Update records from a table in correct sequence that look from 2 tables and loop

Question

Hi!
My question title is kinda unclear but here how it goes.
I created  2 tables for my BOM (Bill of Materials). I have BOM_Header and BOM_Detail. 
Below are my sample data. 
BOM_Header
Product_ID Int
Product_Name Varchar(50)
Cost Numeric(18,2)
Yield Varchar(50)
Select * from BOM_Header
1 Choco Cake 850.00 10
2 Mixed Flour 700.00 30
3 Choco Syrup 160.00 10
4 Egg Formula 2150.00 20
BOM_Detail
Product_ID int
ItemNo Int
ItemName varchar(50)
Quantity int
Unit varchar(50)
ProdCost numeric(18,2)
Select * from BOM_Detail
1 2 Mixed Four 10 Grams 15.00
1 3 Choco Syrup 20 ML 25.00
1 4 Egg Formula 20 Grams 10.00
2 101 Flour 5 packs 80.00
2 4 Egg Formula 5 Grams 60.00
3 201 Cocoa Power 2 kg 20.00
3 202 Sugar 2 kg 60.00
4 301 Whole Egg 10 pcs 85.00
4 302 EP12 Formula 25 ml 52.00
My computation is below.
BOM_Header = a
BOM_Detail = b
a.Cost = b.Quantity  x  b.Product Cost  where a.Product_ID = b.Product_ID
My problem is how can I automatically compute their Food Cost in  sequence from raw materials to finished products.
Based on the data, I need to compute first the Egg Formula because it is used as component of  Mixed Flour then compute the Mixed Flour and other component to get the cost of Choco Cake. 
How can I do this automatically in query to look first in detail if there are ingredients within a sub - ingredients then compute before computing the final cost of the Product. 
This is because cost of ingredients are changing most of the time and need to recalculate to get the most updated cost.
Any suggestion is very much appreciated.
Thank you very much,
Regem

Answer 1

>> My question title is kinda unclear but here how it goes. <<
Then your answers will be unclear, too :(  
You do not know data modeling, so your data types are wrong. What math are you doing with the product_id? None. This is why identifiers are char(n) and not numeric. They should be industry standards if possible. I like the GTIN.
You do not even know that rows are not records. 
Why is the product name fifty characters? Certainly not research! But if you are an ACCESS programmer using a default vale, then you might do this. 
Besides violating ISO-11179 rules, “<vague>_field” makes no sense! It is a quantity that you put in a  string. 
CREATE TABLE BOM_Header
(gtin CHAR(15) NOT NULL PRIMARY KEY,
 product_name   VARCHAR(25) NOT NULL
 unit_cost  DECIMAL(18,2) NOT NULL
  CHECK ( unit_cost >= 0.0.),
 yield_qty INTEGER NOT NULL
   CHECK (yield_qty >= 0));
>> Any suggestion is very much appreciated. <<
Get a copy of my book on TREES in SQL and read the chapter on BOM problems. I am not goingto try to post a whole chaper and diagrams to answer this. You are doing the wrong things and have done them poorly. 
--CELKO-- Books in Celko Series for Morgan-Kaufmann Publishing: Analytics and OLAP in SQL / Data and Databases: Concepts in Practice Data / Measurements and Standards in SQL SQL for Smarties / SQL Programming Style / SQL Puzzles and Answers / Thinking
in Sets / Trees and Hierarchies in SQL