Bird
Raised Fist0
SQLquery~5 mins

RANK and DENSE_RANK difference in SQL - Time & Space Complexity

Choose your learning style10 modes available
LearnWhyDeepVisualTryChallengeProjectRecallTime
Time Complexity: RANK and DENSE_RANK difference
O(n log n)
Understanding Time Complexity

We want to understand how the time it takes to run ranking functions changes as the data grows.

Specifically, we ask: How does using RANK or DENSE_RANK affect the work done by the database?

Scenario Under Consideration

Analyze the time complexity of these SQL queries using ranking functions.

SELECT employee_id, salary,
       RANK() OVER (ORDER BY salary DESC) AS rank_position
FROM employees;

SELECT employee_id, salary,
       DENSE_RANK() OVER (ORDER BY salary DESC) AS dense_rank_position
FROM employees;

These queries assign ranks to employees based on salary, showing two ways to handle ties.

Identify Repeating Operations

Look for repeated work inside the ranking process.

  • Primary operation: Sorting all rows by salary.
  • How many times: Once per query, over all rows.
How Execution Grows With Input

As the number of employees grows, the sorting work grows too.

Input Size (n)Approx. Operations
10About 30 operations
100About 700 operations
1000About 10,000 operations

Pattern observation: Sorting work grows faster than the number of rows, roughly like n log n.

Final Time Complexity

Time Complexity: O(n log n)

This means the time to assign ranks grows a bit faster than the number of rows because sorting is needed first.

Common Mistake

[X] Wrong: "Ranking functions just scan rows once, so they run in linear time."

[OK] Correct: Ranking needs sorting to order rows, and sorting takes more than just scanning once.

Interview Connect

Knowing how ranking functions scale helps you explain query performance clearly and confidently.

Self-Check

What if we added a filter to rank only employees in one department? How would that change the time complexity?

Practice

(1/5)
1. What is the main difference between RANK() and DENSE_RANK() functions in SQL?
easy
A. DENSE_RANK() assigns random ranks, RANK() assigns sequential ranks.
B. RANK() skips rank numbers after ties, DENSE_RANK() does not skip numbers.
C. RANK() and DENSE_RANK() always return the same results.
D. DENSE_RANK() skips rank numbers after ties, RANK() does not skip numbers.

Solution

  1. Step 1: Understand how RANK() works with ties

    RANK() assigns the same rank to tied rows but skips the next rank numbers accordingly. For example, if two rows tie for rank 1, the next rank will be 3.

  2. Step 2: Understand how DENSE_RANK() works with ties

    DENSE_RANK() also assigns the same rank to tied rows but does not skip rank numbers. So after two rows tied at rank 1, the next rank will be 2.

  3. Final Answer:

    RANK() skips rank numbers after ties, DENSE_RANK() does not skip numbers. -> Option B

  4. Quick Check:

    Skipping ranks = RANK() [OK]
Hint: Remember: RANK skips numbers after ties, DENSE_RANK does not [OK]
Common Mistakes:
  • Thinking both functions behave the same with ties
  • Confusing which function skips rank numbers
  • Assuming ranks are assigned randomly
2. Which of the following is the correct syntax to get the rank of employees ordered by salary using DENSE_RANK()?
easy
A. SELECT employee_id, RANK() BY salary FROM employees;
B. SELECT employee_id, DENSE_RANK(salary) FROM employees ORDER BY salary DESC;
C. SELECT employee_id, DENSE_RANK() OVER (ORDER BY salary DESC) AS rank FROM employees;
D. SELECT employee_id, DENSE_RANK() ORDER BY salary FROM employees;

Solution

  1. Step 1: Identify correct DENSE_RANK() syntax

    The correct syntax uses DENSE_RANK() OVER (ORDER BY column) to rank rows based on a column.

  2. Step 2: Check each option

    SELECT employee_id, DENSE_RANK() OVER (ORDER BY salary DESC) AS rank FROM employees; uses the correct window function syntax. Options A, B, and D have syntax errors or misuse the function.

  3. Final Answer:

    SELECT employee_id, DENSE_RANK() OVER (ORDER BY salary DESC) AS rank FROM employees; -> Option C

