MySQL : increment a column by one after insert through a trigger - mysql

Question

So here it is guys,
I have four tables.
My first one :
CREATE TABLE IF NOT EXISTS `Users` (
`Id_User` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`Firstname` varchar(20) NOT NULL,
`Lastname` varchar(20) NOT NULL,
PRIMARY KEY (`Id_User`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1002 ;
It contains the people's data.
My second one :
CREATE TABLE IF NOT EXISTS `Subject` (
`PrimaryKey_Subject` varchar(4) NOT NULL,
`Subject_Name` int(5) NOT NULL,
PRIMARY KEY (`PrimaryKey_Subject`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
It contains the subject name : mathematic, science, biology, etc...
My third one :
CREATE TABLE IF NOT EXISTS `Register` (
`ForeignKey_User` smallint(5) unsigned NOT NULL,
`ForeignKey_Lesson` varchar(4) NOT NULL,
PRIMARY KEY (`ForeignKey_User`,`ForeignKey_Lesson`),
KEY `ForeignKey_User_I` (`ForeignKey_User`),
KEY `ForeignKey_Lesson` (`ForeignKey_Lesson`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
This one allows a user to register to a lesson.
My fourth one :
CREATE TABLE IF NOT EXISTS `Subject_Annex` (
`PrimaryKey_Subject` varchar(4) NOT NULL,
`Number_Registered` int(5) NOT NULL,
PRIMARY KEY (`PrimaryKey_Subject`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
And here it is,
How can I set a trigger that increment the Number_Registered each time someone register to a lesson?
For instance,
I create a user : John Doe --> Users Table.
Then afterwards, I have my Subject Table which contains all the courses: mathematics, science, biology, etc...
I can now, register, through the Register Table my freshly created user to a subject : let's say science
Then now we have the Subject_Annex Table, which through a trigger, and once my user, (John Doe), is linked to a subject, (Science), is supposed to be able to show the amount of users that registered to the subject from, (here Science),and also increase by 1 their number for each time someone register to this subject.
So to make it short,
How can I set a trigger that increments the registered number of user each time someone register to a subject ?
For example :
Primary Key Subject : Science
Number of registered : 1
(Someone else register)
Primary Key Subject : Science
Number of registered : 2
(Someone else register)
Primary Key Subject : Science
Number of registered : 2
Primary Key Subject : Mathematic
Number of registered : 1
Etc...

Answer 1

I managed to solved the problem by myself :
DELIMITER |
create trigger before_insert_subject_annexe
before INSERT
on Register
for each row
BEGIN
INSERT INTO Subject_Annex (PrimaryKey_Subject, Number_Registered)
VALUES (NEW.ForeignKey_Lesson, + 1)
ON DUPLICATE KEY UPDATE
Number_Registered=Number_Registered+1;
END;
DELIMITER |
Then on my Subjet_Annex Table :
CREATE TABLE IF NOT EXISTS `Subject_Annex` (
`PrimaryKey_Subject` varchar(4) NOT NULL,
`Number_Registered` int(5) NOT NULL,
PRIMARY KEY (`PrimaryKey_Subject`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
I added an unique key on Number_Registered