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mysqlunique-constraintcomposite-keydatabase-table
Niyaz
Niyaz

Reputation: 54793

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

I have a table:

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

How can I make the columns user, email, address unique - i.e., ensure that there isn't any pair of rows that has identical values for all three columns?

Upvotes: 1053

Views: 721180

Answers (15)

Harley
Harley

Reputation: 1524

In MySQL Workbench:

  1. Make an Index
  2. Select the Columns
  3. From the Type menu choose UNIQUE

MySQL Workbench GUI - Indexes tab

Upvotes: 0

Frodik
Frodik

Reputation: 15455

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.

Upvotes: 321

jonstjohn
jonstjohn

Reputation: 60276

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`);

Upvotes: 1707

gandalivs
gandalivs

Reputation: 403

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/

Upvotes: 6

bugthefifth
bugthefifth

Reputation: 169

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...

Upvotes: 0

Ganesh Krishnan
Ganesh Krishnan

Reputation: 7395

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.

Upvotes: 0

Devendra Singraul
Devendra Singraul

Reputation: 951

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)
);

Upvotes: 4

Sam Malayek
Sam Malayek

Reputation: 3745

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.

Upvotes: 5

sandeep vanama
sandeep vanama

Reputation: 709

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`);

Upvotes: 3

Cristian Botiza
Cristian Botiza

Reputation: 439

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

Upvotes: 10

Vince K
Vince K

Reputation: 305

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.

Upvotes: 6

rizon
rizon

Reputation: 8187

This works for mysql version 5.5.32

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

Upvotes: 22

kumar
kumar

Reputation: 21

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);

Upvotes: 2

niksoft
niksoft

Reputation: 665

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...

Upvotes: 64

Erick
Erick

Reputation: 6089

Have you tried this ? 

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

Upvotes: 27

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