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

Question

I have a table:

table votes (
    id,
    user,
    email,
    address,
    primary key(id),
);

Now I want to make the columns user

Answer 1

Multi column unique indexes do not work in MySQL if you have a NULL value in row as MySQL treats NULL as a unique value and at least currently has no logic to work around it in multi-column indexes. Yes the behavior is insane, because it limits a lot of legitimate applications of multi-column indexes, but it is what it is... As of yet, it is a bug that has been stamped with "will not fix" on the MySQL bug-track...

Answer 2

this tutorial works for me

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);

https://www.mysqltutorial.org/mysql-unique-constraint/

Answer 3

If you want to avoid duplicates in future. Create another column say id2.

UPDATE tablename SET id2 = id;

Now add the unique on two columns:

alter table tablename add unique index(columnname, id2);

Answer 4

First get rid of existing duplicates

delete a from votes as a, votes as b where a.id < b.id 
and a.user <=> b.user and a.email <=> b.email 
and a.address <=> b.address;

Then add the unique constraint

ALTER TABLE votes ADD UNIQUE unique_index(user, email, address);

Verify the constraint with

SHOW CREATE TABLE votes;

Note that user, email, address will be considered unique if any of them has null value in it.

Answer 5

I have a MySQL table:

CREATE TABLE `content_html` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `id_box_elements` int(11) DEFAULT NULL,
  `id_router` int(11) DEFAULT NULL,
  `content` mediumtext COLLATE utf8_czech_ci NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id_box_elements` (`id_box_elements`,`id_router`)
);

and the UNIQUE KEY works just as expected, it allows multiple NULL rows of id_box_elements and id_router.

I am running MySQL 5.1.42, so probably there was some update on the issue discussed above. Fortunately it works and hopefully it will stay that way.

Answer 6

This works for mysql version 5.5.32

ALTER TABLE  `tablename` ADD UNIQUE (`column1` ,`column2`);