How do I use on delete cascade in mysql? - mysql

Question

I have a database of components. Each component is of a specific type. That means there is a many-to-one relationship between a component and a type. When I delete a type, I would like to delete all the components which has a foreign key of that type. But if I'm not mistaken, cascade delete will delete the type when the component is deleted. Is there any way to do what I described?

Answer 1

Here's what you'd include in your components table.
CREATE TABLE `components` (
`id` int(10) unsigned NOT NULL auto_increment,
`typeId` int(10) unsigned NOT NULL,
`moreInfo` VARCHAR(32),
-- etc
PRIMARY KEY (`id`),
KEY `type` (`typeId`)
CONSTRAINT `myForeignKey` FOREIGN KEY (`typeId`)
REFERENCES `types` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
)
Just remember that you need to use the InnoDB storage engine: the default MyISAM storage engine doesn't support foreign keys.

Answer 2

use this sql
DELETE T1, T2
FROM T1
INNER JOIN T2 ON T1.key = T2.key
WHERE condition

Answer 3

You have to define your Foreign Key constraints as ON DELETE CASCADE.
Note: You need to use InnoDB storage engine, the default MyISAM storage engine not support foreign keys relation.
CREATE TABLE `table2` (
`id` int(11) NOT NULL auto_increment,
`name` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `ids` (`ids`)
CONSTRAINT `foreign` FOREIGN KEY (`ids`)
REFERENCES `table2` (`ids`) ON DELETE CASCADE ON UPDATE CASCADE
)

Answer 4

Step 1. Create the buildings table:
CREATE TABLE buildings (
building_no INT PRIMARY KEY AUTO_INCREMENT,
building_name VARCHAR(255) NOT NULL,
address VARCHAR(255) NOT NULL
);
Step 2. Create the rooms table:
CREATE TABLE rooms (
room_no INT PRIMARY KEY AUTO_INCREMENT,
room_name VARCHAR(255) NOT NULL,
building_no INT NOT NULL,
FOREIGN KEY (building_no)
REFERENCES buildings (building_no)
ON DELETE CASCADE
);
Notice that the ON DELETE CASCADE clause at the end of the foreign key constraint
definition.