Update another table after insert using a trigger?

Question

I\'m currently creating some sort of inventory system.

I have tbl1 where in I save the items. In tbl1, I have column qty or th

Answer 1

If you're not expecting huge amounts of data use a view or a stored procedure to calculate on the fly and return actual quantity. It may save you a lot of headache in the long run.

Your view (actually views) might look like

CREATE VIEW vw_table2_sum AS
SELECT product, SUM(issued_qty) qty
  FROM Table2
 GROUP BY product;

CREATE VIEW vw_table1 AS
SELECT t.id, t.product, t.qty - COALESCE(v.qty, 0) qty
  FROM Table1 t LEFT JOIN vw_table2_sum v
    ON t.product = v.product;

When we do

SELECT * FROM vw_table1;

will get

| ID |  PRODUCT | QTY |
-----------------------
|  1 |    mouse |   8 | -- the quantity is current
|  2 | keyboard |  15 |
|  3 |  monitor |   8 |

Here is SQLFiddle demo

Now if you for some reason want to manage your inventory quantity with a trigger it might look like this

CREATE TRIGGER tg_ai_table2
AFTER INSERT ON table2
FOR EACH ROW
  UPDATE Table1
     SET qty = qty - NEW.issued_qty
   WHERE product = NEW.product;

Here is SQLFiddle demo