Question

I have two tables with the following (simplified) structures:

1. table "Factors" which holds data about purchased goods’ factors and has these columns:

   FactorSerial, PurchaseDate, PurchasedGood

2. table "Prices" which holds goods prices on different dates

   Serial, GoodCode, EvaluationDate, Price

A price is valid until a new row with the same Code but different date is added and thus updates its value

Now, I want to create a table which adds the price to the table 1 according to purchase date.
So if we have:

  PurchaseDate  PurchasedGood
 -----------------------------
  05/20/2011      A111

and:

  GoodCode  EvaluationDate  Price
 --------------------------------
   A111      02/01/2011     100
...
   A111      04/01/2011     110
...
   A111      06/01/2011     120

the result would be

  PurchaseDate  PurchasedGood  Price
 -----------------------------------
  05/20/2011      A111          110

Preferred method is creating the view Prices1 as

 Serial  GoodCode   StartDate  EndDate  Price

and then joining Factors with this view by

  PurchasedDate between StartDate AND EndDate

Can anybody show me how to create view1 (or obtaining the final result with any other method)? Thanks in advance!

P.S. sorry for my bad English!

Answer 1

I want to create a table which adds the price to the table 1 according to purchase date.

Here is a query that returns such data. The syntax is pretty standard SQL, I believe, but this was tested on SQL Server (looks like you may be using PostgreSQL with your "serial" naming).

select a.FactorSerial, a.PurchasedGood, a.PurchaseDate
    , (select max(Price) from Prices where GoodCode = a.PurchasedGood and EvaluationDate = a.EvaluationDate) as Price
from (
    select f.FactorSerial, f.PurchasedGood, f.PurchaseDate, max(p.EvaluationDate) as EvaluationDate
    from Factors as f
        join Prices as p on f.PurchasedGood = p.GoodCode
    where f.PurchaseDate >= p.EvaluationDate
    group by f.FactorSerial, f.PurchasedGood, f.PurchaseDate
) as a

This query assumes that there are no Purchases before a Price existed.

Also, considering:

Preferred method is creating the view Prices1 as

Serial  GoodCode   StartDate  EndDate  Price

and then joining Factors with this view by

 PurchasedDate between StartDate AND EndDate

between is inclusive. Using this method that you’ve described, you would get duplicates if a PurchaseDate lies on the EndDate of one row and the StartDate of another.