Assign a row number to each user, ordered by age descending.

Problem Statement

<p>Assign a row number to each user, ordered by age descending.</p>

Examples

Input: users table: +----+---------+-----+ | id | name | age | +----+---------+-----+ | 1 | Alice | 20 | | 2 | Bob | 40 | | 3 | Charlie | 30 | +----+---------+-----+

Output: +---------+-----+----+ | name | age | rn | +---------+-----+----+ | Bob | 40 | 1 | | Charlie | 30 | 2 | | Alice | 20 | 3 | +---------+-----+----+

Explanation: The window function evaluates the calculation across the partition in the specified ordering.

Complexity

Time Complexity: -

Space Complexity: -

Hints

💡 Hint 1: Use a window function with OVER (...) to compute values across rows related to the current row without collapsing them like GROUP BY. 💡 Hint 2: Check if PARTITION BY is required to split windows by category or if only an ORDER BY inside OVER (...) is needed. 💡 Hint 3: Structure the query as: <window_func>() OVER (PARTITION BY ... ORDER BY ...) AS <alias>;

Editorial & Approach

Problem Overview & Intuition

To solve "Window Function: ROW_NUMBER", we query the relational database engine using declarative SQL. The goal is to assign a row number to each user, ordered by age descending. By formulating an optimal execution plan with appropriate projection and filtering, the database engine executes the query with minimal overhead.

Step-by-Step Approach

Analyze Schema: Identify the target tables, necessary foreign keys, and expected output columns.
Construct Filtering & Logic: Apply row projections to isolate the requested data.
Format & Order: Sort the resulting records according to specified order criteria.

Optimal Implementation (SQL)

SELECT name, age, ROW_NUMBER() OVER (ORDER BY age DESC) AS rn FROM users;

Complexity Analysis

Time Complexity O(N log N) for sorting or partitioning rows.

Space Complexity O(N) for intermediate group hash tables or window buffers.

Key Considerations & Edge Cases

Empty Tables: The query executes safely returning zero rows without syntax error.
NULL Values: Columns containing NULL values are properly handled by standard ANSI SQL semantics.
Case Sensitivity: String comparisons and keywords adhere to PostgreSQL/standard SQL rules.

Window Function: ROW_NUMBER

Medium

Assign a row number to each user, ordered by age descending.

Example Scenarios

1Example 1

Input:

users table

id	name	age
1	Alice	20
2	Bob	40
3	Charlie	30

Output:

name	age	rn
Bob	40	1
Charlie	30	2
Alice	20	3

Explanation:

The window function evaluates the calculation across the partition in the specified ordering.

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