Window Functions Demystified: ROW_NUMBER vs RANK vs DENSE_RANK (Execution Internals)

In high-scale enterprise data engineering, ranking events, deduplicating records, and calculating leaderboards are everyday tasks.

While ROW_NUMBER(), RANK(), and DENSE_RANK() look similar on the surface, choosing the wrong one can corrupt your financial aggregations or deduplication pipelines.

Here is the exact execution breakdown and when to use each.

🔍 The 3 Ranking Functions at a Glance

Function	Tie Handling	Gaps in Sequence?	Primary Production Use-Case
`ROW_NUMBER()`	Assigns arbitrary distinct sequence (1, 2, 3, 4)	❌ No Gaps	Strict Deduplication & Pagination
`RANK()`	Assigns same rank to ties, skips next (1, 2, 2, 4)	⚠️ Has Gaps	Competition Leaderboards & Percentiles
`DENSE_RANK()`	Assigns same rank to ties, no skips (1, 2, 2, 3)	❌ No Gaps	Finding N-th Highest Salary / Top Tiers

💻 Visual SQL Execution

Imagine an Employees table with salaries: [100k, 90k, 90k, 80k].

SELECT 
    EmployeeID,
    Salary,
    ROW_NUMBER() OVER (ORDER BY Salary DESC) AS row_num,
    RANK()       OVER (ORDER BY Salary DESC) AS rnk,
    DENSE_RANK() OVER (ORDER BY Salary DESC) AS dense_rnk
FROM Employees;

📊 Result Set:

Salary | ROW_NUMBER | RANK | DENSE_RANK
-------+------------+------+------------
100k   |     1      |  1   |     1
90k    |     2      |  2   |     2   <-- Tie!
90k    |     3      |  2   |     2   <-- Tie!
80k    |     4      |  4   |     3   <-- Notice RANK jumped to 4, DENSE_RANK went to 3!

⚠️ The #1 Production Interview Trap: "2nd Highest Salary"

If you use RANK() or LIMIT 1 OFFSET 1 to find the 2nd highest salary, and multiple employees tie for the 1st highest salary (e.g. two people earn 100k):

RANK() = 2 returns 0 rows!
DENSE_RANK() = 2 guarantees the correct 2nd highest salary every single time.

WITH RankedSalaries AS (
    SELECT 
        EmployeeID,
        Salary,
        DENSE_RANK() OVER (ORDER BY Salary DESC) AS rnk
    FROM Employees
)
SELECT Salary 
FROM RankedSalaries 
WHERE rnk = 2;