BEFORE INSERT trigger in MySQL
I have dealt with this question before. MySQL has a very flimsy architecture when it comes to triggers.

I have written posts before on how to break triggers midstream. The solution is rather unsavory and would leave a pure SQL developer with a bad taste in the month.

With that said, here are my posts that I hope would help:


Apr 25, 2011 : Trigger in MySQL to prevent insertion
Dec 23, 2011 : check constraint does not work?


Since you are a newbie, writing trigger code requires changing the delimiter to something other than a semicolon and then changing back.

DELIMITER $$

CREATE TRIGGER chk_dates  
BEFORE INSERT ON verlof_aanvragen  
FOR EACH ROW  
BEGIN  
  IF (NEW.datum &lt; CURDATE() + INTERVAL 8 DAY OR NEW.datum &gt; CURDATE() + INTERVAL 365 DAY) 
THEN  
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Any Message';
END IF;
END $$

DELIMITER ;


Please look back at my posts to see how I do this in my code.

UPDATE 2013-06-12 11:34 EDT

Here is your last comment


  I am using above trigger with mysql server v 5.5.24 and it works fine.
  This is my local test site. Now I want to use it at my webhost, they
  have mysql server v 5.1.56 ... and it won; t work ! Am I missing
  something?


The problem is the MySQL version. If you look back in my two trigger posts, I used black market techniques to break the trigger midstream. This is necessary for older versions of MySQL. Notice what I stated on Apr 25, 2011 in Trigger in MySQL to prevent insertion :


  The reason the book suggests preempting the Trigger in this manner
  stems from the fact that the MySQL Stored Procedure Language did not
  have SIGNAL implemented into the language (of course, SIGNAL is ANSI
  standard).
  
  The authors of the book created work arounds by calling on SQL
  statements that are syntactically correct but fail at runtime. Pages
  144-145 (Chapter 6: Error Handling) of the book gives these examples
  on preempting a Stored Procedure directly (Example 6-18) or by SIGNAL
  emulation (Examples 6-19 and 6-20).


I directly quoted Pages 254-256 of the Book



Please note the following:


MySQL 5.5.8 was released as GA Dec 03, 2010
The book was written March 2006


Therefore, signal processing DOES NOT WORK IN MySQL 5.1.

You will have to do the following:


Rewrite the trigger in MySQL 5.1 to break midstream
The trigger you wrote in the MySQL 5.5 database should stay the way it is now


That's why the trigger does not work in MySQL 5.1.

UPDATE 2013-06-12 11:44 EDT

Here is what you can do to the trigger in the MySQL 5.1.56 database

DELIMITER $$

CREATE TRIGGER chk_dates  
BEFORE INSERT ON verlof_aanvragen  
FOR EACH ROW  
BEGIN  
  DECLARE dummy INT;
  IF (NEW.datum &lt; CURDATE() + INTERVAL 8 DAY OR NEW.datum &gt; CURDATE() + INTERVAL 365 DAY) 
THEN  
  SELECT 'Any Message' INTO dummy FROM mysql.user WHERE used = 'anything';
END IF;
END $$

DELIMITER ;


Since mysql.user has no column called used, the query does not execute, breaking the trigger.