Hi there

unique.sql

CREATE TABLE foo1 (id int(10) UNSIGNED NOT NULL, msg varchar(20) NOT NULL);
ALTER TABLE foo1 ADD PRIMARY KEY (id);
INSERT INTO foo1 (id, msg) VALUES (1, 'foo'), (2, 'bar');

/*
MariaDB [sandbox]> SELECT * FROM `foo1`;
+----+-----+
| id | msg |
+----+-----+
|  1 | foo |
|  2 | bar |
+----+-----+
*/

ALTER TABLE foo1 ADD xxx VARCHAR(20) NULL DEFAULT NULL, ADD UNIQUE (xxx);

/*
MariaDB [sandbox]> SELECT * FROM `foo1`;
+----+-----+------+
| id | msg | xxx  |
+----+-----+------+
|  1 | foo | NULL |
|  2 | bar | NULL |
+----+-----+------+
*/