MySQL storing VAT rates and amounts for each invoice - mysql

Question

I have the tables invoice_header, invoice_rows and vat_codes from the billing software based on a MySQL database. I need to add my own table invoice_vat.
For each new invoice (or updated invoice) I need to store (or update) VAT rates, net amounts and VAT amounts into the table invoice_vat, groupped by the vat code.
I have several vate codes and rates. For example I have:
ABC tax rate 5%
DEF tax rate 10%
GHI tax rate 22% etc.
For each invoice I can sell together some products with ABC vat, some products with DEF vat and some other products with GHI vat. Usually we do not use more than 5 different vat rates in one invoice.
For example one of my invoice looks like:
product1, quantity 2pcs, unit_net_price 100.00, row_net_amount 200.00, vat ABC.
product2, quantity 3pcs, unit_net_price 50.00, row_net_amount 150.00, vat ABC.
product3, quantity 1pcs, unit_net_price 90.00, row_net_amount 90.00, vat DEF.
product4, quantity 4pcs, unit_net_price 25.00, row_net_amount 100.00, vat GHI.
For this invoice I need to store the values into the tables invoice_vat:
vat1_code ABC, vat1_rate 5, vat1_net_total_amount 350.00
vat2_code DEF, vat2_rate 10, vat2_net_total_amount 90.00
vat3_code GHI, vat3_rate 22, vat3_net_total_amount 100.00
vat4_code NIL, vat4_rate 0, vat4_net_total_amount 0.00
vat5_code NIL, vat5_rate 0, vat5_net_total_amount 0.00
I can do the following query:
SELECT invoice_rows.vat, sum(invoice_rows.row_net_amount) AS mysum
WHERE invoice_rows.id=655
group by invoice_rows.vat;
Now I need to store the values I get from the query, every time I make a new invoice or I revise/update an old invoice.
Should I use a trigger? How should look the trigger?
Giuseppe

Answer 1

Have table(s) that itemize the components of an invoice. Have another table that shows the invoice amounts as sent to the customer. For accounting and business and maybe legal reasons, this table should be "write once". That is, you should never change it, once it is finalized. At this point you are recording what you sent to the customer; it is not some scratch pad where things can be added/changed/deleted.
Note, in particular, if a item's price changes, or a VAT amount changes, then new computations would lead to an invoice that disagrees with what you sent the customer. Not good.