Top-N per group query in SQL - Step-by-Step Execution

Concept Flow - Top-N per group query

Start with full table

↓

Partition data by group

↓

Order each group by criteria

↓

Assign row numbers within each group

↓

Filter rows where row number <= N

↓

Return top N rows per group

This flow shows how to get the top N rows for each group by partitioning, ordering, numbering, and filtering.

Execution Sample

SQL

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

This query returns the top 2 highest salaries per department from the employees table.

Execution Table

Step	Row	department	employee	salary	ROW_NUMBER() rn	Action	Output Included
1	1	Sales	Alice	90000	1	Assign rn=1 for highest salary in Sales	Yes
2	2	Sales	Bob	85000	2	Assign rn=2 for second highest in Sales	Yes
3	3	Sales	Charlie	70000	3	Assign rn=3, excluded rn>2	No
4	4	HR	Diana	80000	1	Assign rn=1 for highest salary in HR	Yes
5	5	HR	Evan	75000	2	Assign rn=2 for second highest in HR	Yes
6	6	HR	Fiona	60000	3	Assign rn=3, excluded rn>2	No
7	7	IT	George	95000	1	Assign rn=1 for highest salary in IT	Yes
8	8	IT	Hannah	90000	2	Assign rn=2 for second highest in IT	Yes
9	9	IT	Ian	85000	3	Assign rn=3, excluded rn>2	No
10	Exit					All rows processed, filtered to rn <= 2	Query ends

💡 Rows with rn > 2 are excluded, so only top 2 salaries per department are returned.

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3	After Row 4	After Row 5	After Row 6	After Row 7	After Row 8	After Row 9	Final
rn (row number per group)	N/A	1 (Sales)	2 (Sales)	3 (Sales)	1 (HR)	2 (HR)	3 (HR)	1 (IT)	2 (IT)	3 (IT)	Final assigned per row

Key Moments - 2 Insights

Why do some rows have rn values greater than 2 but are not included in the output?

Rows with rn > 2 are filtered out by the WHERE clause (rn <= 2), so only the top 2 rows per group remain, as shown in execution_table rows 3, 6, and 9.

How does the ROW_NUMBER() function know to restart counting for each department?

Because of PARTITION BY department in the window function, ROW_NUMBER() resets to 1 for each new department group, as seen in execution_table steps 4 and 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the rn value for employee 'Evan' in the HR department?

A 1

B 3

C 2

D None

💡 Hint

Check execution_table row 5 under the 'ROW_NUMBER() rn' column.

At which step does the query exclude a row because rn > 2?

A Step 5

B Step 3

C Step 7

D Step 9

💡 Hint

Look for rows marked 'excluded rn>2' in the Action column in execution_table.

If we change rn <= 2 to rn <= 3 in the WHERE clause, how would the output change?

A More rows per group would be included

B Fewer rows per group would be included

C No change in output

D Query would error

💡 Hint

Refer to the exit_note and see which rows are currently excluded due to rn > 2.

Concept Snapshot

Top-N per group query syntax:
Use ROW_NUMBER() OVER (PARTITION BY group_col ORDER BY criteria) to number rows per group.
Filter with WHERE rn <= N to get top N rows per group.
This returns the highest (or lowest) N rows per group based on ordering.
Commonly used for ranking or limiting results within groups.

Full Transcript

This visual execution trace shows how to write a SQL query to get the top N rows per group. The query uses ROW_NUMBER() with PARTITION BY to assign a row number to each row within its group, ordered by a criteria such as salary descending. Then it filters to keep only rows where the row number is less than or equal to N, effectively selecting the top N rows per group. The execution table walks through each row, showing the assigned row number and whether it is included in the output. The variable tracker shows how the row number resets for each group. Key moments clarify why some rows are excluded and how partitioning works. The quiz tests understanding of row numbers and filtering. The snapshot summarizes the syntax and behavior for quick reference.

Practice

1. What does the ROW_NUMBER() OVER (PARTITION BY column ORDER BY column) clause do in a SQL query?

easy

A. Assigns a unique rank number to each row within each group defined by the partition column.

B. Counts the total number of rows in the entire table.

C. Filters rows to only include those with the highest value in the column.

D. Joins two tables based on the partition column.

Solution

1. Step 1: Understand ROW_NUMBER() function

   ROW_NUMBER() assigns a unique sequential number to rows in the result set.

