Are multiple foreign keys allowed? - mysql

Question

I have multiple tables and I want multiple foreign keys.
Users Table
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| idUsers | int(11) | NO | PRI | NULL | |
| Username | varchar(45) | YES | | NULL | |
| Password | varchar(45) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
Friends Table
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| idUser | int(11) | YES | MUL | NULL | |
| idFriend | int(11) | YES | MUL | NULL | |
| Status | varchar(45) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
Groups Table
+-----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| idGroup | int(11) | NO | PRI | NULL | |
| GroupName | varchar(45) | YES | | NULL | |
+-----------+-------------+------+-----+---------+-------+
Members Table
+----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------+------+-----+---------+-------+
| idGroup | int(11) | YES | MUL | NULL | |
| idMember | int(11) | YES | MUL | NULL | |
+----------+---------+------+-----+---------+-------+
This is the foreign key for my Members Table so that the idGroup and idMember both correspond to the Users table idUsers.
ALTER TABLE `GroupOrder`.`Friends`
ADD CONSTRAINT `idFriend`
FOREIGN KEY (`idFriend`)
REFERENCES `GroupOrder`.`Users` (`idUsers`)
ON DELETE CASCADE
ON UPDATE CASCADE;
I want to make another foreign key in the Friends table that corresponds to the Users table. Am I'm doing in wrong. Everytime I do this I get an error.
ALTER TABLE `GroupOrder`.`Friends`
ADD CONSTRAINT `idFriend`
FOREIGN KEY (`idUser` , `idFriend`)
REFERENCES `GroupOrder`.`Users` (`idUsers` , `idUsers`)
ON DELETE CASCADE
ON UPDATE CASCADE;
Error
ERROR 1005: Can't create table 'GroupOrder.#sql-2fce_1e' (errno: 150)
SQL Statement:
ALTER TABLE `GroupOrder`.`Friends`
ADD CONSTRAINT `idFriend`
FOREIGN KEY (`idUser` , `idFriend`)
REFERENCES `GroupOrder`.`Users` (`idUsers` , `idUsers`)
ON DELETE CASCADE
ON UPDATE CASCADE
ERROR: Error when running failback script. Details follow.
ERROR 1050: Table 'Friends' already exists
SQL Statement:
CREATE TABLE `Friends` (
`idUser` int(11) DEFAULT NULL,
`idFriend` int(11) DEFAULT NULL,
`Status` varchar(45) DEFAULT NULL,
KEY `idUser_idx` (`idFriend`),
KEY `idUser_idx1` (`idUser`,`idFriend`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

Answer 1

I am assuming friends is a many to many mapping from users to users. This sort of implies friendship can be one-way, but that isn't necessarily wrong.
You already mapped idfriend in friends to idusers in users, now map iduser in friends to idusers in users.
ALTER TABLE `GroupOrder`.`Friends`
ADD CONSTRAINT `idFriend2`
FOREIGN KEY (`idUser`)
REFERENCES `GroupOrder`.`Users` (`idUsers`)
ON DELETE CASCADE
ON UPDATE CASCADE;
Now, you probably want to enforce uniqueness of iduser and idfriend in friends so you don't have repetition.
alter table `grouporder`.friends`
add unique(oduser, idfriend);