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.

Practice

1. What does the ROW_NUMBER() function do in MySQL window functions?

easy

A. Assigns a unique sequential number to each row in the result set.

B. Calculates the sum of values in a column.

C. Filters rows based on a condition.

D. Joins two tables based on a key.

Solution

1. Step 1: Understand the purpose of ROW_NUMBER()

   ROW_NUMBER() assigns a unique number to each row in the order specified by ORDER BY.

2. Step 2: Differentiate from other functions

   It does not sum, filter, or join; it only numbers rows uniquely.

3. Final Answer:

   Assigns a unique sequential number to each row in the result set. -> Option A

4. Quick Check:

   ROW_NUMBER() = unique row numbering [OK]

Hint: ROW_NUMBER() gives each row a unique number in order [OK]

Common Mistakes:

• Confusing ROW_NUMBER() with aggregate functions like SUM()
• Thinking ROW_NUMBER() filters rows
• Assuming ROW_NUMBER() joins tables

2. Which of the following is the correct syntax to assign row numbers partitioned by department and ordered by salary DESC?

easy

A. ROW_NUMBER(PARTITION BY department ORDER BY salary DESC)

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

C. ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC)

D. ROW_NUMBER() BY department ORDER BY salary DESC

Solution

1. Step 1: Recall correct ROW_NUMBER() syntax

   The correct syntax uses OVER() with PARTITION BY and ORDER BY inside parentheses.

2. Step 2: Identify correct option

   ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) correctly uses ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC).

3. Final Answer:

   ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) -> Option C

4. Quick Check:

   Correct syntax includes OVER() with PARTITION BY and ORDER BY [OK]

Hint: ROW_NUMBER() always needs OVER() with clauses inside [OK]

Common Mistakes:

• Omitting OVER() keyword
• Placing PARTITION BY outside parentheses
• Using incorrect keywords order

3. Given the table Employees with columns id, department, salary, what will be the output of this query?

SELECT id, department, salary, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rn FROM Employees;

Assuming data:
id | department | salary
1 | Sales | 5000
2 | Sales | 7000
3 | HR | 6000
4 | HR | 5500

medium

A. [{id:2, department:'Sales', salary:7000, rn:2}, {id:1, department:'Sales', salary:5000, rn:1}, {id:3, department:'HR', salary:6000, rn:2}, {id:4, department:'HR', salary:5500, rn:1}]

B. [{id:1, department:'Sales', salary:5000, rn:2}, {id:2, department:'Sales', salary:7000, rn:1}, {id:3, department:'HR', salary:6000, rn:1}, {id:4, department:'HR', salary:5500, rn:2}]

C. [{id:1, department:'Sales', salary:5000, rn:1}, {id:2, department:'Sales', salary:7000, rn:2}, {id:4, department:'HR', salary:5500, rn:1}, {id:3, department:'HR', salary:6000, rn:2}]

D. [{id:1, department:'Sales', salary:5000, rn:1}, {id:2, department:'Sales', salary:7000, rn:1}, {id:3, department:'HR', salary:6000, rn:1}, {id:4, department:'HR', salary:5500, rn:1}]

Solution

1. Step 1: Partition rows by department

   Rows are grouped into Sales and HR departments separately.

2. Step 2: Order each partition by salary descending and assign row numbers

   Within Sales: id 2 (7000) gets rn=1, id 1 (5000) rn=2; Within HR: id 3 (6000) rn=1, id 4 (5500) rn=2.

3. Final Answer:

   [{id:1, department:'Sales', salary:5000, rn:2}, {id:2, department:'Sales', salary:7000, rn:1}, {id:3, department:'HR', salary:6000, rn:1}, {id:4, department:'HR', salary:5500, rn:2}] -> Option B

4. Quick Check:

   Partition by department + order by salary DESC = correct row numbers [OK]

Hint: Partition groups restart numbering; order controls row number sequence [OK]

Common Mistakes:

• Ignoring PARTITION BY and numbering globally
• Ordering ascending instead of descending
• Assigning same row number to multiple rows

4. Identify the error in this query:

SELECT id, ROW_NUMBER() PARTITION BY department ORDER BY salary FROM Employees;

medium

A. Missing OVER() keyword before PARTITION BY clause.

B. ORDER BY cannot be used inside ROW_NUMBER().

C. PARTITION BY should be after ORDER BY.

D. ROW_NUMBER() cannot be used with SELECT.

Solution

1. Step 1: Check ROW_NUMBER() syntax

   ROW_NUMBER() requires OVER() before PARTITION BY and ORDER BY clauses.

2. Step 2: Identify missing OVER()

   The query misses OVER(), causing syntax error.

3. Final Answer:

   Missing OVER() keyword before PARTITION BY clause. -> Option A

4. Quick Check:

   ROW_NUMBER() must have OVER() [OK]

Hint: Always use OVER() with ROW_NUMBER() [OK]

Common Mistakes:

• Omitting OVER() keyword
• Placing PARTITION BY outside OVER()
• Misordering clauses inside OVER()

5. You want to select the top 2 highest paid employees per department from the Employees table. Which query correctly uses ROW_NUMBER() to achieve this?

hard

A. SELECT id, department, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) AS rn FROM Employees WHERE rn <= 2;

B. SELECT *, ROW_NUMBER() FROM Employees WHERE rn <= 2 PARTITION BY department ORDER BY salary DESC;

C. SELECT id, department, salary FROM Employees ORDER BY salary DESC LIMIT 2 PARTITION BY department;

D. SELECT * FROM (SELECT *, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rn FROM Employees) AS t WHERE rn <= 2;

Solution

1. Step 1: Use ROW_NUMBER() with PARTITION BY department and ORDER BY salary DESC

   This assigns row numbers restarting at 1 for each department, ordered by salary descending.

2. Step 2: Filter rows where row number is 2 or less

   Wrap the window function query as a subquery and filter with WHERE rn <= 2 to get top 2 per department.

3. Final Answer:

   SELECT * FROM (SELECT *, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rn FROM Employees) AS t WHERE rn <= 2; -> Option D

4. Quick Check:

   Use subquery with ROW_NUMBER() and filter rn <= 2 [OK]

Hint: Use subquery with ROW_NUMBER() and filter rn <= 2 [OK]

Common Mistakes:

• Filtering rn in outer query without subquery
• Using LIMIT with PARTITION BY incorrectly
• Omitting PARTITION BY causing global numbering