I have 2 tables: "products" and "pieces"

PRODUCTS

idProd
product
price

PIECES

id
idProdMain
idProdChild
quant

idProdMain and idProdChild are related with the table: "products".

Other considerations is that 1 product can have some pieces and 1 product can be a piece.

Price product equal a sum of quantity * price of all their pieces.

"Products" table contains all products (p

EXAMPLE:

TABLE PRODUCTS (idProd - product - price)

1 - Computer - 300€
2 - Hard Disk - 100€
3 - Memory - 50€
4 - Main Board - 100€
5 - Software - 50€
6 - CDroms 100 un. - 30€

TABLE PIECES (id - idProdMain - idProdChild - Quant.)

1 - 1 - 2 - 1 
2 - 1 - 3 - 2
3 - 1 - 4 - 1

WHAT I NEED?

I need update the price of the main product when the price of the product child (piece) is changed. Following the previous example, if I change the price of this product "memory" (is a piece too) to 60€, then product "Computer" will must change his price to 320€

How I can do it using queries?

Already I have tried this to obtain the price of the main product, but not runs. This query not returns any value:

SELECT Sum(products.price*pieces.quant) AS Expr1
FROM products LEFT JOIN pieces ON (products.idProd = pieces.idProdChild) AND (products.idProd = pieces.idProdChild) AND (products.idProd = pieces.idProdMain)
WHERE (((pieces.idProdMain)=5));

MORE INFO

The table "products" contains all the products to sell that it is in the shop.
The table "pieces" is to take a control of the compound products. To know those who are the products children. For example of compound product: computers. This product is composed by other products (motherboard, hard disk, memory, cpu, etc.)