Trigger in MYSQL that delete - mysql

Question

I have posted this code in another threat, but this question is slightly different. I have to make a trigger. The question sounds like this:
"Create a trigger that removes the season if unused whenever
information about when and who produces a product is deleted (i.e.,
whenever a record is removed from the 'Produces' table, check if there
are other records about product being produced for the same season, if
not, remove also the corresponding record from the season table)"
My MYSQL code:
the MYSQL code is here:
CREATE TABLE IF NOT EXISTS Dvds(
Serial integer NOT NULL,
Name varchar(50),
Year integer,
Genre varchar(50),
Price integer,
PRIMARY KEY (Serial));
CREATE TABLE IF NOT EXISTS Shops(
Id integer NOT NULL,
Name varchar(50),
Address varchar(50),
PRIMARY KEY (Id));
CREATE TABLE IF NOT EXISTS Customers(
CNo integer NOT NULL,
AccNo integer,
Time varchar(50),
PRIMARY KEY (CNo));
CREATE TABLE IF NOT EXISTS ContactPersons(
Id integer NOT NULL,
Name varchar(50),
Phone integer,
PRIMARY KEY (Id));
CREATE TABLE IF NOT EXISTS Seasons(
StartDate date NOT NULL,
EndDate date NOT NULL,
PRIMARY KEY (StartDate,EndDate));
CREATE TABLE IF NOT EXISTS WebShops(
Id integer NOT NULL,
Url varchar(50),
FOREIGN KEY (Id) REFERENCES Shops (Id),
PRIMARY KEY (Id));
CREATE TABLE IF NOT EXISTS Producer(
Id integer NOT NULL,
Address varchar(50),
Name varchar(50),
PRIMARY KEY (Id));
CREATE TABLE IF NOT EXISTS Sold(
Id integer NOT NULL,
CNo integer NOT NULL,
Serial integer NOT NULL,
FOREIGN KEY (Id) REFERENCES Shops (Id),
FOREIGN KEY (CNo) REFERENCES Customers (CNo),
FOREIGN KEY (Serial) REFERENCES Dvds (Serial),
PRIMARY KEY (Id,CNo,Serial));
CREATE TABLE IF NOT EXISTS Has(
Id integer NOT NULL,
Serial integer NOT NULL,
FOREIGN KEY (Id) REFERENCES Shops (Id),
FOREIGN KEY (Serial) REFERENCES Dvds (Serial),
PRIMARY KEY (Id,Serial));
CREATE TABLE IF NOT EXISTS Has2(
Serial integer NOT NULL,
Producer_Id integer NOT NULL,
StartDate date NOT NULL,
EndDate date NOT NULL,
ContactPersons_Id integer NOT NULL,
FOREIGN KEY (Serial) REFERENCES Dvds (Serial),
FOREIGN KEY ( Producer_Id) REFERENCES Producer (Id),
FOREIGN KEY (StartDate) REFERENCES Seasons (StartDate),
FOREIGN KEY (EndDate) REFERENCES Seasons (EndDate),
FOREIGN KEY (ContactPersons_Id) REFERENCES ContactPersons (Id),
PRIMARY KEY (Serial,Producer_Id,StartDate,EndDate,ContactPersons_Id));
The trigger:
CREATE TRIGGER `Seasons_before_delete`
AFTER DELETE ON `Seasons`
FOR EACH ROW
BEGIN
DELETE FROM seasonstart
WHERE seasonstart.???????
DELETE FROM seasonend
WHERE seasonend.??????
END
but I really don't know. My teacher told me that I could use IF , ELSEIF, GOTOEND? But am i on the right track? I'm really blank right now what to do, so hope someone have a suggestion, what I could do to solve this?

Answer 1

In a trigger you have 2 virtual tables.
NEW and OLD.
Obviously in a create trigger there is no old and in a delete trigger there is no new.
So if you want to use the data of the table where the change happened (the table the trigger is attached to) you use those two tables.
Because this is a delete trigger you'll need to use the old table.
So your trigger will look something like this:
CREATE TRIGGER `Seasons_before_delete` AFTER DELETE ON `Seasons`
FOR EACH ROW
BEGIN
DELETE FROM seasonstart
WHERE seasonstart.seasonID = old.ID
DELETE FROM seasonend
WHERE seasonend.seasonID = old.ID
END
Note that it makes little sense to call a AFTER DELETE trigger "something_something_BEFORE" :-).
Further note that I have no idea why you need an IF in this trigger, the where in the delete statement already takes care of that.