SQL Questions Asked in an American Express Interview

How I Would Solve These Tricky SQL Questions Asked in American Express Interview

SQL is a fundamental skill for any data analyst, and mastering complex queries is key to standing out in interviews. Below, I break down how I would approach solving the tricky SQL questions mentioned. Each of these challenges is designed to test both your technical proficiency and your problem-solving ability. Let’s dive into the solutions.

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1. Find the Second-Highest Salary in a Table Without Using LIMIT or TOP

This is a classic problem that requires creativity. My solution:

sql

SELECT MAX(salary) 
FROM employees 
WHERE salary < (SELECT MAX(salary) FROM employees);

Here, the subquery finds the maximum salary, and the outer query selects the highest salary below that.

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2. Find All Employees Who Earn More Than Their Managers

Joining the table to itself is the key here:

sql
SELECT e1.employee_name 
FROM employees e1 
JOIN employees e2 
ON e1.manager_id = e2.employee_id 
WHERE e1.salary > e2.salary;

This query compares employees' salaries with their managers' salaries.

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3. Find Duplicate Rows Without Using GROUP BY

Use a ROW_NUMBER() window function to isolate duplicates:

sql
WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY id) AS row_num
    FROM table_name
)
SELECT * 
FROM cte 
WHERE row_num > 1;

This identifies duplicates without relying on GROUP BY.

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4. Find the Top 10% Earners in a Table

Using PERCENTILE_CONT:

sql
WITH percentiles AS (
    SELECT PERCENTILE_CONT(0.9) WITHIN GROUP (ORDER BY salary) AS percentile_90
    FROM employees
)
SELECT * 
FROM employees, percentiles 
WHERE salary > percentile_90;

This query extracts the 90th percentile and selects employees earning above that threshold.

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5. Find the Cumulative Sum of a Column

A window function simplifies this task:

sql
SELECT employee_id, 
       salary, 
       SUM(salary) OVER (ORDER BY employee_id) AS cumulative_sum
FROM employees;

The SUM() function with OVER computes the cumulative sum across rows.

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6. Find Employees Who Have Never Taken a Leave

A NOT IN query solves this effectively:

sql
SELECT * 
FROM employees 
WHERE employee_id NOT IN (SELECT employee_id FROM leaves);

This ensures only employees without leave records are selected.

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7. Difference Between Current Row and Next Row

Using the LEAD() window function:

sql
SELECT employee_id, 
       salary, 
       salary - LEAD(salary) OVER (ORDER BY employee_id) AS diff
FROM employees;

The LEAD() function retrieves the next row value for the comparison.

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8. Find Departments With More Than One Employee

Classic aggregation with a HAVING clause:

sql
SELECT department, COUNT(*) AS employee_count 
FROM employees 
GROUP BY department 
HAVING COUNT(*) > 1;

This identifies departments with multiple employees.

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9. Maximum Value for Each Group Without Using GROUP BY

Using a WINDOW FUNCTION:

sql
SELECT * 
FROM (
    SELECT *, MAX(column_name) OVER (PARTITION BY group_column) AS max_in_group 
    FROM table_name
) subquery 
WHERE column_name = max_in_group;

This method avoids the need for explicit GROUP BY.

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10. Employees Taking More Than 3 Leaves in a Month

Group the leave data and filter by HAVING:

sql
SELECT employee_id, MONTH(leave_date) AS leave_month, COUNT(*) AS leave_count
FROM leaves 
GROUP BY employee_id, MONTH(leave_date) 
HAVING COUNT(*) > 3;

This query isolates employees with excessive leave.

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Reflections and Key Takeaways

Each of these questions tests a specific SQL skill, from basic operations to advanced window functions and subqueries. Here’s how I prepared for such challenges:

• Practice Daily: I honed my skills on platforms like HackerRank and LeetCode.
• Master Window Functions: Functions like ROW_NUMBER(), LEAD(), and SUM() OVER are crucial for handling complex scenarios.
• Understand Business Context: These questions often simulate real-world scenarios, so understanding the "why" behind the query is essential.

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Pro Tip: In interviews, explain your thought process clearly. Employers value logical problem-solving as much as correct answers.

If you're preparing for SQL interviews, focus on efficiency, creativity, and understanding the intent behind queries. Master these skills, and you’ll be ready to ace your interview! 🚀

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