50 Essential SQL Interview Questions: If you’re preparing for an SQL interview, mastering key concepts like database management, queries, joins, normalization, indexing, and stored procedures is crucial. This guide provides 50 of the most commonly asked SQL interview questions and answers, making it ideal for both beginners and experienced professionals.
SQL Interview Questions for Freshers
50 Essential SQL Interview Questions::
1. What is DBMS?
A Database Management System (DBMS) is software that enables users to create, store, manage, and retrieve data efficiently.
2. What is RDBMS?
Relational Database Management System (RDBMS) stores data in tables with relationships between them. Example: MySQL, PostgreSQL.
3. What is SQL?
Structured Query Language (SQL) is used to manage and manipulate relational databases.
4. What is a Database?
A structured collection of data that allows easy access and management.
5. What are Tables and Fields?
Tables store data in rows and columns. Columns are called fields, and rows are called records.
6. What is a Primary Key?
A unique identifier for each record in a table, ensuring data integrity.
7. What is a Unique Key?
A constraint that ensures all values in a column are unique, but unlike a primary key, it allows NULL values.
8. What is a Foreign Key?
A key in one table that references the primary key in another table, maintaining referential integrity.
9. What is a Join?
A SQL operation used to combine rows from two or more tables based on a related column.
10. What are the types of Joins?
- 50 Essential SQL Interview Questions: INNER JOIN – Returns matching records from both tables.
- LEFT JOIN – Returns all records from the left table and matching records from the right.
- RIGHT JOIN – Returns all records from the right table and matching records from the left.
- FULL JOIN – Returns all records from both tables.
SQL Interview Questions for 3+ Years Experience
50 Essential SQL Interview Questions:
11. What is Normalization?
50 Essential SQL Interview Questions: The process of structuring a database to minimize redundancy and dependency.
12. What is Denormalization?
The process of combining tables to improve read performance at the cost of redundancy.
13. What are different Normal Forms?
- 1NF – Removes duplicate columns.
- 2NF – Eliminates partial dependencies.
- 3NF – Removes transitive dependencies.
- 4NF & 5NF – Further eliminate redundancy and anomalies.
14. What is a View?
A virtual table based on the result of a query, improving security and performance.
50 Essential SQL Interview Questions::
15. What is an Index?
A database object that improves query performance by allowing faster data retrieval.
16. What are the types of Indexes?
- Clustered Index – