Top SQL Interview Questions (2026)
SQL is one of the most common topics asked in backend interviews. To strengthen your preparation, also check system design concepts and backend developer roadmap.
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1. What are different types of JOINs?
SQL supports various types of JOINs to combine data from multiple tables:
- INNER JOIN: Returns records with matching values in both tables.
- LEFT JOIN: Returns all records from the left table and matched records from the right table.
- RIGHT JOIN: Returns all records from the right table and matched records from the left table.
- FULL JOIN: Returns records when there is a match in either left or right table.
2. What is Indexing?
Indexing is a technique to improve query performance by creating data structures that allow faster search and retrieval. Example: CREATE INDEX idx_name ON users(name);
3. What is Normalization?
Normalization organizes database tables to minimize redundancy and dependency. Common normal forms include 1NF, 2NF, 3NF, and BCNF.
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4. What is a Primary Key?
A primary key uniquely identifies each record in a table. A table can have only one primary key.
5. What is a Foreign Key?
A foreign key is a column that creates a link between two tables, enforcing referential integrity.
6. What are Transactions?
Transactions are a sequence of operations performed as a single unit. They support ACID properties (Atomicity, Consistency, Isolation, Durability). Example: BEGIN; UPDATE accounts SET balance=balance-100 WHERE id=1; COMMIT;
7. What is the difference between WHERE and HAVING?
WHERE: Filters rows before aggregation.
HAVING: Filters groups after aggregation.
8. What are Views?
Views are virtual tables based on the result of a query. They simplify complex queries and enhance security by restricting data access.
9. What is Denormalization?
Denormalization is the process of combining tables or adding redundant data to improve read performance, often used in reporting or analytics.
10. How to optimize SQL queries?
Use indexes, avoid SELECT *, use joins efficiently, analyze query execution plans, and limit nested subqueries.
Practice these questions using our AI quick Preparation methods.
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