row_number over partition by sql

The `ROW_NUMBER()` window function is a powerful tool in SQL for assigning unique sequential numbers to rows within a partition. It's particularly useful when you need to rank items within specific groups or categories. The `PARTITION BY` clause is essential because it defines the groups over which the numbering takes place. Without `PARTITION BY`, `ROW_NUMBER()` would simply assign a unique number to every row in the entire result set, effectively creating a global ranking. Imagine you have sales data for different regions. Using `ROW_NUMBER()` with `PARTITION BY` region allows you to assign a rank to each sale within that specific region. This is different from a global rank, where the highest sale across all regions would be ranked 1.Crucially, `ROW_NUMBER()` assigns numbers consecutively within each partition. If you have two sales in the same region with the same amount, they will receive consecutive numbers. This is distinct from other ranking functions like `RANK()` or `DENSE_RANK()`, which might assign the same rank to rows with equal values. This is important because `ROW_NUMBER()` ensures that every row gets a unique number, which is essential for tasks like assigning unique IDs within categories.Understanding the `PARTITION BY` clause is key. It divides the data into groups, and `ROW_NUMBER()` operates independently on each group. This allows for customized ranking within specific subsets of your data, making it highly versatile for various analytical tasks. For example, you might want to identify the top-performing salesperson in each region or assign unique identifiers to customer orders within a specific month.