Question

I am just learning about foreign keys. I am simplifying my tables to this:

Table:  item
Columns:  item_id (PK), qty

Table:  item_change_log
Columns:  id (PK), item_id (indexed), change_value

Both tables are INNODB.

Everytime a change in the item levels is made, the quantity changed is logged in the item_change_log.change_value. I want the change_value to automatically add/subtract item.qty.

The intended behavior:

User creates Item X. item.item_id = X, qty = 0. No record is made in item_change_log yet.
User changes quantity of X on hand to 2. item_change_log: id = 1, item_id = X, change_value = 2. This change_value automatically increases item.qty = 2 via foreign keys.

I know how to make the foreign keys for the item id’s see each other, but I don’t know how to add the “math” element, or if that is possible.

Thanks!

Answer 1

What you need is a trigger, but which?

• Since it is the item_change_log table your user actually modifies,
  you need a trigger on item_change_log
• Since you allways insert into item_change_log, you need an INSERT
  trigger
• Since You only want to update item only, if the insert to item_change_log
  really works, you want an AFTER INSERT trigger

So here we go

CREATE TRIGGER update_item_table AFTER INSERT ON item_change_log
  FOR EACH ROW BEGIN
    UPDATE item SET qty=qty+NEW.qty WHERE item_id=NEW.item_id;
  END;