Why window functions are needed in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the running total of sales.

SQL

SELECT sales_date, sales_amount, SUM(sales_amount) OVER (ORDER BY sales_date) AS running_total FROM sales ORDER BY sales_date [1];

Drag options to blanks, or click blank then click option'

ADESC

BGROUP BY

CASC

DHAVING

Attempts:

3 left

💡 Hint

Common Mistakes

Using DESC will reverse the order and give a running total backward.

Using GROUP BY or HAVING here is incorrect because we want a window function.

The running total should be ordered from earliest to latest date, so we use ASC.

2fill in blank

medium

Complete the code to find the average salary per department without collapsing rows.

SQL

SELECT employee_id, department_id, salary, AVG(salary) OVER (PARTITION BY [1]) AS avg_dept_salary FROM employees;

Drag options to blanks, or click blank then click option'

Adepartment_id

Bemployee_id

Csalary

Dhire_date

Attempts:

3 left

💡 Hint

Common Mistakes

Partitioning by employee_id will give average salary per employee, which is just the salary itself.

Partitioning by salary or hire_date does not group by department.

We partition by department_id to calculate average salary per department while keeping all rows.

3fill in blank

hard

Fix the error in the code to rank employees by salary within each department.

SQL

SELECT employee_id, department_id, salary, RANK() OVER (PARTITION BY department_id ORDER BY [1]) AS salary_rank FROM employees;

Drag options to blanks, or click blank then click option'

Aemployee_id

Bsalary

Cdepartment_id

Dhire_date

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by employee_id or department_id does not rank salaries.

Ordering by hire_date ranks by date, not salary.

Ranking should be based on salary, so ORDER BY salary is correct.

4fill in blank

hard

Fill both blanks to calculate the difference between each sale and the previous sale.

SQL

SELECT sales_date, sales_amount, sales_amount - LAG([1]) OVER (ORDER BY [2]) AS diff FROM sales;

Drag options to blanks, or click blank then click option'

Asales_amount

Bsales_date

Cemployee_id

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using employee_id or department_id in LAG or ORDER BY will not give correct previous sale.

Not ordering by sales_date will give wrong previous row.

LAG(sales_amount) gets the previous sale amount ordered by sales_date.

5fill in blank

hard

Fill all three blanks to assign a row number to each employee per department ordered by hire date.

SQL

SELECT employee_id, department_id, hire_date, ROW_NUMBER() OVER (PARTITION BY [1] ORDER BY [2] [3]) AS row_num FROM employees;

Drag options to blanks, or click blank then click option'

Adepartment_id

Bhire_date

CASC

DDESC

Attempts:

3 left

💡 Hint

Common Mistakes

Using DESC will number from latest to earliest hire date.

Partitioning by hire_date or employee_id is incorrect for grouping by department.

Partition by department_id to group employees, order by hire_date ascending to number from earliest hire.

Practice

1. Why are window functions needed in SQL?
SELECT with window functions can do what that regular aggregation cannot?

easy

A. They speed up query execution by skipping indexes.

B. They allow calculations across rows without hiding individual rows.

C. They automatically create new tables for results.

D. They replace the need for JOIN operations.

Solution

1. Step 1: Understand aggregation vs window functions

   Regular aggregation groups rows and returns fewer rows, hiding details.

2. Step 2: Identify window function behavior

   Window functions calculate values across rows but keep all original rows visible.

3. Final Answer:

   They allow calculations across rows without hiding individual rows. -> Option B

4. Quick Check:

   Window functions keep rows visible = A [OK]

Hint: Window functions keep all rows, aggregations reduce rows [OK]

Common Mistakes:

• Thinking window functions create new tables
• Confusing window functions with JOINs
• Believing window functions speed up queries

2. Which of the following is the correct syntax to use a window function in SQL?

easy

A. SELECT name, RANK() OVER (ORDER BY score) FROM players;

B. SELECT name, RANK(score) FROM players ORDER BY score;

C. SELECT name, RANK() FROM players GROUP BY score;

D. SELECT name, RANK OVER (ORDER BY score) FROM players;

Solution

1. Step 1: Recall window function syntax

   Window functions use function() OVER (PARTITION BY ... ORDER BY ...).

