MySQL reference non-unique value in parent table - mysql

Question

I have an approval tag and would like it to be populated down when it is changed in the parent table, however I cannot figure out how to reference the parent from the child to pull the value down.
For reference I want the value approval in reservations to be populated down to reservedTickets when I update the approval from 0 to 1 in reservations. However, to my understanding I cannot use a standard FOREIGN KEY or REFERENCE reference to reservations since approval is not unique.
Before anyone says anything about "why not have the value in only one table" it is to separate concerns between the administrator who has the ability to update the reservations table and a non-admin updating reservedTickets. Also, due to the high number of FOREIGN KEY constraints in reservedTickets having to join with the reservations table to track approval can be tricky depending on my starting point.
CREATE TABLE reservations(
rid int NOT NULL AUTO_INCREMENT,
aid int NOT NULL,
approval tinyint(1) NOT NULL,
creationTime TIMESTAMP
DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (rid),
FOREIGN KEY (aid) REFERENCES accounts (aid)
ON DELETE CASCADE
);
CREATE TABLE reservedTickets(
rid int NOT NULL,
tid int NOT NULL,
hid int NOT NULL,
approval tinyint(1) NOT NULL,
PRIMARY KEY (tid),
FOREIGN KEY (rid) REFERENCES reservations (rid)
ON DELETE CASCADE,
FOREIGN KEY (tid) REFERENCES tickets (tid)
ON DELETE CASCADE,
FOREIGN KEY (hid) REFERENCES people (hid)
ON DELETE CASCADE,
FOREIGN KEY (approval) REFERENCES reservations (approval)
ON UPDATE CASCADE
);

Answer 1

Personally I'd avoid cascade updates because of many locks the engine issues behind the scene, but if you really want it you can have a unique constraint on reservations(rid,approval) and foreign key in reservedTickets that references it. As far as I remember Mysql, it comes with a cost of extra index, but it gives you what you want.
On the other hand , you can implement desired functionality with trigger on reservation, or leave it up to application (or update table only through stored procedure that takes care of carrying this flag).
CREATE TABLE reservations(
rid int NOT NULL AUTO_INCREMENT,
aid int NOT NULL,
approval tinyint(1) NOT NULL,
creationTime TIMESTAMP
DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (rid),
CONSTRAINT UQ_RESERVATION_COMPOSITE UNIQUE(rid, approval),
...
);
CREATE TABLE reservedTickets(
rid int NOT NULL,
tid int NOT NULL,
hid int NOT NULL,
approval tinyint(1) NOT NULL,
PRIMARY KEY (tid),
FOREIGN KEY (rid,approval) REFERENCES reservations (rid,approval)
ON DELETE CASCADE ON UPDATE CASCADE,
FOREIGN KEY (tid) REFERENCES tickets (tid)
ON DELETE CASCADE,
FOREIGN KEY (hid) REFERENCES people (hid)
ON DELETE CASCADE , ....
);

Answer 2

It's not so much that you cannot use a foreign key on a non-unique column, because you can in InnoDB. However, the problem that arises with this would be if you have automatic updating or deleting on the foreign key, because the database would update all keys that were the same.
What you may be able to do, is use a prepared statement that allows you to update the approval column when updated from the reservations table, etc.