[mysql] MySQL foreign key constraints, cascade delete

I got confused by the answer to this question, so I created a test case in MySQL, hope this helps

-- Schema
CREATE TABLE T1 (
    `ID` int not null auto_increment,
    `Label` varchar(50),
    primary key (`ID`)
);

CREATE TABLE T2 (
    `ID` int not null auto_increment,
    `Label` varchar(50),
    primary key (`ID`)
);

CREATE TABLE TT (
    `IDT1` int not null,
    `IDT2` int not null,
    primary key (`IDT1`,`IDT2`)
);

ALTER TABLE `TT`
    ADD CONSTRAINT `fk_tt_t1` FOREIGN KEY (`IDT1`) REFERENCES `T1`(`ID`) ON DELETE CASCADE,
    ADD CONSTRAINT `fk_tt_t2` FOREIGN KEY (`IDT2`) REFERENCES `T2`(`ID`) ON DELETE CASCADE;

-- Data
INSERT INTO `T1` (`Label`) VALUES ('T1V1'),('T1V2'),('T1V3'),('T1V4');
INSERT INTO `T2` (`Label`) VALUES ('T2V1'),('T2V2'),('T2V3'),('T2V4');
INSERT INTO `TT` (`IDT1`,`IDT2`) VALUES
(1,1),(1,2),(1,3),(1,4),
(2,1),(2,2),(2,3),(2,4),
(3,1),(3,2),(3,3),(3,4),
(4,1),(4,2),(4,3),(4,4);

-- Delete
DELETE FROM `T2` WHERE `ID`=4; -- Delete one field, all the associated fields on tt, will be deleted, no change in T1
TRUNCATE `T2`; -- Can't truncate a table with a referenced field
DELETE FROM `T2`; -- This will do the job, delete all fields from T2, and all associations from TT, no change in T1

Examples related to mysql

Implement specialization in ER diagram How to post query parameters with Axios? PHP with MySQL 8.0+ error: The server requested authentication method unknown to the client Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver' phpMyAdmin - Error > Incorrect format parameter? Authentication plugin 'caching_sha2_password' is not supported How to resolve Unable to load authentication plugin 'caching_sha2_password' issue Connection Java-MySql : Public Key Retrieval is not allowed How to grant all privileges to root user in MySQL 8.0 MySQL 8.0 - Client does not support authentication protocol requested by server; consider upgrading MySQL client

Examples related to foreign-keys

Migration: Cannot add foreign key constraint The ALTER TABLE statement conflicted with the FOREIGN KEY constraint Can a foreign key refer to a primary key in the same table? Introducing FOREIGN KEY constraint may cause cycles or multiple cascade paths - why? MySQL Error 1215: Cannot add foreign key constraint MySQL Cannot Add Foreign Key Constraint Delete rows with foreign key in PostgreSQL How to remove constraints from my MySQL table? MySQL - Cannot add or update a child row: a foreign key constraint fails How to remove foreign key constraint in sql server?

Examples related to innodb

How can I rebuild indexes and update stats in MySQL innoDB? Database corruption with MariaDB : Table doesn't exist in engine How to regex in a MySQL query mysqldump exports only one table TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT maximum storage sizes What's the difference between MyISAM and InnoDB? Why is MySQL InnoDB insert so slow? How to debug Lock wait timeout exceeded on MySQL? How to change value for innodb_buffer_pool_size in MySQL on Mac OS? #1025 - Error on rename of './database/#sql-2e0f_1254ba7' to './database/table' (errno: 150)