MySQL cascade deletion and update

(1) cascading deletion using foreign keys

1. Create a test database first

CREATE TABLE `roottb` (  `id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,  `data` VARCHAR(100) NOT NULL DEFAULT '',  PRIMARY KEY (`id`)) TYPE=InnoDB;CREATE TABLE `subtb` (  `id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,  `rootid` INT(11) UNSIGNED NOT NULL DEFAULT '0',  `data` VARCHAR(100) NOT NULL DEFAULT '',  PRIMARY KEY (`id`),  INDEX (`rootid`),  FOREIGN KEY (`rootid`) REFERENCES roottb(`id`) ON DELETE CASCADE) TYPE=InnoDB;

Note: Data Tables must use the InnoDB engine.

The foreign key must be indexed.

The foreign key binding relationship uses "on Delete cascade"

2. Insert Test Data

INSERT INTO `roottb` (`id`,`data`)  VALUES ('1', 'test root line 1'),         ('2', 'test root line 2'),         ('3', 'test root line 3');INSERT INTO `subtb` (`id`,`rootid`,`data`)  VALUES ('1', '1', 'test sub line 1 for root 1'),         ('2', '1', 'test sub line 2 for root 1'),         ('3', '1', 'test sub line 3 for root 1'),         ('4', '2', 'test sub line 1 for root 2'),         ('5', '2', 'test sub line 2 for root 2'),         ('6', '2', 'test sub line 3 for root 2'),         ('7', '3', 'test sub line 1 for root 3'),         ('8', '3', 'test sub line 2 for root 3'),         ('9', '3', 'test sub line 3 for root 3');

3. view the status of the data table

4. Test the cascade deletion function

Delete only the data records with the ID of 2 in the roottb table to see if the related sub-records with the ID of 2 in the subtb table are automatically deleted!

mysql>; delete from `roottb` where `id`='2';Query OK, 1 row affected (0.03 sec)mysql>; select * from `roottb`;+----+------------------+| id | data             |+----+------------------+|  1 | test root line 1 ||  3 | test root line 3 |+----+------------------+2 rows in set (0.00 sec)mysql>; select * from `subtb`;+----+--------+----------------------------+| id | rootid | data                       |+----+--------+----------------------------+|  1 |      1 | test sub line 1 for root 1 ||  2 |      1 | test sub line 2 for root 1 ||  3 |      1 | test sub line 3 for root 1 ||  7 |      3 | test sub line 1 for root 3 ||  8 |      3 | test sub line 2 for root 3 ||  9 |      3 | test sub line 3 for root 3 |+----+--------+----------------------------+6 rows in set (0.01 sec)

The above part is transferred from: bbs.chinaunix.net/forum.php? MoD = viewthread & tid = 462977.

(2) cascade deletion using triggers

The following example is provided:

1. Create a Test Database

CREATE TABLE `root_trigger` (  `id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,  `data` VARCHAR(100) NOT NULL DEFAULT '',  PRIMARY KEY (`id`));CREATE TABLE `sub_trigger` (  `id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,  `rootid` INT(11) UNSIGNED NOT NULL DEFAULT '0',  `data` VARCHAR(100) NOT NULL DEFAULT '',  PRIMARY KEY (`id`)  );

2. Insert Test Data

INSERT INTO `root_trigger` (`id`,`data`)  VALUES ('1', 'test root line 1'),         ('2', 'test root line 2'),         ('3', 'test root line 3');INSERT INTO `sub_trigger` (`id`,`rootid`,`data`)  VALUES ('1', '1', 'test sub line 1 for root 1'),         ('2', '1', 'test sub line 2 for root 1'),         ('3', '1', 'test sub line 3 for root 1'),         ('4', '2', 'test sub line 1 for root 2'),         ('5', '2', 'test sub line 2 for root 2'),         ('6', '2', 'test sub line 3 for root 2'),         ('7', '3', 'test sub line 1 for root 3'),         ('8', '3', 'test sub line 2 for root 3'),         ('9', '3', 'test sub line 3 for root 3');

3. Create a trigger for (cascade/synchronize) Deletion

drop trigger if exists t_afterdelete_on_sub;create trigger t_afterdelete_on_subafter delete on root_triggerfor each rowbegin       delete from sub_trigger where rootid=old.id;      end;

4. Delete the record with ID = 2 in the root_trigger table, and check whether the record with rootid = 2 in sub_trigger is deleted.

The deletion is successful, and the trigger is deleted cascade!