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?
ALTER TABLE students ADD FOREIGN KEY (student_id) REFERENCES points(id); To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax: ALTER TABLE students ADD CONSTRAINT fk_student_id FOREIGN KEY (student_id) REFERENCES points(id);
Use the ADD CONSTRAINT clause to specify a primary key, foreign key, referential, unique, or check constraint on a new or existing column or on a set of columns. This syntax fragment is part of the ALTER TABLE statement. Notes: For NULL and NOT NULL constraints, use instead the MODIFY Clause.
Here's the syntax to create foreign key in MySQL. ALTER TABLE table_name ADD CONSTRAINT constraint_name FOREIGN KEY (foreign_key_name,...) REFERENCES parent_table(column_name,...); In the above query, table_name is the the table where you want to add foreign key.
The syntax for creating a unique constraint using an ALTER TABLE statement in MySQL is: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n); table_name.
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`);
The FOREIGN_KEY_CHECKS
is a great tools but if your need to know how to do this without dropping and recreating your tables. You can use a SELECT
statement ON information_schema.TABLE_CONSTRAINTS
to determine if the foreign key exists:
IF NOT EXISTS (
SELECT NULL
FROM information_schema.TABLE_CONSTRAINTS
WHERE
CONSTRAINT_SCHEMA = DATABASE() AND
CONSTRAINT_NAME = 'fk_rabbits_main_page' AND
CONSTRAINT_TYPE = 'FOREIGN KEY'
)
THEN
ALTER TABLE `rabbits`
ADD CONSTRAINT `fk_rabbits_main_page`
FOREIGN KEY (`main_page_id`)
REFERENCES `rabbit_pages` (`id`);
END IF
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
For a non-MariaDB, this worked for me:
SET @dbname = DATABASE();
SET @tablename = "my_table";
SET @constraintname = "my_constraint_name";
SET @columnname = "my_column";
SET @othertablename = "other_table";
SET @othercolumnname = "other_column_name";
SET @deleteaction = "CASCADE";
SET @updateaction = "RESTRICT";
SET @preparedStatement = (SELECT IF(
(
SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
WHERE
(table_name = @tablename)
AND (table_schema = @dbname)
AND (constraint_name = @constraintname)
) > 0,
"SELECT 1",
CONCAT("ALTER TABLE ",@tablename,
" ADD CONSTRAINT ",@constraintname,
" FOREIGN KEY(",@columnname,")
REFERENCES ",@othertablename,"(",@othercolumnname,")
ON DELETE ",@deleteaction,
" ON UPDATE ",@updateaction)));
PREPARE alterIfNotExists FROM @preparedStatement;
EXECUTE alterIfNotExists;
DEALLOCATE PREPARE alterIfNotExists;
This is a solution, is made from a similar problem: https://stackoverflow.com/a/31989541/3589448
Add parameters as needed. @deleteaction and @updateaction can have: "RESTRICT", "CASCADE", "SET NULL" or "NO ACTION".
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