Conditional Trigger

I have a trigger that updates values in a table when col2 is updated by a stored procedure.

I'd like to make this a conditonal trigger to prevent negative values from being inserted in the table. I'm not familiar with the syntax of triggers enough to get this to work and I've search for a reference with no luck.

Here's the current trigger code:


Code:

FOR INSERT, UPDATE
AS
UPDATE table1
SET col4= col1 - col2 - col3



I need to evaulate (col1 - col2 - col3) to see if it's less than zero. If it is, I want to set col4=0.

This is what I've tried, but it doesn't work. Any help is appreciated:


Code:

FOR INSERT, UPDATE
AS
select col1, col2, col3 from table1
if (col1 - col2 - col3) < 0
UPDATE table1
SET col4 = 0
else
UPDATE table1
SET col4= col1 - col2 - col3