABLE `members` ADD UNIQUE (