How do I specify unique constraint for multiple columns in MySQL?
MysqlUnique ConstraintComposite KeyDatabase TableMysql Problem Overview
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.
Mysql Solutions
Solution 1 - Mysql
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 - Mysql
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 - Mysql
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 - Mysql
Have you tried this ?
UNIQUE KEY `thekey` (`user`,`email`,`address`)
Solution 5 - Mysql
This works for mysql version 5.5.32
ALTER TABLE `tablename` ADD UNIQUE (`column1` ,`column2`);
Solution 6 - Mysql
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
Solution 7 - Mysql
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.
Solution 8 - Mysql
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
.
Solution 9 - Mysql
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)
);
Solution 10 - Mysql
this tutorial works for me
ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);
Solution 11 - Mysql
For adding unique index following are required:
-
table_name
-
index_name
-
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`);
Solution 12 - Mysql
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);
Solution 13 - Mysql
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.
Solution 14 - Mysql
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...