  4. Quick Check:

    Window function syntax = SELECT employee_id, DENSE_RANK() OVER (ORDER BY salary DESC) AS rank FROM employees; [OK]
Hint: Use OVER (ORDER BY ...) with ranking functions [OK]
Common Mistakes:
  • Omitting OVER clause
  • Passing column inside function parentheses
  • Using wrong keywords like BY instead of ORDER BY
3. Given the table Scores with values:
Player | Score
Alice | 100
Bob | 90
Carol | 90
Dave | 80
What will be the output of this query?
SELECT Player, Score, RANK() OVER (ORDER BY Score DESC) AS Rank, DENSE_RANK() OVER (ORDER BY Score DESC) AS DenseRank FROM Scores;
medium
A. [{"Player":"Alice","Score":100,"Rank":1,"DenseRank":1},{"Player":"Bob","Score":90,"Rank":2,"DenseRank":2},{"Player":"Carol","Score":90,"Rank":2,"DenseRank":2},{"Player":"Dave","Score":80,"Rank":4,"DenseRank":3}]
B. [{"Player":"Alice","Score":100,"Rank":1,"DenseRank":1},{"Player":"Bob","Score":90,"Rank":3,"DenseRank":2},{"Player":"Carol","Score":90,"Rank":3,"DenseRank":2},{"Player":"Dave","Score":80,"Rank":4,"DenseRank":3}]
C. [{"Player":"Alice","Score":100,"Rank":1,"DenseRank":1},{"Player":"Bob","Score":90,"Rank":2,"DenseRank":3},{"Player":"Carol","Score":90,"Rank":2,"DenseRank":3},{"Player":"Dave","Score":80,"Rank":3,"DenseRank":4}]
D. [{"Player":"Alice","Score":100,"Rank":1,"DenseRank":1},{"Player":"Bob","Score":90,"Rank":2,"DenseRank":2},{"Player":"Carol","Score":90,"Rank":3,"DenseRank":2},{"Player":"Dave","Score":80,"Rank":4,"DenseRank":3}]

Solution

  1. Step 1: Assign RANK() values

    Alice has highest score 100, rank 1. Bob and Carol tie at 90, both get rank 2. Because of tie at rank 2, next rank is 4 for Dave.

  2. Step 2: Assign DENSE_RANK() values

    Alice is dense rank 1. Bob and Carol tie at 90, dense rank 2. Dave has next dense rank 3 (no skipping).

  3. Final Answer:

    [{"Player":"Alice","Score":100,"Rank":1,"DenseRank":1},{"Player":"Bob","Score":90,"Rank":2,"DenseRank":2},{"Player":"Carol","Score":90,"Rank":2,"DenseRank":2},{"Player":"Dave","Score":80,"Rank":4,"DenseRank":3}] -> Option A

  4. Quick Check:

    RANK skips numbers, DENSE_RANK does not [OK]
Hint: Check how ranks jump after ties for RANK vs DENSE_RANK [OK]
Common Mistakes:
  • Assigning dense ranks with gaps
  • Not skipping ranks in RANK after ties
  • Mixing up rank numbers for tied rows
4. You wrote this query:
SELECT name, RANK() ORDER BY score DESC FROM players;

But it gives a syntax error. What is the problem?
medium
A. Missing OVER() clause after RANK() function.
B. ORDER BY cannot be used inside SELECT clause.
C. RANK() requires a parameter inside parentheses.
D. The table name is missing in the query.

Solution

  1. Step 1: Check RANK() function usage

    RANK() is a window function and must be followed by OVER() clause specifying order.

  2. Step 2: Identify missing OVER() clause

    The query uses RANK() without OVER(), causing syntax error.

  3. Final Answer:

    Missing OVER() clause after RANK() function. -> Option A

  4. Quick Check:

    Window functions need OVER() [OK]
Hint: Always add OVER() after window functions like RANK() [OK]
Common Mistakes:
  • Forgetting OVER() clause
  • Trying to put ORDER BY inside SELECT without OVER()
  • Assuming RANK() takes parameters
5. You have a sales table:
Salesperson | SalesAmount
John | 500
Jane | 700
Joe | 700
Jill | 600
You want to assign ranks so that tied salespeople share the same rank and the next rank number is not skipped. Which query correctly achieves this?
hard
A. SELECT Salesperson, SalesAmount, RANK() OVER (ORDER BY SalesAmount DESC) AS Rank FROM sales;
B. SELECT Salesperson, SalesAmount, RANK() OVER (PARTITION BY SalesAmount ORDER BY SalesAmount DESC) AS Rank FROM sales;
C. SELECT Salesperson, SalesAmount, ROW_NUMBER() OVER (ORDER BY SalesAmount DESC) AS Rank FROM sales;
D. SELECT Salesperson, SalesAmount, DENSE_RANK() OVER (ORDER BY SalesAmount DESC) AS Rank FROM sales;

Solution

  1. Step 1: Understand ranking requirements

    Tied salespeople should share the same rank, and the next rank number should not skip any numbers.

  2. Step 2: Choose correct function

    DENSE_RANK() assigns same rank to ties and does not skip ranks. RANK() skips ranks after ties. ROW_NUMBER() never assigns same rank to ties. SELECT Salesperson, SalesAmount, RANK() OVER (PARTITION BY SalesAmount ORDER BY SalesAmount DESC) AS Rank FROM sales; misuses PARTITION BY.

  3. Final Answer:

    SELECT Salesperson, SalesAmount, DENSE_RANK() OVER (ORDER BY SalesAmount DESC) AS Rank FROM sales; -> Option D

  4. Quick Check:

    DENSE_RANK() for no skipped ranks after ties [OK]
Hint: Use DENSE_RANK() to avoid skipping ranks after ties [OK]
Common Mistakes:
  • Using RANK() when no skipping is wanted
  • Using ROW_NUMBER() which never ties
  • Incorrect use of PARTITION BY in ranking
Recommended

Ready to prove it?

Pass the test and this pattern moves unknown strong while earning +1 Grip toward your certificate.

10 questions~5 mineasy hard