ROW_NUMBER, RANK, DENSE_RANK in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign a unique row number to each employee ordered by salary.

PostgreSQL

SELECT employee_id, salary, ROW_NUMBER() OVER (ORDER BY [1]) AS row_num FROM employees;

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

Asalary

Bemployee_id

Cdepartment

Dhire_date

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column not present in the table.

Leaving the ORDER BY clause empty.

Confusing ROW_NUMBER() with RANK().

The ROW_NUMBER() function assigns a unique number to each row ordered by the specified column. Here, ordering by salary numbers employees from lowest to highest salary.

2fill in blank

medium

Complete the code to rank employees by salary, allowing ties to have the same rank.

PostgreSQL

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

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

Aemployee_id

Bhire_date

Csalary

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by ascending salary when descending is needed.

Using ROW_NUMBER() instead of RANK() for ties.

Forgetting DESC for highest salary first.

The RANK() function assigns ranks to rows ordered by salary descending. Employees with the same salary get the same rank, and gaps appear in ranks after ties.

3fill in blank

hard

Complete the code to assign dense ranks to employees by salary within each department.

PostgreSQL

SELECT employee_id, department, salary, DENSE_RANK() OVER (PARTITION BY [1] ORDER BY [2] DESC) AS dept_salary_rank FROM employees;

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

Aemployee_id

Bsalary

Cdepartment

Dhire_date

Attempts:

3 left

💡 Hint

Common Mistakes

Using salary in PARTITION BY instead of department.

Omitting PARTITION BY when grouping is needed.

Confusing DENSE_RANK() with RANK() behavior.

The PARTITION BY department clause groups rows by department. The DENSE_RANK() ORDER BY salary DESC ranks salaries within each department with no gaps after ties.

4fill in blank

hard

Fill both blanks to rank employees by salary within each department using RANK(), ordering salaries descending.

PostgreSQL

SELECT employee_id, department, salary, RANK() OVER (PARTITION BY [1] ORDER BY [2] DESC) AS dept_rank FROM employees;

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

Adepartment

Bsalary

Cemployee_id

Dhire_date

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping partition and order columns.

Ordering ascending instead of descending.

Using ROW_NUMBER() instead of RANK() for ties.

The PARTITION BY groups employees by department. The ORDER BY orders salaries descending within each department for ranking.

5fill in blank

hard

Fill all three blanks to assign dense ranks to employees by department and salary, and filter to show only top 3 ranks per department.

PostgreSQL

WITH ranked_employees AS (SELECT employee_id, department, salary, DENSE_RANK() OVER (PARTITION BY [1] ORDER BY [2] DESC) AS dense_rank FROM employees) SELECT * FROM ranked_employees WHERE dense_rank [3] 3;

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

Adepartment

Bsalary

C<=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '<=' to include top 3 ranks.

Swapping partition and order columns.

Forgetting to filter ranks in the WHERE clause.

Partition by department to group employees. Order by salary descending to rank salaries. Filter ranks less than or equal to 3 to get top 3 per department.

Practice

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

easy

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

B. Assigns the same rank to rows with identical values, skipping ranks after ties.

C. Assigns ranks to rows without skipping numbers after ties.

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

Solution

1. Step 1: Understand ROW_NUMBER() purpose

   ROW_NUMBER() gives each row a unique sequential number, starting at 1.

2. Step 2: Compare with RANK() and DENSE_RANK()

   Unlike RANK() or DENSE_RANK(), ROW_NUMBER() never assigns the same number to multiple rows.

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 always gives unique numbers, no ties allowed [OK]

Common Mistakes:

• Confusing ROW_NUMBER with RANK or DENSE_RANK
• Thinking ROW_NUMBER skips numbers after ties
• Assuming ROW_NUMBER counts total rows

2. Which of the following is the correct syntax to assign a rank using RANK() over a column named score ordered descending?

easy

A. SELECT RANK(score) OVER (ORDER BY DESC) FROM table_name;

B. SELECT RANK() ORDER BY score DESC FROM table_name;

C. SELECT RANK() OVER (ORDER BY score DESC) FROM table_name;

D. SELECT RANK() OVER (PARTITION BY score DESC) FROM table_name;

Solution

1. Step 1: Recall correct RANK() syntax

   RANK() is used with OVER() clause specifying ORDER BY column and direction.

2. Step 2: Check each option

   SELECT RANK() OVER (ORDER BY score DESC) FROM table_name; correctly uses RANK() OVER (ORDER BY score DESC). Others misuse syntax or clauses.

