NTH_VALUE function in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of NTH_VALUE with simple partition

Given the table sales with columns region and amount, what is the output of the following query?

SELECT region, amount, NTH_VALUE(amount, 2) OVER (PARTITION BY region ORDER BY amount ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS second_amount FROM sales;

SQL

CREATE TABLE sales (region VARCHAR(10), amount INT);
INSERT INTO sales VALUES
('East', 100), ('East', 200), ('East', 300),
('West', 150), ('West', 250);

SELECT region, amount, NTH_VALUE(amount, 2) OVER (PARTITION BY region ORDER BY amount ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS second_amount FROM sales ORDER BY region, amount;

A [{'region': 'East', 'amount': 100, 'second_amount': 200}, {'region': 'East', 'amount': 200, 'second_amount': 200}, {'region': 'East', 'amount': 300, 'second_amount': 200}, {'region': 'West', 'amount': 150, 'second_amount': 250}, {'region': 'West', 'amount': 250, 'second_amount': 250}]

B [{'region': 'East', 'amount': 100, 'second_amount': 100}, {'region': 'East', 'amount': 200, 'second_amount': 200}, {'region': 'East', 'amount': 300, 'second_amount': 300}, {'region': 'West', 'amount': 150, 'second_amount': 150}, {'region': 'West', 'amount': 250, 'second_amount': 250}]

C [{'region': 'East', 'amount': 100, 'second_amount': 100}, {'region': 'East', 'amount': 200, 'second_amount': 100}, {'region': 'East', 'amount': 300, 'second_amount': 100}, {'region': 'West', 'amount': 150, 'second_amount': 150}, {'region': 'West', 'amount': 250, 'second_amount': 150}]

D [{'region': 'East', 'amount': 100, 'second_amount': null}, {'region': 'East', 'amount': 200, 'second_amount': null}, {'region': 'East', 'amount': 300, 'second_amount': null}, {'region': 'West', 'amount': 150, 'second_amount': null}, {'region': 'West', 'amount': 250, 'second_amount': null}]

💡 Hint

Remember that NTH_VALUE returns the value of the nth row in the window frame, ordered by the ORDER BY clause.

The NTH_VALUE(amount, 2) function returns the second smallest amount within each region partition. For 'East', the amounts ordered are 100, 200, 300, so the second is 200. For 'West', the amounts are 150, 250, so the second is 250. This value is repeated for all rows in the partition.

🧠 Conceptual

intermediate

Understanding frame specification impact on NTH_VALUE

What happens if you run the query below on a table orders with columns customer_id and order_date? Assume multiple orders per customer.

SELECT customer_id, order_date, NTH_VALUE(order_date, 3) OVER (PARTITION BY customer_id ORDER BY order_date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS third_order_date FROM orders;

Which statement is true?

A The function returns the third earliest order date for each customer for all rows.

B The function returns the third order date only if the frame includes at least three rows; otherwise, it returns NULL.

C The function always returns the third order date of the entire table, ignoring partitions.

D The function returns NULL for the first two orders and the third order date starting from the third row onward per customer.

💡 Hint

Consider how the frame defined by ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW changes as the window moves.

The frame grows from the first row to the current row. For the first two rows, the frame has fewer than three rows, so NTH_VALUE returns NULL. Starting from the third row, the frame includes at least three rows, so the third order date in the frame is returned.

📝 Syntax

advanced

Identify the syntax error in NTH_VALUE usage

Which of the following queries will cause a syntax error?

A SELECT NTH_VALUE(salary, 1) OVER (ORDER BY salary) FROM employees;

B SELECT NTH_VALUE(salary, 3) OVER (ORDER BY salary ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) FROM employees;

C SELECT NTH_VALUE(salary, 2) OVER (PARTITION BY department ORDER BY salary) FROM employees;

D SELECT NTH_VALUE(salary, 0) OVER (ORDER BY salary) FROM employees;

💡 Hint

Check the valid range for the second argument of NTH_VALUE.

The second argument to NTH_VALUE must be a positive integer starting from 1. Using 0 is invalid and causes a syntax error.

