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SQL Interview Questions and Answers 2026 - Top 10 Database Query Questions - Admin - 03-21-2026
SQL (Structured Query Language) is an essential skill tested in technical interviews at every IT company in 2026, especially at companies like Oracle, Microsoft, Amazon, and all data-driven organizations. Whether you're preparing for Database Developer, Data Analyst, or Backend Engineer roles, these top 10 SQL interview questions are most frequently asked.
Keywords: SQL interview questions 2026, SQL query interview, SQL joins interview, SQL subquery interview, SQL aggregate functions interview questions
1. What are the different types of SQL commands?
SQL commands are categorized into: DDL (Data Definition Language - CREATE, ALTER, DROP, TRUNCATE), DML (Data Manipulation Language - SELECT, INSERT, UPDATE, DELETE), DCL (Data Control Language - GRANT, REVOKE), TCL (Transaction Control Language - COMMIT, ROLLBACK, SAVEPOINT). DDL auto-commits while DML requires explicit commit.
2. What is the difference between WHERE and HAVING clauses?
WHERE filters rows before grouping and cannot use aggregate functions. HAVING filters groups after GROUP BY and can use aggregate functions. Example: SELECT dept, COUNT(*) FROM employees WHERE salary > 50000 GROUP BY dept HAVING COUNT(*) > 5. WHERE is applied first, then GROUP BY, then HAVING.
3. Explain different types of JOINs in SQL.
INNER JOIN returns only matching rows from both tables. LEFT JOIN returns all rows from left table plus matching rows from right. RIGHT JOIN returns all rows from right table plus matching rows from left. FULL OUTER JOIN returns all rows from both tables. CROSS JOIN returns Cartesian product. SELF JOIN joins a table with itself using aliases.
4. What are subqueries and their types?
A subquery is a query nested inside another query. Types include: Single-row subquery (returns one row), Multi-row subquery (returns multiple rows, used with IN, ANY, ALL), Correlated subquery (references outer query, executes once per outer row), and Scalar subquery (returns single value). Subqueries can appear in SELECT, FROM, WHERE, and HAVING clauses.
5. What are aggregate functions in SQL?
Aggregate functions perform calculations on a set of values and return a single value. Common functions: COUNT() counts rows, SUM() adds values, AVG() calculates average, MAX() finds maximum, MIN() finds minimum. They are used with GROUP BY to group results. NULL values are ignored by all aggregate functions except COUNT(*).
6. What is the difference between UNION and UNION ALL?
UNION combines result sets of two SELECT statements and removes duplicate rows. UNION ALL combines result sets without removing duplicates, making it faster. Both require the same number of columns with compatible data types. Use UNION when you need unique results and UNION ALL when duplicates are acceptable or performance is critical.
7. What are indexes and how do they improve query performance?
An index is a database structure that speeds up data retrieval by creating a sorted reference to table rows. Types include Clustered Index (determines physical order of data, one per table), Non-Clustered Index (separate structure pointing to data, multiple per table), Unique Index, and Composite Index. Indexes speed up SELECT but slow down INSERT, UPDATE, and DELETE operations.
8. What are Views in SQL?
A View is a virtual table based on a SELECT query. It doesn't store data physically but provides a simplified interface to complex queries. Views can restrict access to specific columns, provide data abstraction, and simplify complex joins. Updatable views allow INSERT, UPDATE, DELETE operations. Materialized views store results physically and need periodic refresh.
9. What are window functions in SQL?
Window functions perform calculations across a set of rows related to the current row without grouping them. Key functions include ROW_NUMBER() (sequential numbering), RANK() (ranking with gaps), DENSE_RANK() (ranking without gaps), LEAD/LAG (access next/previous rows), and SUM/AVG OVER (running totals/averages). They use PARTITION BY and ORDER BY within the OVER clause.
10. What is normalization and denormalization?
Normalization organizes data to reduce redundancy through normal forms (1NF, 2NF, 3NF, BCNF). It improves data integrity but may require complex joins. Denormalization intentionally adds redundancy to improve read performance by reducing joins. OLTP systems typically use normalized schemas while OLAP/data warehouse systems use denormalized schemas like star or snowflake schemas.
RE: SQL Interview Questions and Answers 2026 - Top 10 Database Query Questions - indian - 03-22-2026
SQL mastery is non-negotiable for any database-related role. Understanding JOINs, subqueries, window functions, and query optimization gives candidates a huge edge in technical interviews. Practicing on real datasets is the best preparation strategy.
RE: SQL Interview Questions and Answers 2026 - Top 10 Database Query Questions - mohan - 03-22-2026
SQL knowledge is critical for almost every tech role today. Beyond the basics, make sure you practice writing complex JOINs, subqueries, and window functions like ROW_NUMBER and RANK. Understanding query optimization and indexing strategies can set you apart in interviews. I recommend practicing on sites like SQLZoo and LeetCode SQL problems to get comfortable with real-world query scenarios.