SQL Interview Questions for Data Engineers

All SQL questions

SQL interview questions
#	Question	Difficulty	Type	Practise
1	Explain INNER JOIN vs LEFT JOIN with a practical example.	Easy	conceptual, coding	—
2	Find customers who never placed an order, and explain NOT IN vs NOT EXISTS.	Easy	coding, debugging	LeetCode (opens in a new tab)
3	How do window functions differ from GROUP BY?	Easy	conceptual, coding	—
4	Average Order Value: SQL Case Study with 8 Approaches	Medium	coding, optimization, scenario	—
5	Customer Lifetime Value: SQL Case Study with 8 Approaches	Medium	coding, optimization, scenario	—
6	Daily Active Users: SQL Case Study with 8 Approaches	Medium	coding, optimization, scenario	—
7	Find the second-highest salary without using a simple MAX approach.	Medium	coding	LeetCode (opens in a new tab)
8	How would you detect and remove duplicate records safely?	Medium	coding, scenario	LeetCode (opens in a new tab)
9	Lead Scoring: SQL Case Study with 7 Approaches	Medium	coding, scenario, optimization	—
10	New vs Returning Customers: SQL Case Study with 8 Approaches	Medium	coding, optimization, scenario	—
11	Refund Analysis: SQL Case Study with 7 Approaches	Medium	coding, optimization, scenario	—
12	Return the top three salaries in each department, including ties.	Medium	coding	LeetCode (opens in a new tab)
13	Revenue doubled after you joined a second table. What happened and how do you fix it?	Medium	debugging, coding	—
14	Session Duration: SQL Case Study with 8 Approaches	Medium	coding, optimization, scenario	—
15	Subscription Renewals: SQL Case Study with 7 Approaches	Medium	coding, optimization, scenario	—
16	Top Selling Products: SQL Case Study with 8 Approaches	Medium	coding, optimization, scenario	—
17	A/B Test Results: SQL Case Study with 7 Approaches	Hard	coding, scenario, optimization	—
18	Churn Rate: SQL Case Study with 8 Approaches	Hard	coding, optimization, scenario	—
19	Conversion Funnel: SQL Case Study with 8 Approaches	Hard	coding, optimization, scenario	—
20	Employee Hierarchy: SQL Case Study with 8 Approaches	Hard	coding, optimization, scenario	—
21	Fraud Pattern Detection: SQL Case Study with 8 Approaches	Hard	coding, optimization, scenario	—
22	How would you optimize a slow analytical SQL query?	Hard	optimization, scenario	—
23	Inventory Turnover: SQL Case Study with 8 Approaches	Hard	coding, optimization, scenario	—
24	Monthly Revenue: SQL Case Study with 8 Approaches	Hard	coding, optimization, scenario	—
25	Write an SCD Type 2 upsert in SQL that closes old versions and inserts new ones.	Hard	coding, scenario	—

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