3. Final Answer:

   SELECT RANK() OVER (ORDER BY score DESC) FROM table_name; -> Option C

4. Quick Check:

   RANK() OVER (ORDER BY column DESC) syntax [OK]

Hint: RANK() needs OVER with ORDER BY column [OK]

Common Mistakes:

• Putting column inside RANK() function
• Using PARTITION BY incorrectly
• Omitting OVER() clause

3. Given the table players with columns name and score, what will be the output of this query?

SELECT name, score, RANK() OVER (ORDER BY score DESC) AS rank FROM players;

Data:
name | score -----|------ Alice | 100 Bob | 90 Carol | 90 Dave | 80

medium

A. [{"name": "Alice", "score": 100, "rank": 1}, {"name": "Bob", "score": 90, "rank": 1}, {"name": "Carol", "score": 90, "rank": 1}, {"name": "Dave", "score": 80, "rank": 2}]

B. [{"name": "Alice", "score": 100, "rank": 1}, {"name": "Bob", "score": 90, "rank": 2}, {"name": "Carol", "score": 90, "rank": 2}, {"name": "Dave", "score": 80, "rank": 4}]

C. [{"name": "Alice", "score": 100, "rank": 1}, {"name": "Bob", "score": 90, "rank": 2}, {"name": "Carol", "score": 90, "rank": 2}, {"name": "Dave", "score": 80, "rank": 3}]

D. [{"name": "Alice", "score": 100, "rank": 1}, {"name": "Bob", "score": 90, "rank": 2}, {"name": "Carol", "score": 90, "rank": 3}, {"name": "Dave", "score": 80, "rank": 4}]

Solution

1. Step 1: Understand RANK() behavior with ties

   RANK() assigns the same rank to tied scores and skips ranks after ties.

2. Step 2: Apply ranks to scores

   Alice has highest score 100 -> rank 1. Bob and Carol tie at 90 -> both rank 2. Next rank after tie skips to 4 for Dave.

3. Final Answer:

   [{"name": "Alice", "score": 100, "rank": 1}, {"name": "Bob", "score": 90, "rank": 2}, {"name": "Carol", "score": 90, "rank": 2}, {"name": "Dave", "score": 80, "rank": 4}] -> Option B

4. Quick Check:

   RANK() skips ranks after ties [OK]

Hint: RANK() skips numbers after ties, DENSE_RANK() does not [OK]

Common Mistakes:

• Assuming ranks after ties are consecutive
• Confusing RANK() with DENSE_RANK()
• Misordering rows by score

4. Identify the error in this query:

SELECT name, DENSE_RANK(score) OVER (ORDER BY score DESC) FROM players;

medium

A. DENSE_RANK() should not have a column inside the parentheses.

B. DENSE_RANK() cannot be used without PARTITION BY clause.

C. ORDER BY clause is missing in the OVER() clause.

D. The query is correct and will run without errors.

Solution

1. Step 1: Check DENSE_RANK() syntax

   DENSE_RANK() does not take any arguments inside parentheses; it is called as DENSE_RANK() OVER (...).

2. Step 2: Identify the error in the query

   The query incorrectly uses DENSE_RANK(score), which causes a syntax error.

3. Final Answer:

   DENSE_RANK() should not have a column inside the parentheses. -> Option A

4. Quick Check:

   DENSE_RANK() has empty parentheses [OK]

Hint: Window functions have empty parentheses, column goes in ORDER BY [OK]

Common Mistakes:

• Putting column inside DENSE_RANK()
• Thinking PARTITION BY is mandatory
• Omitting OVER() clause

5. You want to assign ranks to employees based on their sales, but you want no gaps in ranking numbers even if some employees have the same sales. Which function should you use and why?

Options:
A) ROW_NUMBER() because it gives unique numbers.
B) RANK() because it skips ranks after ties.
C) DENSE_RANK() because it assigns ranks without gaps after ties.
D) COUNT() because it counts rows.

hard

A. DENSE_RANK() because it assigns ranks without gaps after ties.

B. RANK() because it skips ranks after ties.

C. ROW_NUMBER() because it gives unique numbers.

D. COUNT() because it counts rows.

Solution

1. Step 1: Understand ranking requirements

   The requirement is to assign ranks with no gaps even if employees tie in sales.

2. Step 2: Match functions to behavior

   ROW_NUMBER() gives unique numbers, ignoring ties. RANK() skips ranks after ties causing gaps. DENSE_RANK() assigns ranks without gaps after ties.

3. Step 3: Choose correct function

   DENSE_RANK() fits the requirement perfectly.

4. Final Answer:

   DENSE_RANK() because it assigns ranks without gaps after ties. -> Option A

5. Quick Check:

   DENSE_RANK() = no gaps in ranks [OK]

Hint: Use DENSE_RANK() to avoid gaps in ranking numbers [OK]

Common Mistakes:

• Choosing RANK() and expecting no gaps
• Using ROW_NUMBER() which ignores ties
• Confusing COUNT() with ranking functions