Solving the Gender-based Alternating Row Problem in SQL: A Data Engineering Interview Question asked by MAANG

The Problem Statement

Given a table named Employee with the following structure:

Name   Sex
A      M
B      M
C      M
X      F
Y      F
Z      F

We need to generate an output that alternates between male and female employees, ordered by their names. The expected output is:

Name
A
X
B
Y
C
Z

Analyzing the Problem

The challenge here is to partition the data by gender and then interleave the rows such that the output alternates between male and female employees. This requires the use of window functions and common table expressions (CTEs) to assign row numbers within each partition and then order them accordingly.

The Solution

To achieve the desired output, we can use the following SQL approach:

1. Partition the Data: Use the ROW_NUMBER() window function to assign a unique number to each row within its gender partition.
2. Combine the Data: Use a CTE (Common Table Expression) to handle the partitioned data and then combine it in a way that alternates the rows based on the assigned row numbers.

Here’s the SQL query that accomplishes this:

WITH CTE AS (
    SELECT Name, 
           ROW_NUMBER() OVER (PARTITION BY Sex ORDER BY Name) AS rownumberforEmp 
    FROM Employee
)
SELECT Name 
FROM CTE 
ORDER BY rownumberforEmp, 
         CASE WHEN Sex = 'M' THEN 1 ELSE 2 END;