Logic behind loading dimension tables

Question

How do I populate Dim_tbls from a relational source?

These example tables are given:

tbl_sales:    id_sales, fk_id_customer, fk_id_product, country, timestamp   
tbl_customer: id_customer, name, adress, zip, city
tbl_product:  id_product, price, product

My goal is to get these attributes into a start-schema. The problem I have is the logic behind loading the dimension tables. I mean, what data would I load into the Dim_Product? All the products that are in tbl_product? But how would I know how many Sales are done with a specific product?

Analysis I would like to do are:

 How many people bought product x.
 How many sales are made from city x.
 How many sales were made between Time x and y. 

---

Example data:

 tbl_sales: id_sales | fk_id_customer | fk_id_product | country | timestamp 
                1    |       2        |      1        |   UK    | 19.11.2013 10:23:22
                2    |       1        |      2        |   FR    | 20.11.2013 06:04:22

 tbl_customer: id_customer | name | adress | zip | city
                      1    | Frank|Street X| 211 | London
                      2    | Steve|Street Y| 431 | Paris

 tbl_customer: id_product| Price | product
                      1  | 100,00| Hammer
                      2  |  50,00| Saw

Answer 1

Let's start with a very simple star schema model; for example, I assumed you don't need to worry about handling changes to dimensions' attributes.

factSales

  DateKey
  CustomerKey
  ProductKey
  Counter (=1; this is a factless fact table)

dimDate

  DateKey
  Date
  Year
  Quarter
  Month
  ...

dimCustomer

  CustomerKey
  Name
  Address
  Zip
  City

dimProduct

  ProductKey
  Name
  Price (if it changes, you need move it to factSales)

---

How many people bought product x.

SELECT DISTINCT CustomerKey
FROM factSales
WHERE ProductKey IN ( SELECT ProductKey
                      FROM dimProduct
                      WHERE Name = 'Product X' )

How many sales are made from city x.

SELECT SUM(Counter)
FROM factSales
WHERE CustomerKey IN ( SELECT CustomerKey
                       FROM dimCustomer
                       WHERE City = 'City X' )

How many sales were made between Time x and y.

SELECT SUM(Counter)
FROM factSales
WHERE DateKey IN ( SELECT DateKey
                   FROM dimDate
                   WHERE Date BETWEEN DateX AND DateY )