How to Implement a Composite Foreign Key in MySQL?

Composite Foreign Key in SQL

Database design often involves creating relationships between tables. When the primary key of one table is composed of multiple columns, it becomes challenging to link it to a foreign key in another table. This question explores how to establish a relationship between two tables with a composite primary key and a foreign key.

Problem Statement

Consider the following two tables:

CREATE TABLE IF NOT EXISTS `tutorial` (
  `beggingTime` time NOT NULL,
  `day` varchar(8) NOT NULL,
  `tutorId` int(3) NOT NULL,
  `maxMembers` int(2) NOT NULL,
  `minMembers` int(1) NOT NULL,
  PRIMARY KEY (`beggingTime`,`day`,`tutorId`),
  KEY `tutorId` (`tutorId`)
);

CREATE TABLE IF NOT EXISTS `group` (
  `groupId` tinyint(3) NOT NULL AUTO_INCREMENT,
  `status` varchar(20) NOT NULL,
  `groupName` varchar(50) NOT NULL,
  PRIMARY KEY (`groupId`)
);

The objective is to create a field in the group table that links to the composite primary key in the tutorial table. The question arises: how can we relate these tables effectively?

Solution

According to MySQL documentation, it is possible to set up a foreign key mapping to composite keys. This requires creating multiple columns in the foreign key table:

ALTER TABLE `group` ADD COLUMN `beggingTime` time NOT NULL;
ALTER TABLE `group` ADD COLUMN `day` varchar(8) NOT NULL;
ALTER TABLE `group` ADD COLUMN `tutorId` int(3) NOT NULL;

ALTER TABLE `group` ADD FOREIGN KEY (`beggingTime`,`day`,`tutorId`) 
    REFERENCES tutorial(`beggingTime`,`day`,`tutorId`);

This approach creates additional columns in the group table that correspond to the components of the composite primary key in the tutorial table.

Recommendation

While creating a composite foreign key is possible, it is not always the best design choice. As other responses have suggested, it is recommended to reconsider the table design and use a single-column primary key for the tutorial table. This will improve database performance and simplify relationship management.

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