Retrieving Cost on a Certain Date in SQL: A Comprehensive Guide

Introduction

SQL (Structured Query Language) is a standard language for managing relational databases. In this guide, we’ll explore how to retrieve the cost of an order on a specific date, taking into account the price history of items from that date forward.

We’ll start by examining the given tables and their relationships:

• Order: stores information about each order, including orderID, date, and shipID.
• Shipping: contains details about shipping costs, with shipID as the primary key.
• PO (Purchase Order): keeps track of the products purchased in an order, along with their quantities (amount) and corresponding orderID.
• Price: stores historical prices for each product, starting from a specific date (fromDate).

Understanding the Problem

Our goal is to calculate the total cost of an order on a given date, considering the price history of items up to that point. We can break this down into two parts:

1. Cost of items: retrieve the price of each item at the specified date.
2. Shipping costs: add the shipping cost for each order.

Using Subqueries to Retrieve Prices

The solution provided uses a subquery to get the latest price for each product up to the given date. Let’s break it down:

SELECT o.orderId, SUM(pr.price) + SUM(s.price) AS orderCost
FROM [Order] o 
INNER JOIN Shipping s ON o.shipId = s.shipId
inner join PO po on o.shipId   = po.shipId 
inner join (select top 1 price from Price pr where fromDate = (select max(fromDate)from Price pr where fromDate <= [date])) pr 
on pr.prodID  = po.prodID
GROUP BY  o.orderId;

This subquery is executed first, and its results are used in the main query. The innermost subquery:

SELECT top 1 price from Price pr where fromDate = (select max(fromDate)from Price pr where fromDate <= [date])

• finds the maximum fromDate that’s less than or equal to the specified date.
• selects the corresponding price value.

The outer subquery:

SELECT top 1 price from Price pr where fromDate = (select max(fromDate)from Price pr where fromDate <= [date])

• finds all Price records with a fromDate equal to the maximum found in the inner subquery.
• selects only the top row, which corresponds to the latest price up to the given date.

The main query then joins this result with other tables and calculates the total order cost by summing shipping costs and prices.

Using Window Functions (Alternative Solution)

Another approach uses window functions to find the maximum fromDate for each product that’s less than or equal to the specified date, and then selects the corresponding price.

SELECT o.orderId, SUM(s.price) + SUM(pr.price) AS orderCost
FROM [Order] o 
INNER JOIN Shipping s ON o.shipId = s.shipId
inner join PO po on o.shipId   = po.shipId 
left join (
    SELECT prodID, price,
           ROW_NUMBER() OVER (PARTITION BY prodID ORDER BY fromDate DESC) AS row_num
    FROM Price
) pr ON po.prodID  = pr.prodID AND pr.row_num = 1
GROUP BY  o.orderId;

In this solution:

• We use ROW_NUMBER() with a partition by product ID and ordering by fromDate in descending order to get the latest price for each product.
• The outer query joins this result with other tables and calculates the total order cost.

Conclusion

Retrieving the cost of an order on a specific date requires careful consideration of price history. By using subqueries or window functions, we can efficiently calculate the total cost, including shipping costs, while taking into account the price evolution over time.

Remember to always consider performance implications when choosing between these approaches and ensure that your database schema is optimized for efficient data retrieval.

Last modified on 2024-12-18