Tables Used

orderdetails

Column	Type
orderdetailid	bigint
orderid	bigint
productid	bigint
quantity	bigint
unitprice	double precision
currencyid	bigint

orders

Column	Type
orderid	bigint
customerid	bigint
orderdate	timestamp without time zone
totalamount	double precision
status	text

products

Column	Type
productid	bigint
name	text
description	text
category	text
price	double precision
supplierid	bigint
stockquantity	bigint

Expected Output

Column
category
productname
previousproduct

Reverse Product Lineage :Compare Each Product Sold with the One Before It in the Same Category Intermediate

Q: Do I need advanced SQL to attempt this?

This problem is labeled as Intermediate. It assumes you’ve written a few window function queries before, but you don’t need to be an expert. Use the hints and explanations if you get stuck — they’re there to help you think through the logic.

Track each product sold in reverse, showing its category and the item before it.

Xorthax wants to track the sequence of products sold in reverse. Write a query to display the current product and the previous product sold in each category.

Write Your Query

Answer:

          select p.category,
       p.name as productname,
       lag(p.name) over (partition by p.category
                         order by o.orderdate asc) as previousproduct
from orderdetails od
join orders o on od.orderid = o.orderid
join products p on od.productid = p.productid;

This reflects sales tracking where analysts study the order of product sales to understand category flow and buying sequences.

You’ll practice comparing records within grouped sequences to identify prior entries and analyze historical order relationships.

This problem is labeled as Intermediate. It assumes you’ve written a few window function queries before, but you don’t need to be an expert. Use the hints and explanations if you get stuck — they’re there to help you think through the logic.

Yes. Every problem comes with optional hints you can reveal one at a time, plus a fully worked step-by-step solution. You decide how much help you want while practicing.

All problems on PracticeWindowFunctions.com are completely free and can be solved without creating an account. Right now there are over 80 practice problems, with new ones added regularly.

SQL for Data Analysis (O’Reilly)

SQL techniques tailored for analytics and reporting workflows.

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Tables Used

Expected Output

Reverse Product Lineage :Compare Each Product Sold with the One Before It in the Same Category Intermediate

Track each product sold in reverse, showing its category and the item before it.

What real-world scenario does this represent?

What SQL concepts will I practice here?

Do I need advanced SQL to attempt this?

Will I get hints and step-by-step explanations if I get stuck?

Are the problems free to access, and how many are available?

SQL for Data Analysis (O’Reilly)