SQL Interview Challenges with Real Examples
💡 Concept Name
SQL Interview Challenges – Real-world SQL problems designed to test understanding of joins, subqueries, aggregations, and query optimization in technical interviews.
📘 Quick Intro
SQL interviews often focus less on syntax and more on solving actual business problems through data manipulation. You’re typically asked to retrieve insights using joins, subqueries, groupings, and window functions. Challenges range from finding top-performing products to detecting duplicates or gaps in datasets. Solving these efficiently demonstrates depth, not just knowledge.
🧠 Analogy / Short Story
Think of SQL interview questions like escape rooms—every query puzzle has a hidden path, and the clues lie in the data structure. Some require unlocking doors with joins, others need careful use of windows and filters. Rushing without observing can trap you in Cartesian chaos. Winning is not just escaping but doing so elegantly and with performance in mind.
🔧 Technical Explanation
- 🔍 Interview challenges often test inner vs outer joins, correlated vs uncorrelated subqueries, and grouping techniques.
- 🪟 Window functions like RANK(), ROW_NUMBER(), and LEAD() are commonly evaluated.
- ⚡ Focus is also placed on query efficiency, indexing, and avoiding N+1 subqueries.
- 📐 Real examples can include finding duplicates, top-N per category, gaps, running totals, or change tracking.
- ✅ Clean logic, readable aliases, and modular subqueries often win over clever hacks.
🎯 Purpose & Use Case
- ✅ Assess ability to transform business requirements into SQL queries.
- ✅ Validate familiarity with best practices in joining and aggregating data.
- ✅ Test optimization instincts through rewriting inefficient queries.
- ✅ Reveal knowledge of edge cases like nulls, duplicates, or grouped conditions.
💻 Real Code Example
-- Find second highest salary (MySQL)
SELECT MAX(Salary) AS SecondHighest
FROM Employees
WHERE Salary < (SELECT MAX(Salary) FROM Employees);
-- Customers who placed more than 3 orders (SQL Server)
SELECT c.CustomerName
FROM Customers c
JOIN Orders o ON c.CustomerId = o.CustomerId
GROUP BY c.CustomerName
HAVING COUNT(o.OrderId) > 3;
-- Running total of sales
SELECT OrderId, OrderDate, Amount,
SUM(Amount) OVER (ORDER BY OrderDate) AS RunningTotal
FROM Orders;
❓ Interview Q&A
Q1: How would you find the second highest value in a column?
A: Use a subquery to filter values less than the max, then apply MAX() again.
Q2: What’s a window function?
A: A function that performs calculations across rows related to the current row without collapsing them.
Q3: Difference between WHERE and HAVING?
A: WHERE filters rows before grouping; HAVING filters after grouping.
Q4: How do you handle NULLs in aggregations?
A: Use functions like COALESCE() to replace nulls or understand how COUNT() ignores them.
Q5: How do you find duplicates in a table?
A: Use GROUP BY on suspect columns and HAVING COUNT(*) > 1.
Q6: What is the difference between RANK() and DENSE_RANK()?
A: RANK() skips numbers for ties; DENSE_RANK() does not.
Q7: How to find gaps in a sequence?
A: Use a CTE with ROW_NUMBER() and compare it with actual ID values.
Q8: What’s an anti-join and how is it done?
A: It returns rows in A not in B, typically via LEFT JOIN with IS NULL.
Q9: What’s the purpose of a CTE?
A: Common Table Expressions (CTEs) make queries modular and readable, useful for recursion or reuse.
Q10: How do you find top N records per group?
A: Use ROW_NUMBER() OVER (PARTITION BY ...) and filter by row number.
📝 MCQs
Q1. Which SQL function returns running totals?
- COUNT()
- MAX()
- SUM() OVER (ORDER BY ...)
- GROUP_TOTAL()
Q2. Which clause is used to filter grouped results?
- WHERE
- HAVING
- GROUP
- ORDER BY
Q3. Which operator finds missing matches in a join?
- INNER JOIN
- RIGHT JOIN
- LEFT JOIN + IS NULL
- FULL JOIN
Q4. How do you find the second highest salary?
- Use DISTINCT
- Sort with LIMIT 2
- Use subquery with MAX()
- Use AVG()
Q5. Which keyword begins a window function?
- FROM
- GROUP
- OVER
- INTO
Q6. ROW_NUMBER() is used to...
- Sum totals
- Rank results globally
- Assign sequence numbers per partition
- Delete duplicates
Q7. To find duplicates, use...
- WHERE IN()
- HAVING IN()
- GROUP BY + HAVING COUNT(*) > 1
- LEFT JOIN
Q8. A CTE is initiated using...
- START
- DEFINE
- WITH
- INIT
Q9. What clause allows sorting in window functions?
- WHERE
- SORT
- ORDER BY
- GROUP BY
Q10. Which function ranks without gaps?
- RANK()
- ROW_NUMBER()
- DENSE_RANK()
- COUNT()
💡 Bonus Insight
Always try to write queries that scale well with increasing data. Consider using indexes on filter columns, avoiding unnecessary subqueries, and analyzing query execution plans. Interviewers appreciate clean logic with clear intention over overly clever tricks.
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