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.

Answer 1

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

Answer 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 `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 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...

Answer 4

Have you tried this ?

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

Answer 5

This works for mysql version 5.5.32

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

Answer 6

You can add multiple-column unique indexes via phpMyAdmin. (I tested in version 4.0.4)

Navigate to the structure page for your target table. Add a unique index to one of the columns. Expand the Indexes list on the bottom of the structure page to see the unique index you just added. Click the edit icon, and in the following dialog you can add additional columns to that unique index.

Answer 7

MySql 5 or higher behaves like this (I've just tested):

• you can define unique constraints involving nullable columns. Say you define a constraint unique (A, B) where A is not nullable but B is
• when evaluating such a constraint you can have (A, null) as many times you want (same A value!)
• you can only have one (A, not null B) pair

Example: PRODUCT_NAME, PRODUCT_VERSION 'glass', null 'glass', null 'wine', 1

Now if you try to insert ('wine' 1) again it will report a constraint violation Hope this helps

Answer 8

I do it like this:

CREATE UNIQUE INDEX index_name ON TableName (Column1, Column2, Column3);

My convention for a unique index_name is TableName_Column1_Column2_Column3_uindex.

Answer 9

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 10

For adding unique index following are required:

1) table_name
2) index_name
3) columns on which you want to add index

ALTER TABLE  `tablename` 
ADD UNIQUE index-name
(`column1` ,`column2`,`column3`,...,`columnN`);

In your case we can create unique index as follows:

ALTER TABLE `votes`ADD 
UNIQUE <votesuniqueindex>;(`user` ,`email`,`address`);

Answer 11

If You are creating table in mysql then use following :

create table package_template_mapping (
mapping_id  int(10) not null auto_increment  ,
template_id int(10) NOT NULL ,
package_id  int(10) NOT NULL ,
remark      varchar(100),
primary key (mapping_id) ,
UNIQUE KEY template_fun_id (template_id , package_id)
);