Many-to-many relationships examples

Question

I haven't found any MYSQL many-to-many relationships examples here and in google. What I am looking is to see a very simple example with php+mysql showing database's results. Can anybody write a very simple example?

Answer 1

Example scenario: students and courses at a university. A given student might be on several courses, and naturally a course will usually have many students.

Example tables, simple design:

CREATE TABLE `Student` (     `StudentID` INT UNSIGNED NOT NULL AUTO_INCREMENT,     `FirstName` VARCHAR(25),     `LastName` VARCHAR(25) NOT NULL,     PRIMARY KEY (`StudentID`) ) ENGINE=INNODB CHARACTER SET utf8 COLLATE utf8_general_ci  CREATE TABLE `Course` (     `CourseID` SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,     `Code` VARCHAR(10) CHARACTER SET ascii COLLATE ascii_general_ci NOT NULL,     `Name` VARCHAR(100) NOT NULL,     PRIMARY KEY (`CourseID`) ) ENGINE=INNODB CHARACTER SET utf8 COLLATE utf8_general_ci  CREATE TABLE `CourseMembership` (     `Student` INT UNSIGNED NOT NULL,     `Course` SMALLINT UNSIGNED NOT NULL,     PRIMARY KEY (`Student`, `Course`),     CONSTRAINT `Constr_CourseMembership_Student_fk`         FOREIGN KEY `Student_fk` (`Student`) REFERENCES `Student` (`StudentID`)         ON DELETE CASCADE ON UPDATE CASCADE,     CONSTRAINT `Constr_CourseMembership_Course_fk`         FOREIGN KEY `Course_fk` (`Course`) REFERENCES `Course` (`CourseID`)         ON DELETE CASCADE ON UPDATE CASCADE ) ENGINE=INNODB CHARACTER SET ascii COLLATE ascii_general_ci 

Find all students registered for a course:

SELECT     `Student`.* FROM     `Student`     JOIN `CourseMembership` ON `Student`.`StudentID` = `CourseMembership`.`Student` WHERE     `CourseMembership`.`Course` = 1234 

Find all courses taken by a given student:

SELECT     `Course`.* FROM     `Course`     JOIN `CourseMembership` ON `Course`.`CourseID` = `CourseMembership`.`Course` WHERE     `CourseMembership`.`Student` = 5678 

Answer 2

Here's a quick and dirty example of the SQL involved. I don't see any need to muddy up the concept with php. Just retrieve the set like you would any other.

In this example, there are many names, and many colors. People are allowed to have more than one favorite color, and many people can have the same favorite color. Hence many to many.

 ***** Tables **********  person -------- id - int  name - varchar  favColor ------------- id - int  color - varchar  person_color ------------ person_id - int (matches an id from person) color_id - int (matches an id from favColor)    ****** Sample Query ******  SELECT name, color  FROM person      LEFT JOIN person_color ON (person.id=person_id)     LEFT JOIN favColor ON (favColor.id=color_id)   ****** Results From Sample Query *******  Name - Color --------------- John - Blue John - Red Mary - Yellow Timmy - Yellow Suzie - Green Suzie - Blue etc... 

Does that help?