How do I specify unique constraint for multiple columns in MySQL? I have a table: table votes ( id, user, email, address, primary key(id), ); Now I want to make the columns user, email, address unique (together). How do I do this in MySql? Of course the example is just... an example. So please don't worry about the semantics. Solution: ALTER TABLE `votes` ADD UNIQUE `unique_index`(`user`, `email`, `address`); http://stackoverflow.com/questions/635937/how-do-i-specify-unique-constraint-for-multiple-columns-in-mysql