Nesting of groups or elements - mysql

Question

I'm fairly new in Mysql, but I have problem that I cannot solve. I will give you an example to demonstrate it. Please note that I know that (for current example) there are other simpler and more efficient ways to solve it... but just take it as an example of the required procedure.
First the data: The data would be the name of a Person.
CREATE TABLE person(
id INT,
name VARCHAR(100)
) TYPE=innodb;
Second: Group Creation... So this is fairly simple... and could easily done using a table 'group' with a foreignkey to person. These groups could be arbitrary, containing any number of persons, duplicated... or not... (that is simple!!)
Third: MY REAL PROBLEM--- I also would like to have Groups that have other Groups as elements (instead of persons). This is where a really get stuck, because I know how to create a groups of persons, a group of groups (having a self-referencing foreign key)... but I don't know how to create a group that MAY HAVE persons AND Groups.
I appreciate any suggestion to solve this issue.
Thank you very much for your comments.
Regards
ACombo

Answer 1

I'd go with firstly setting up the myGroup and person tables.
Secondly, I'd set up a myGroupGroup table with columns myGroupId, parentMyGroupId. This will allow you to relate group rows to child group rows i.e. "this group has these groups within it". If a group has no rows in this table then it has no child groups within it.
Thirdly, I'd set up a personGroup table with columns personId, myGroupId. This will allow you to relate person rows to a given group. If a group has no rows in this table then it has no persons within it.
CREATE TABLE person(
id INT UNSIGNED PRIMARY KEY,
name VARCHAR(100)
) ENGINE=innodb;
CREATE TABLE myGroup(
id INT UNSIGNED PRIMARY KEY,
groupName VARCHAR(100)
) ENGINE=innodb;
-- Holds groups within groups
CREATE TABLE myGroupGroup(
id INT UNSIGNED PRIMARY KEY,
myGroupId INT UNSIGNED,
parentMyGroupId INT UNSIGNED DEFAULT NULL,
CONSTRAINT `fk_myGroupGroup_group1` FOREIGN KEY (`parentMyGroupId`) REFERENCES `myGroup` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `fk_myGroupGroup_group2` FOREIGN KEY (`myGroupId`) REFERENCES `myGroup` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=innodb;
-- Holds persons within a group
CREATE TABLE personGroup(
id INT,
personId int UNSIGNED NOT NULL,
myGroupId int UNSIGNED NOT NULL,
CONSTRAINT `fk_personGroup_group1` FOREIGN KEY (`myGroupId`) REFERENCES `myGroup` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `fk_personGroup_person1` FOREIGN KEY (`personId`) REFERENCES `person` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=innodb;
I've tweaked your SQL a bit:
1) Replaced TYPE with ENGINE
2) Replaced table name group with myGroup (GROUP is a reserved word)
Good luck!

Answer 2

Alternative:
CREATE TABLE Entity
( EntityId INT --- this id could be AUTO_INCREMENT
, PRIMARY KEY (EntityId)
) ENGINE = InnoDB ;
CREATE TABLE Person
( PersonId INT --- but not this id
, PersonName VARCHAR(100)
, PRIMARY KEY (PersonId)
, FOREIGN KEY (PersonId)
REFERENCES Entity(EntityId)
) ENGINE = InnoDB ;
CREATE TABLE Grouping
( GroupingId INT --- and neither this id
, GroupingName VARCHAR(100)
, PRIMARY KEY (GroupingId)
, FOREIGN KEY (GroupingId)
REFERENCES Entity(EntityId)
) ENGINE = InnoDB ;
CREATE TABLE Belongs
( EntityId INT
, GroupingID INT
, PRIMARY KEY (EntityId, GroupingId)
, FOREIGN KEY (EntityId)
REFERENCES Entity(EntityId)
, FOREIGN KEY (GroupingID)
REFERENCES Grouping(GroupingId)
) ENGINE = InnoDB ;