2. Step 2: Check each option

   SELECT name, RANK() OVER (ORDER BY score) FROM players; correctly uses RANK() OVER (ORDER BY score). Others miss parentheses or OVER keyword.

3. Final Answer:

   SELECT name, RANK() OVER (ORDER BY score) FROM players; -> Option A

4. Quick Check:

   Window function syntax = function() OVER (...) [OK]

Hint: Window functions always use OVER() clause [OK]

Common Mistakes:

• Omitting parentheses after function name
• Forgetting the OVER keyword
• Using GROUP BY instead of window functions

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

SELECT id, region, amount, SUM(amount) OVER (PARTITION BY region ORDER BY id) AS running_total FROM sales ORDER BY id;

medium

A. A running total of sales amount per region, ordered by id, showing all rows.

B. Total sales amount per region, one row per region only.

C. Sum of all sales amounts ignoring region, one total value.

D. Syntax error due to missing GROUP BY clause.

Solution

1. Step 1: Understand SUM() OVER with PARTITION and ORDER

   SUM() OVER (PARTITION BY region ORDER BY id) calculates a running total per region ordered by id.

2. Step 2: Check output rows

   Window functions keep all rows, so all sales rows appear with running totals.

3. Final Answer:

   A running total of sales amount per region, ordered by id, showing all rows. -> Option A

4. Quick Check:

   Running total per group = A [OK]

Hint: SUM() OVER with PARTITION BY gives running totals per group [OK]

Common Mistakes:

• Thinking it returns one row per group
• Confusing window function with GROUP BY aggregation
• Expecting syntax error without GROUP BY

4. Identify the error in this SQL query using a window function:

SELECT employee_id, department, ROW_NUMBER() ORDER BY salary FROM employees;

medium

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

B. employee_id must be inside ROW_NUMBER() function.

C. ROW_NUMBER() requires GROUP BY clause.

D. Missing OVER() clause after ROW_NUMBER() function.

Solution

1. Step 1: Check window function syntax

   ROW_NUMBER() must be followed by OVER() with optional ORDER BY inside it.

2. Step 2: Identify missing OVER()

   The query uses ROW_NUMBER() ORDER BY salary without OVER(), causing syntax error.

3. Final Answer:

   Missing OVER() clause after ROW_NUMBER() function. -> Option D

4. Quick Check:

   Window functions need OVER() [OK]

Hint: Always add OVER() after window functions [OK]

Common Mistakes:

• Placing ORDER BY outside OVER()
• Thinking GROUP BY is needed for ROW_NUMBER()
• Misunderstanding function arguments

5. You want to find each employee's salary rank within their department, but also show all employees with their salaries and ranks. Which SQL approach best achieves this?

hard

A. Use a subquery to calculate ranks and join back to employees table.

B. Use GROUP BY department, salary with RANK() to get ranks per department.

C. Use RANK() OVER (PARTITION BY department ORDER BY salary DESC) to rank salaries per department while keeping all rows.

D. Use ORDER BY salary DESC with LIMIT to get top salaries per department.

Solution

1. Step 1: Understand the requirement

   We want ranks per department and all employees listed with their ranks.

2. Step 2: Evaluate options

   Use RANK() OVER (PARTITION BY department ORDER BY salary DESC) to rank salaries per department while keeping all rows. uses RANK() OVER (PARTITION BY department ORDER BY salary DESC) which ranks salaries per department and keeps all rows visible.

3. Final Answer:

   Use RANK() OVER (PARTITION BY department ORDER BY salary DESC) to rank salaries per department while keeping all rows. -> Option C

4. Quick Check:

   Window function with PARTITION keeps all rows and ranks [OK]

Hint: Use RANK() OVER PARTITION BY to rank within groups [OK]

Common Mistakes:

• Using GROUP BY loses individual rows
• Using subqueries unnecessarily complicates query
• LIMIT filters rows, not ranks all