How do I use on delete cascade in 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.