PARTITION BY: Grouping Inside Your Window

Welcome to **Day 63**. Today we combine the power of grouping with the flexibility of windows using `PARTITION BY`.

What is a Partition?

A partition tells the window function: "Reset your calculation every time this column changes."

Real-World Example: Top Product inside each Category

If you want to find the top-selling product in *each* category (Electronics, Clothing, Food), a global rank won't work.

SELECT

category,

product_name,

sales,

RANK() OVER(PARTITION BY category ORDER BY sales DESC) as rank_in_category

FROM products;

How it works

The database "buckets" the rows by category, sorts them by sales within that category, and assigns ranks. Then it starts over for the next category.

Why analysts love it

It allows you to answer complex questions like "Who is the highest paid person in every department?" or "What was the latest order for every customer?" in a single query.

Your Task for Today

Rank your employees by salary, but partitioned by their department.

*Day 64: LAG & LEAD—Looking into the Past and Future.*