Window functions (ROW_NUMBER) in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the ROW_NUMBER() window function do in SQL?

ROW_NUMBER() assigns a unique sequential number to each row within a partition of a result set, starting at 1 for the first row.

beginner

How do you reset the row numbering for each group in a table using ROW_NUMBER()?

Use the PARTITION BY clause inside the OVER() clause to restart numbering for each group.

intermediate

Write a simple SQL query using ROW_NUMBER() to number employees ordered by their salary descending.

SELECT employee_id, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) AS rank FROM employees;

beginner

Can ROW_NUMBER() be used without PARTITION BY? What happens then?

Yes, if PARTITION BY is omitted, ROW_NUMBER() numbers all rows in the entire result set sequentially.

intermediate

Why is ROW_NUMBER() useful compared to using LIMIT or OFFSET for pagination?

ROW_NUMBER() allows numbering rows in a stable order and can be combined with filtering to implement efficient pagination without skipping rows.

What does ROW_NUMBER() return for each row?

A A unique sequential number starting at 1

B The total count of rows

C The sum of values in the row

D The average of values in the partition

ROW_NUMBER() assigns a unique sequential number to each row starting at 1.

Which clause is used with ROW_NUMBER() to restart numbering for each group?

A PARTITION BY

B ORDER BY

C GROUP BY

D HAVING

PARTITION BY divides rows into groups and restarts ROW_NUMBER() for each group.

What happens if you omit PARTITION BY in ROW_NUMBER()?

A It causes an error

B Rows are grouped by default

C Rows are numbered across the entire result set

D Rows are numbered randomly

Without PARTITION BY, ROW_NUMBER() numbers all rows sequentially in the full result set.

How can ROW_NUMBER() help with pagination?

A By grouping rows

B By counting total rows

C By sorting rows alphabetically

D By numbering rows to filter specific pages

ROW_NUMBER() numbers rows so you can select a range for a page, like rows 11 to 20.

Which of these is a valid ROW_NUMBER() usage?

A ROW_NUMBER() GROUP BY department

B ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC)

C ROW_NUMBER() ORDER BY salary

D ROW_NUMBER() WHERE salary > 50000

ROW_NUMBER() requires OVER() with optional PARTITION BY and ORDER BY clauses.

Explain how ROW_NUMBER() works and how you can use PARTITION BY with it.

Think about numbering rows within groups like pages in a book.

Describe a real-life scenario where ROW_NUMBER() can help solve a problem.

Imagine you want to show top salespeople per region.