ROW_NUMBER function in SQL

Introduction

The ROW_NUMBER function helps you give a unique number to each row in your data, making it easy to order or pick specific rows.

When you want to number rows in a list, like ranking students by score.

When you need to pick the top N items from each group, like top 3 sales per region.

When you want to remove duplicate rows but keep one copy.

When you want to paginate results, showing rows page by page.

Syntax

SQL

ROW_NUMBER() OVER (PARTITION BY column1, column2, ... ORDER BY column3 [ASC|DESC], ...)

PARTITION BY divides the data into groups to restart numbering for each group.

ORDER BY decides the order in which rows are numbered.

Examples

Numbers all employees by salary from highest to lowest.

SQL

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

Numbers employees within each department by their hire date.

SQL

SELECT department, employee_name, ROW_NUMBER() OVER (PARTITION BY department ORDER BY hire_date) AS hire_order FROM employees;

Gives a simple row number to all products ordered by their id.

SQL

SELECT ROW_NUMBER() OVER (ORDER BY id) AS row_num, * FROM products;

Sample Program

This query ranks employees by their sales amount within each department, giving the highest sales person rank 1 in their department.

SQL

WITH sales_data AS (
  SELECT employee_id, department, sales_amount
  FROM sales
)
SELECT employee_id, department, sales_amount,
       ROW_NUMBER() OVER (PARTITION BY department ORDER BY sales_amount DESC) AS sales_rank
FROM sales_data;

Important Notes

ROW_NUMBER always gives unique numbers, even if there are ties in ordering values.

If you want the same number for ties, consider using RANK() or DENSE_RANK() instead.

ROW_NUMBER requires an OVER clause with ORDER BY; without ORDER BY, numbering is not defined.

Summary

ROW_NUMBER assigns a unique number to each row in a result set.

Use PARTITION BY to restart numbering for groups.

ORDER BY controls the numbering order.

Practice

1. What does the ROW_NUMBER() function do in SQL?

easy

A. Counts the total number of rows in a table.

B. Filters rows based on a condition.

C. Calculates the sum of values in a column.

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

Solution

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

   The ROW_NUMBER() function assigns a unique sequential integer to rows in the result set, starting at 1.

2. Step 2: Compare with other options

   Counting rows, summing values, or filtering rows are done by other SQL functions like COUNT(), SUM(), or WHERE clause, not ROW_NUMBER().

3. Final Answer:

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

4. Quick Check:

   ROW_NUMBER() = unique row numbers [OK]

Hint: ROW_NUMBER() gives each row a unique number starting at 1 [OK]

Common Mistakes:

• Confusing ROW_NUMBER() with COUNT()
• Thinking ROW_NUMBER() filters rows
• Assuming ROW_NUMBER() sums values

2. Which of the following is the correct syntax to assign row numbers ordered by salary in descending order?

easy

A. SELECT ROW_NUMBER() FROM employees ORDER BY salary DESC;

B. SELECT ROW_NUMBER() OVER (ORDER BY salary DESC) AS rn FROM employees;

C. SELECT ROW_NUMBER() ORDER BY salary DESC FROM employees;

D. SELECT ROW_NUMBER(salary DESC) FROM employees;

Solution

1. Step 1: Identify correct ROW_NUMBER() syntax

   The correct syntax uses ROW_NUMBER() OVER (ORDER BY column) to assign numbers ordered by a column.

2. Step 2: Check each option

   Only SELECT ROW_NUMBER() OVER (ORDER BY salary DESC) AS rn FROM employees; correctly uses OVER (ORDER BY salary DESC). The other options omit the OVER clause, pass arguments directly to ROW_NUMBER(), or misplace the ORDER BY clause.

3. Final Answer:

   SELECT ROW_NUMBER() OVER (ORDER BY salary DESC) AS rn FROM employees; -> Option B

4. Quick Check:

   ROW_NUMBER() needs OVER with ORDER BY [OK]

Hint: ROW_NUMBER() must use OVER() with ORDER BY inside [OK]

Common Mistakes:

• Omitting OVER() clause
• Placing ORDER BY outside OVER()
• Passing column directly to ROW_NUMBER()

3. Given the table sales with columns region and amount, what will this query return?

SELECT region, amount, ROW_NUMBER() OVER (PARTITION BY region ORDER BY amount DESC) AS rn FROM sales;

medium

A. The total count of sales per region.

B. A unique row number for all sales ignoring region, ordered by amount ascending.

C. A unique row number for each sale, restarting at 1 for each region, ordered by highest amount first.

D. The sum of amounts per region.

Solution

1. Step 1: Understand PARTITION BY effect

   Using PARTITION BY region restarts the row numbering for each region group separately.

2. Step 2: Understand ORDER BY inside ROW_NUMBER()

   Ordering by amount DESC means the highest amount in each region gets row number 1, next highest 2, and so on.

3. Final Answer:

   A unique row number for each sale, restarting at 1 for each region, ordered by highest amount first. -> Option C

4. Quick Check:

   ROW_NUMBER() with PARTITION BY = numbering per group [OK]

Hint: PARTITION BY restarts numbering per group [OK]

Common Mistakes:

• Ignoring PARTITION BY and numbering all rows globally
• Confusing ORDER BY direction
• Thinking ROW_NUMBER() counts rows

4. Identify the error in this query:

SELECT name, ROW_NUMBER() ORDER BY salary FROM employees;

medium

A. Missing OVER() clause after ROW_NUMBER().

B. ORDER BY cannot be used in SELECT clause.

C. ROW_NUMBER() cannot be used without GROUP BY.

D. salary column does not exist.

Solution

1. Step 1: Check ROW_NUMBER() syntax

   The ROW_NUMBER() function requires an OVER() clause specifying ordering or partitioning.

2. Step 2: Analyze the query

   The query uses ROW_NUMBER() without OVER(), which causes a syntax error. ORDER BY inside SELECT is invalid without OVER().

3. Final Answer:

   Missing OVER() clause after ROW_NUMBER(). -> Option A

4. Quick Check:

   ROW_NUMBER() must have OVER() [OK]

Hint: ROW_NUMBER() always needs OVER() clause [OK]

Common Mistakes:

• Forgetting OVER() after ROW_NUMBER()
• Placing ORDER BY outside OVER()
• Assuming ROW_NUMBER() works without ordering

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 * FROM (SELECT *, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rn FROM employees) sub WHERE rn <= 2;

B. SELECT *, ROW_NUMBER() FROM employees WHERE salary IN (SELECT MAX(salary) FROM employees GROUP BY department);

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

D. SELECT department, salary FROM employees ORDER BY salary DESC LIMIT 2;

Solution

1. Step 1: Use ROW_NUMBER() with PARTITION BY department

   To get top 2 per department, assign row numbers restarting for each department ordered by salary descending.

2. Step 2: Filter rows with row number less than or equal to 2

   Wrap the query as a subquery and filter where rn <= 2 to get top 2 employees per department.

3. Final Answer:

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

4. Quick Check:

   ROW_NUMBER() + PARTITION + filter = top N per group [OK]

Hint: Use ROW_NUMBER() with PARTITION BY and filter rn <= N [OK]

Common Mistakes:

• Not using PARTITION BY to group by department
• Filtering on rn in the same SELECT without subquery
• Using LIMIT without partitioning