❓ optimization

advanced

Optimizing NTH_VALUE with large datasets

You have a large transactions table with millions of rows. You want to find the 5th highest transaction amount per customer using NTH_VALUE. Which approach is most efficient?

A Use NTH_VALUE(amount, 5) OVER (PARTITION BY customer_id ORDER BY amount DESC) without frame clause.

B Use NTH_VALUE(amount, 5) OVER (PARTITION BY customer_id ORDER BY amount DESC ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW).

C Use a subquery with ROW_NUMBER() to filter the 5th row per customer instead of NTH_VALUE.

D Use NTH_VALUE(amount, 5) OVER (ORDER BY amount DESC) without partitioning.

💡 Hint

Consider performance implications of window functions on large data and alternatives.

Using ROW_NUMBER() in a subquery to filter the 5th row per customer is often more efficient than NTH_VALUE on large datasets because it avoids computing the window function for all rows and allows filtering early.

🔧 Debug

expert

Debugging unexpected NULLs from NTH_VALUE

You run this query:

SELECT employee_id, salary, NTH_VALUE(salary, 3) OVER (PARTITION BY department ORDER BY salary) AS third_salary FROM employees;

But you notice that third_salary is NULL for some employees even though their department has more than three employees. What is the most likely cause?

A The default window frame is ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, so the frame may not include the third row yet.

B NTH_VALUE always returns NULL if the partition has more than 3 rows.

C The ORDER BY clause is missing in the window function.

D The salary column contains NULL values causing NTH_VALUE to return NULL.

💡 Hint

Think about how the default frame affects which rows are visible to the function at each row.

By default, the frame is from the first row to the current row. For the first and second rows, the frame has fewer than three rows, so NTH_VALUE returns NULL. Only from the third row onward does the frame include the third row, returning a non-NULL value.

Practice

1. What does the SQL function NTH_VALUE(column, n) do when used with an OVER() clause?

easy

A. Returns the maximum value in the column.

B. Returns the total count of rows in the table.

C. Returns the value at the nth position in the ordered set of rows.

D. Returns the first value in the column without ordering.

Solution

1. Step 1: Understand the function purpose

   The NTH_VALUE function is designed to return a value from a specific position (n) in an ordered set of rows.

2. Step 2: Recognize the role of OVER()

   The OVER() clause defines how rows are ordered and grouped, which is essential for NTH_VALUE to pick the correct nth value.

3. Final Answer:

   Returns the value at the nth position in the ordered set of rows. -> Option C

4. Quick Check:

   NTH_VALUE returns nth value in order [OK]

Hint: NTH_VALUE picks the nth row's value after ordering [OK]

Common Mistakes:

• Confusing NTH_VALUE with aggregate functions like COUNT or MAX
• Ignoring the ordering inside OVER() clause
• Assuming it returns the nth row without ordering

2. Which of the following is the correct syntax to get the 3rd highest salary using NTH_VALUE in SQL?

easy

A. SELECT NTH_VALUE(salary, 3) OVER (PARTITION BY salary) FROM employees;

B. SELECT NTH_VALUE(salary, 3) FROM employees ORDER BY salary DESC;

C. SELECT NTH_VALUE(3, salary) OVER (ORDER BY salary DESC) FROM employees;

D. SELECT NTH_VALUE(salary, 3) OVER (ORDER BY salary DESC) FROM employees;

Solution

1. Step 1: Check the function parameters

   The correct order is NTH_VALUE(column, n), so salary first, then 3.

2. Step 2: Verify the OVER clause

   Ordering by salary descending (ORDER BY salary DESC) ensures the highest salaries come first, so the 3rd highest is the 3rd row.

