Hi . i want to create a trigger for display error when my product number equal 2

Create trigger trg_4
No cascade before insert on sales
For each row mode db2sql
when (select pro_num from product,sales where product.pro_id = sales.pro_id and product.pro_num= 2)
Signal sqlstate '80000' ('Insufficient stock on hand!');

in this trigger select statement exactly work but db2 return error for when
why ???

WHEN must be followed by a condition.

e.g.

when (( select COUNT(*) from product,sales where product.pro_id = sales.pro_id and product.pro_num= 2) < 2 )
Signal sqlstate '80000' ('Insufficient stock on hand!');

would work.

you need to add "EXISTS" after "WHEN" statement.