2. Step 2: Understand PARTITION BY usage

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

3. Final Answer:

   Assigns a unique rank number to each row within each group defined by the partition column. -> Option A

4. Quick Check:

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

Hint: ROW_NUMBER() with PARTITION BY ranks rows per group [OK]

Common Mistakes:

• Thinking ROW_NUMBER() counts total rows
• Confusing PARTITION BY with filtering
• Assuming it filters rows automatically

2. Which of the following SQL queries correctly selects the top 2 highest salaries per department from an employees table?

easy

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

B. SELECT * FROM employees ORDER BY salary DESC LIMIT 2 PARTITION BY department;

C. SELECT * FROM employees WHERE ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) <= 2;

D. SELECT TOP 2 * FROM employees PARTITION BY department ORDER BY salary DESC;

Solution

1. Step 1: Understand ROW_NUMBER() usage in subquery

   ROW_NUMBER() must be used in a subquery or CTE to assign ranks before filtering.

2. Step 2: Filter rows by rank in outer query

   The outer query filters rows where the rank (rn) is 2 or less 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:

   Use subquery with ROW_NUMBER() and filter by rank [OK]

Hint: Use subquery with ROW_NUMBER() then filter by rank [OK]

Common Mistakes:

• Trying to filter ROW_NUMBER() directly in WHERE clause
• Using LIMIT with PARTITION BY (not valid syntax)
• Using TOP without proper partitioning

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

SELECT region, salesperson, amount FROM (SELECT region, salesperson, amount, ROW_NUMBER() OVER (PARTITION BY region ORDER BY amount DESC) AS rn FROM sales) AS ranked WHERE rn = 1;

medium

A. All sales records sorted by amount descending.

B. All sales records where amount equals 1.

C. The highest sale amount record for each region.

D. The lowest sale amount record for each region.

Solution

1. Step 1: Analyze ROW_NUMBER() with PARTITION BY region

   Rows are grouped by region and ranked by amount descending, so rn=1 is highest amount per region.

2. Step 2: Filter WHERE rn = 1

   This keeps only the top-ranked row per region, i.e., the highest sale amount record.

3. Final Answer:

   The highest sale amount record for each region. -> Option C

4. Quick Check:

   ROW_NUMBER() = 1 per group returns top record [OK]

Hint: Filter WHERE rn=1 to get top record per group [OK]

Common Mistakes:

• Thinking it returns all rows sorted
• Confusing rn=1 with lowest amount
• Assuming amount = 1 filters rows

4. Identify the error in this SQL query that tries to get the top 3 recent orders per customer from orders table:

SELECT customer_id, order_id, order_date, ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY order_date DESC) AS rank FROM orders WHERE rank <= 3;

medium

A. Missing GROUP BY clause for customer_id.

B. ROW_NUMBER() cannot be used with PARTITION BY.

C. ORDER BY must be ascending, not descending.

D. You cannot filter on an alias in the WHERE clause directly.

Solution

1. Step 1: Understand alias usage in WHERE clause

   Aliases like 'rank' defined in SELECT cannot be used in WHERE clause directly because WHERE is evaluated before SELECT.

2. Step 2: Correct approach using subquery or CTE

   Use a subquery or CTE to assign ROW_NUMBER(), then filter on 'rank' in outer query.

3. Final Answer:

   You cannot filter on an alias in the WHERE clause directly. -> Option D

4. Quick Check:

   Filter on ROW_NUMBER() alias in outer query, not WHERE [OK]

Hint: Filter ROW_NUMBER() in outer query, not WHERE clause [OK]

Common Mistakes:

• Using alias in WHERE clause directly
• Thinking ORDER BY DESC is invalid
• Adding unnecessary GROUP BY

5. You have a products table with columns category, product_id, and price. You want to find the top 2 most expensive products per category, but if two products have the same price, include both even if it means more than 2 per category. Which query achieves this?

hard

A. Use ROW_NUMBER() partitioned by category ordered by price DESC and filter WHERE rn <= 2.

B. Use RANK() partitioned by category ordered by price DESC and filter WHERE rank <= 2.

C. Use DENSE_RANK() partitioned by category ordered by price ASC and filter WHERE dense_rank <= 2.

D. Use COUNT() partitioned by category ordered by price DESC and filter WHERE count <= 2.

Solution

1. Step 1: Understand difference between ROW_NUMBER(), RANK(), and DENSE_RANK()

   ROW_NUMBER() assigns unique ranks, so ties get different numbers. RANK() assigns same rank to ties and skips ranks after. DENSE_RANK() assigns same rank to ties but no gaps.

2. Step 2: Choose RANK() to include ties at second place

   RANK() allows including all products tied at rank 2, possibly more than 2 rows per category.

3. Final Answer:

   Use RANK() partitioned by category ordered by price DESC and filter WHERE rank <= 2. -> Option B

4. Quick Check:

   RANK() includes ties with same rank [OK]

Hint: Use RANK() to include ties in top N per group [OK]

Common Mistakes:

• Using ROW_NUMBER() excludes ties
• Using DENSE_RANK() includes ties but no gaps
• Using COUNT() is invalid for ranking