📽 Watch Video!
1. What is SQL?
SQL stands for Structured Query Language. It is used to communicate with and manage databases. SQL allows you to insert, query, update, and delete data.
2. What are the different types of SQL commands?
SQL commands are divided into several categories:
- DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE
- DDL (Data Definition Language): CREATE, ALTER, DROP, TRUNCATE
- DCL (Data Control Language): GRANT, REVOKE
- TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT
3. What is the difference between WHERE and HAVING?
Use WHERE to filter rows before aggregation and HAVING to filter groups after aggregation.
4. Explain JOIN types in SQL.
SQL joins are used to combine rows from two or more tables based on a related column:
- INNER JOIN: Returns rows with matching values in both tables
- LEFT JOIN: Returns all rows from the left table and matched rows from the right table
- RIGHT JOIN: Returns all rows from the right table and matched rows from the left table
- FULL OUTER JOIN: Returns all rows when there is a match in either table
5. What are aggregate functions in SQL?
Aggregate functions perform calculations on multiple rows and return a single result. Common functions include:
- SUM()
- COUNT()
- AVG()
- MIN()
- MAX()
6. What is a subquery?
A subquery is a query nested inside another query. It is used to perform operations that require multiple steps, such as filtering data based on aggregated results.
7. Difference between PRIMARY KEY and UNIQUE key
- PRIMARY KEY: Uniquely identifies each row, cannot be NULL, only one per table
- UNIQUE KEY: Ensures unique values in a column, can accept one NULL, multiple per table allowed
8. What is normalization?
Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. Common normal forms include 1NF, 2NF, 3NF, and BCNF.
9. How do you retrieve the top N records in SQL?
Use TOP or LIMIT depending on your database:
-- SQL Server SELECT TOP 5 * FROM Orders ORDER BY OrderDate DESC; -- MySQL / PostgreSQL SELECT * FROM Orders ORDER BY OrderDate DESC LIMIT 5;
10. Explain the difference between DELETE and TRUNCATE
- DELETE: Removes specific rows, can use WHERE, logs changes, can be rolled back
- TRUNCATE: Removes all rows, cannot use WHERE, minimal logging, usually cannot be rolled back
11. What is a foreign key?
A foreign key is a column (or combination of columns) that establishes a link between data in two tables. It ensures referential integrity between parent and child tables.
12. What is an index?
An index improves query performance by allowing faster retrieval of records. However, excessive indexing can slow down write operations.
13. What are views in SQL?
A view is a virtual table based on the result of a SQL query. Views do not store data physically but can simplify complex queries and improve security.
14. What is a transaction?
A transaction is a sequence of SQL statements executed as a single unit of work. Transactions must follow ACID properties: Atomicity, Consistency, Isolation, Durability.
15. Practice Tips for SQL Interviews
- Practice writing queries on sample databases
- Understand the difference between SQL commands and when to use them
- Review JOINs, subqueries, and aggregate functions
- Use online platforms for hands-on SQL exercises
- Be prepared to explain your reasoning clearly during interviews
Conclusion
These beginner-friendly SQL interview questions cover the most common topics you will encounter. By practicing these questions and understanding the concepts behind them, you can confidently face SQL interviews and improve your job prospects.
For more interview preparation and practical exercises enroll in our complete I Want To Learn SQL courses for hands-on experience and a certificate.
⭐ Need an SQL Certificate, CEUs, or a FREE start with Hands-On Practice! → Get Started Here!