Question

Let’s say I have two tables as shown:

user
id    plan     course_limit     username 
10                  0            ahmad

note: plan is enum field containing '','a','b','c'.

course
id    user_id   username
1       10        ahmad

Now I want when a user insert into course as shown I want the course_limit to increment by 1 for that user so that I can apply a limit.

Answer 1

You can create a trigger with the following code.

CREATE 
DEFINER = 'root'@'localhost'
TRIGGER databasename.AI_course_each
AFTER INSERT
ON databasename.course
FOR EACH ROW
BEGIN 

  UPDATE user SET user.course_limit = user.course_limit + 1
  WHERE user.user_id = new.user_id; 

END;

Some explanation
You create a trigger that fires once for every row FOR EACH ROW that is inserted AFTER INSERT into table course.

When you insert into the table you can trigger BEFORE and AFTER the insert is done.
If you want to be able to prevent the insert you fire before, if you just want to do useful work, you fire after.
The inserted fields can be accessed via a dummy table ‘new’.

So here after each insert the UPDATE statement gets executed.

More trigger options
You can also use triggers BEFORE UPDATE, AFTER UPDATE,BEFORE DELETE and AFTER DELETE.

In the update and delete cases you get an extra dummy table old that you can use to refer to the data in the table before the update or delete happened.

Lots more is possible, but I’ll keep it simple for now.

See http://forge.mysql.com/wiki/Triggers for more info.