3. Final Answer:

   SELECT NTH_VALUE(salary, 3) OVER (ORDER BY salary DESC) FROM employees; -> Option D

4. Quick Check:

   Syntax: NTH_VALUE(column, n) OVER (ORDER BY col DESC) [OK]

Hint: NTH_VALUE(column, n) needs OVER with ORDER BY for ranking [OK]

Common Mistakes:

• Swapping parameters inside NTH_VALUE
• Missing OVER() clause or ORDER BY inside it
• Using PARTITION BY incorrectly without ORDER BY

3. Given the table Scores with columns student and score, what will this query return?

SELECT student, score, NTH_VALUE(score, 2) OVER (ORDER BY score DESC) AS second_highest FROM Scores;

medium

A. Each row shows the student's score and the 2nd highest score overall repeated.

B. Each row shows the student's score and their own 2nd highest score.

C. The query returns only the row with the 2nd highest score.

D. The query returns an error because NTH_VALUE needs PARTITION BY.

Solution

1. Step 1: Understand the window function without PARTITION

   Since there is no PARTITION BY, the entire table is treated as one group ordered by score descending.

2. Step 2: Effect of NTH_VALUE(score, 2) OVER (ORDER BY score DESC)

   This returns the 2nd highest score value from the whole table, repeated for every row.

3. Final Answer:

   Each row shows the student's score and the 2nd highest score overall repeated. -> Option A

4. Quick Check:

   NTH_VALUE without PARTITION repeats nth value for all rows [OK]

Hint: Without PARTITION, NTH_VALUE repeats same nth value for all rows [OK]

Common Mistakes:

• Thinking NTH_VALUE filters rows to nth only
• Assuming PARTITION BY is mandatory
• Confusing NTH_VALUE with ROW_NUMBER or RANK

4. Identify the error in this SQL query:

SELECT employee_id, NTH_VALUE(salary, 1) OVER (PARTITION BY department_id) FROM employees;

medium

A. Missing ORDER BY clause inside OVER() causes error.

B. NTH_VALUE cannot be used with PARTITION BY.

C. The function parameters are in wrong order.

D. No error; query is correct.

Solution

1. Step 1: Check the OVER() clause requirements

   NTH_VALUE requires an ORDER BY clause inside OVER() to define the row order for nth value selection.

2. Step 2: Identify missing ORDER BY

   The query uses PARTITION BY but lacks ORDER BY, which causes a syntax error.

3. Final Answer:

   Missing ORDER BY clause inside OVER() causes error. -> Option A

4. Quick Check:

   NTH_VALUE needs ORDER BY in OVER() [OK]

Hint: Always include ORDER BY inside OVER() with NTH_VALUE [OK]

Common Mistakes:

• Omitting ORDER BY inside OVER()
• Thinking PARTITION BY alone orders rows
• Swapping function parameters

5. You want to find the 2nd earliest order date for each customer in the Orders table with columns customer_id and order_date. Which query correctly uses NTH_VALUE to achieve this?

hard

A. SELECT customer_id, order_date, NTH_VALUE(order_date, 2) FROM Orders GROUP BY customer_id;

B. SELECT customer_id, order_date, NTH_VALUE(order_date, 2) OVER (PARTITION BY customer_id ORDER BY order_date ASC) AS second_earliest FROM Orders;

C. SELECT customer_id, order_date, NTH_VALUE(order_date, 2) OVER (ORDER BY customer_id, order_date ASC) AS second_earliest FROM Orders;

D. SELECT customer_id, NTH_VALUE(order_date, 2) FROM Orders ORDER BY order_date ASC;

Solution

1. Step 1: Use PARTITION BY to group by customer

   To find the 2nd earliest order per customer, partition rows by customer_id.

2. Step 2: Order each partition by order_date ascending

   Ordering by order_date ASC ensures earliest dates come first.

3. Step 3: Apply NTH_VALUE to get 2nd earliest date

   NTH_VALUE with n=2 picks the 2nd row's order_date in each partition.

4. Final Answer:

   SELECT customer_id, order_date, NTH_VALUE(order_date, 2) OVER (PARTITION BY customer_id ORDER BY order_date ASC) AS second_earliest FROM Orders; -> Option B

5. Quick Check:

   Partition by customer, order by date ASC, NTH_VALUE(2) [OK]

Hint: Partition by group, order ascending, then NTH_VALUE(n) [OK]

Common Mistakes:

• Missing PARTITION BY to group customers
• Ordering by wrong columns or descending order
• Using NTH_VALUE without OVER() clause