How do I specify unique constraint for multiple columns in MySQL?

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`);

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