How to validate with before insert trigger in sqlite

Question

I've been searching on the web how to use "before trigger" in sqlite. I know properly on using "after trigger". I found the one that said that "before trigger" was used to validate before any changes made to database (insert/updat row in table). To clear the expression, I wrote a trigger in which type is before and event is insert to validate mark before inserting. My trigger is

create trigger mark_insert before insert on exam
when mark<=50
begin
insert into exam(mark) values(50);
end; 

Maybe it is wrong trigger. So, could you please point me out the errors in it? I want to check the mark whether it is less than or equal 50. If so, I want to insert default value(in this case, 50). I'm not much familiar with sql and trigger and starting to learn.

My exam table has following columns,

id(pk) | subject | mark

Answer 1

if you want to check the mark whether it is less than or equal 50

CREATE TRIGGER trigger_min_mark AFTER INSERT ON exam
WHEN New.mark<=50
BEGIN
  UPDATE exam SET mark=50 WHERE id = New.id;
END

Be carefull you in the when statement you must specify New.ColumnName