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.

score 1683 · Accepted Answer · edited Mar 18 '21 at 15:12

1683

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`);

edited Mar 18 '21 at 15:12

ScottishTapWater

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answered Mar 11 '09 at 19:14

jonstjohn

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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" – Pakman Jun 03 '13 at 20:42
17

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 http://stackoverflow.com/questions/9537710/is-there-a-way-to-use-on-duplicate-key-to-update-all-that-i-wanted-to-insert – Joe Dec 25 '13 at 23:06

score 313 · Answer 2 · edited Oct 26 '21 at 06:53

313

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.

edited Oct 26 '21 at 06:53

T.Todua

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answered Aug 09 '10 at 14:56

Frodik

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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. :) – GazB Jan 27 '12 at 11:46
Why `UNIQUE KEY my_uniq_id`? Just `UNIQUE` is fine (on current MariaDB, anyway). – EML Jan 26 '23 at 18:49

score 62 · Answer 3 · answered May 07 '10 at 21:21

62

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...

answered May 07 '10 at 21:21

niksoft

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Very interesting, I can confirm that I can have multiple null values in unique columns in MySQL 5.7.37 – Adam Fowler Feb 17 '22 at 16:20
This is not a bug you can fix this problem by composite primary key – Syed Waqas Bukhary Aug 05 '22 at 07:33

score 27 · Answer 4 · answered Mar 11 '09 at 19:14

27

Have you tried this ?

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

answered Mar 11 '09 at 19:14

Erick

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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. – WQC Dec 11 '12 at 15:30

score 21 · Answer 5 · edited Feb 05 '14 at 08:55

21

This works for mysql version 5.5.32

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

edited Feb 05 '14 at 08:55

Code Lღver

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answered Jan 02 '14 at 19:00

rizon

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score 7 · Answer 6 · answered Aug 03 '14 at 12:56

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

score 6 · Answer 7 · answered Mar 20 '14 at 00:40

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.

gandalivs · Answer 8 · 2020-08-31T06:50:17.900

6

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/

edited Aug 31 '20 at 06:50

answered Aug 31 '20 at 02:57

gandalivs

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score 5 · Answer 9 · answered Jun 27 '18 at 21:20

5

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.

answered Jun 27 '18 at 21:20

Sam Malayek

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score 4 · Answer 10 · answered Oct 09 '18 at 05:50

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)
);

score 3 · Answer 11 · edited Nov 22 '16 at 14:07

3

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`);

edited Nov 22 '16 at 14:07

TheGameiswar

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answered Jul 10 '14 at 08:22

sandeep vanama

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score 2 · Answer 12 · edited Mar 09 '13 at 23:01

2

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);

edited Mar 09 '13 at 23:01

Taryn

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answered Mar 09 '13 at 13:04

kumar

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score 0 · Answer 13 · answered Jan 13 '20 at 18:32

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.

score 0 · Answer 14 · edited May 06 '20 at 11:39

0

For PostgreSQL... It didn't work for me with index; it gave me an error, so I did this:

alter table table_name
add unique(column_name_1,column_name_2);

PostgreSQL gave unique index its own name. I guess you can change the name of index in the options for the table, if it is needed to be changed...

edited May 06 '20 at 11:39

David Buck

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answered May 06 '20 at 11:01

bugthefifth

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

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