To add a unique constraint, you need to use two components:
ALTER TABLE
– to change the table schema and,
ADD UNIQUE
– to add the unique constraint.
You then can define your new unique key with the format 'name'('column1', 'column2'...)
So for your particular issue, you could use this command:
ALTER TABLE `votes` ADD UNIQUE `unique_index`(`user`, `email`, `address`);