Top 10 Difficult SQL Questions for Experienced Professionals 2025

10 April, 2025 admin 0 Comments 1 category

When you’re an experienced SQL professional—whether you’re a data analyst, backend developer, or database architect—interviewers don’t just want to test your syntax. They want to test logic, performance tuning, real-time scenarios, and your problem-solving ability under pressure.

In this post, we cover the top difficult SQL questions for experienced candidates that are often asked in high-level technical interviews, especially for those with 4+ years of experience.

1. Find the Nth Highest Salary Without Using LIMIT/TOP

sqlCopyEditSELECT DISTINCT salary 
FROM employees e1
WHERE N - 1 = (
  SELECT COUNT(DISTINCT salary) 
  FROM employees e2 
  WHERE e2.salary > e1.salary
);

🧠 Why it’s difficult: Requires nested subqueries and a strong grasp of ranking logic without relying on predefined SQL functions.

2. Detect Cycles in a Hierarchical Table

Imagine a table where employees report to managers. You need to detect if there’s a loop.

💡 Hint: Use recursive CTEs and maintain a visited path list.

sqlCopyEditWITH RECURSIVE org_tree AS (
  SELECT employee_id, manager_id, ARRAY[employee_id] AS path
  FROM employees
  WHERE manager_id IS NULL
  UNION ALL
  SELECT e.employee_id, e.manager_id, path || e.employee_id
  FROM employees e
  JOIN org_tree ot ON e.manager_id = ot.employee_id
  WHERE NOT e.employee_id = ANY(path)
)
SELECT * FROM org_tree;

3. Query to Remove Duplicate Rows Keeping the Latest One

sqlCopyEditDELETE FROM orders
WHERE order_id NOT IN (
  SELECT MAX(order_id)
  FROM orders
  GROUP BY customer_id, product_id
);

🧠 Tricky part: Retaining only the latest based on business logic like timestamp or ID.

4. Find Consecutive Login Days for Each User

sqlCopyEditWITH user_dates AS (
  SELECT user_id, login_date,
         ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY login_date) AS rn
  FROM logins
),
grouped AS (
  SELECT user_id, login_date, login_date - INTERVAL '1 day' * rn AS grp
  FROM user_dates
)
SELECT user_id, MIN(login_date) AS start_date, MAX(login_date) AS end_date,
       COUNT(*) AS consecutive_days
FROM grouped
GROUP BY user_id, grp;

🧠 Why it’s asked: Shows your grasp of date arithmetic, grouping, and sequence analysis.

5. Find Missing Values in a Sequence

sqlCopyEditSELECT t1.id + 1 AS missing_id
FROM numbers t1
LEFT JOIN numbers t2 ON t1.id + 1 = t2.id
WHERE t2.id IS NULL;

📌 Works well for ID tracking, invoice series, or log integrity.

6. Query to Pivot Rows to Columns Without PIVOT Keyword

sqlCopyEditSELECT 
  customer_id,
  MAX(CASE WHEN month = 'Jan' THEN revenue ELSE 0 END) AS Jan_Revenue,
  MAX(CASE WHEN month = 'Feb' THEN revenue ELSE 0 END) AS Feb_Revenue
FROM sales
GROUP BY customer_id;

7. Recursive Sum in SQL

Calculate the cumulative salary increase each year using CTEs.

sqlCopyEditWITH RECURSIVE salary_growth AS (
  SELECT employee_id, salary, year
  FROM salaries
  WHERE year = 2020
  UNION ALL
  SELECT s.employee_id, sg.salary + s.increase, s.year
  FROM salaries s
  JOIN salary_growth sg ON s.employee_id = sg.employee_id AND s.year = sg.year + 1
)
SELECT * FROM salary_growth;

8. Detect Gaps in Date Ranges (Leave or Attendance)

sqlCopyEditSELECT 
  employee_id, 
  MAX(end_date) AS last_day_before_gap
FROM attendance a1
WHERE NOT EXISTS (
  SELECT 1 FROM attendance a2
  WHERE a2.start_date = a1.end_date + INTERVAL '1 day'
    AND a1.employee_id = a2.employee_id
)
GROUP BY employee_id;

9. Find the Percentage of Rows Where a Condition Holds True

sqlCopyEditSELECT 
  ROUND(100.0 * SUM(CASE WHEN status = 'active' THEN 1 ELSE 0 END) / COUNT(*), 2) AS active_ratio
FROM users;

🔍 Useful for analytics and reporting dashboards.

10. Explain the Use of Indexes with Examples

You might not be asked to write a query but explain:

What kind of indexes exist (B-tree, bitmap)
When to use composite indexes
Drawbacks of over-indexing (slow inserts, larger storage)

🔍 Real Interview Tip:

“Most difficult SQL questions for experienced professionals” don’t just test your SQL writing ability—but also:

How you think through data problems
How you write efficient, readable code
How you optimize performance
How you explain your thought process

✅ FAQs: Difficult SQL Questions for Experienced

Q1. Are these SQL questions for data engineers as well?
Yes! Many of these apply to data engineers, analysts, and backend developers working with large datasets.

Q2. How can I practice these questions?
Use platforms like LeetCode, StrataScratch, and HackerRank. Also try real-world datasets like those from Kaggle.

Q3. Are these asked in MNCs like TCS, Accenture, or Capgemini?
Absolutely. These are common in interviews across top IT companies, especially in L2/L3 roles.

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