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, 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 1

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`);

Solution 2

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 `my_uniq_id` (`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.

Solution 3

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...

Solution 4

Have you tried this ?

UNIQUE KEY `thekey` (`user`,`email`,`address`)

Solution 5

This works for mysql version 5.5.32

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

View more solutions

Comments

[GazB]

I wanted to mark this answer up as it does show you how to create a new table with a unique index where as jonstjohn's answer above tells you how to update an existing table. They do the same thing though just depends on if your creating a new table or updating an existing one. :)

[WQC]

What Erick said works just fine for me: UNIQUE KEY thekey` (user,email,address)`. I'm using MYSQL version 5.5.24 You can set this even in PHPMYADMIN if you are using it. I'm using the 3.5.1 version of PMA.

[Pakman]

For those people using MySQL workbench: go to the Indexes tab and select UNIQUE as your type. Give your index a name and check the appropriate columns under the section "Index Columns"

[Lawrence]

Tried it with MySql 5.6. Works from the first try. Thanks Erick!

[Joe]

Like me, if anyone else found this from search engines... and to answer Clizzin's question about if ON DUPLICATE KEY will work with a composite key - it absolutely will, however it will require some minor adjustments to the syntax. see stackoverflow.com/questions/9537710/…

[Adam F]

Very interesting, I can confirm that I can have multiple null values in unique columns in MySQL 5.7.37