MySQL: Add constraint if not exists

In my create script for my database create script looking something like this:

CREATE TABLE IF NOT EXISTS `rabbits`
(
    `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
    `name` VARCHAR(255) NOT NULL,
    `main_page_id` INT UNSIGNED COMMENT 'What page is the main one',
    PRIMARY KEY (`id`),
    KEY `main_page_id` (`main_page_id`)
)
ENGINE=InnoDB;

CREATE TABLE IF NOT EXISTS `rabbit_pages`
(
    `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
    `rabbit_id` INT UNSIGNED NOT NULL,
    `title` VARCHAR(255) NOT NULL,
    `content` TEXT NOT NULL,
    PRIMARY KEY (`id`),
    KEY `rabbit_id` (`rabbit_id`),
    CONSTRAINT `fk_rabbits_pages` FOREIGN KEY (`rabbit_id`) REFERENCES `rabbits` (`id`)
)
ENGINE=InnoDB;

ALTER TABLE `rabbits`
    ADD CONSTRAINT `fk_rabbits_main_page` FOREIGN KEY (`main_page_id`) REFERENCES `rabbit_pages` (`id`);

This runs fine the first time, but if I run it again it fails on the last line there with "Duplicate key on write or update".

Is there a way I can do sort of a ADD CONSTRAINT IF NOT EXISTS or something like that? Like I can do with the CREATE TABLE query?

 Answers

Interesting question. You may want to disable foreign keys before you call your CREATE TABLE statements and enable them afterwards. This will allow you to define the foreign keys directly in the CREATE TABLE DDL:

Example:

SET FOREIGN_KEY_CHECKS = 0;
Query OK, 0 rows affected (0.00 sec)

CREATE TABLE IF NOT EXISTS `rabbits` (
    `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
    `name` VARCHAR(255) NOT NULL,
    `main_page_id` INT UNSIGNED COMMENT 'What page is the main one',
    PRIMARY KEY (`id`),
    KEY `main_page_id` (`main_page_id`),
    CONSTRAINT `fk_rabbits_main_page` FOREIGN KEY (`main_page_id`) REFERENCES `rabbit_pages` (`id`)
) ENGINE=InnoDB;
Query OK, 0 rows affected (0.04 sec)

CREATE TABLE IF NOT EXISTS `rabbit_pages` (
    `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
    `rabbit_id` INT UNSIGNED NOT NULL,
    `title` VARCHAR(255) NOT NULL,
    `content` TEXT NOT NULL,
    PRIMARY KEY (`id`),
    KEY `rabbit_id` (`rabbit_id`),
    CONSTRAINT `fk_rabbits_pages` FOREIGN KEY (`rabbit_id`) REFERENCES `rabbits` (`id`)
) ENGINE=InnoDB;
Query OK, 0 rows affected (0.16 sec)

SET FOREIGN_KEY_CHECKS = 1;
Query OK, 0 rows affected (0.00 sec)

Test case:

INSERT INTO rabbits (name, main_page_id) VALUES ('bobby', NULL);
Query OK, 1 row affected (0.02 sec)

INSERT INTO rabbit_pages (rabbit_id, title, content) VALUES (1, 'My Main Page', 'Hello');
Query OK, 1 row affected (0.00 sec)

SELECT * FROM rabbits;
+----+-------+--------------+
| id | name  | main_page_id |
+----+-------+--------------+
|  1 | bobby | NULL         |
+----+-------+--------------+
1 row in set (0.00 sec)

SELECT * FROM rabbit_pages;
+----+-----------+--------------+---------+
| id | rabbit_id | title        | content |
+----+-----------+--------------+---------+
|  1 |         1 | My Main Page | Hello   |
+----+-----------+--------------+---------+
1 row in set (0.00 sec)

UPDATE rabbits SET main_page_id = 2 WHERE id = 1;
ERROR 1452 (23000): A foreign key constraint fails

UPDATE rabbits SET main_page_id = 1 WHERE id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

UPDATE rabbit_pages SET rabbit_id = 2 WHERE id = 1;
ERROR 1452 (23000): A foreign key constraint fails

---

MariaDB supports this syntax in 10.0.2 or later:

ALTER TABLE `rabbits`
ADD CONSTRAINT `fk_rabbits_main_page` FOREIGN KEY IF NOT EXISTS
(`main_page_id`) REFERENCES `rabbit_pages` (`id`);

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

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.

 Answers

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

---

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...

---

This works for mysql version 5.5.32

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

---

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

---

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`);