Needs to be
```
ADD CONSTRAINT `fk_users_user_id`
```

Also a minor OCD correction, inconsistent mix of spaces and tabs:
```
[spaces]ADD UNIQUE INDEX `unq_sets_name` (`name`),
[spaces]ADD CONSTRAINT `fk_users_user_id`
[tab]FOREIGN KEY (`user_id`)
[tab]REFERENCES `users` (`id`)
[tab]ON DELETE CASCADE
```
spaces to indent the query is fine.. but good to be consistent between lines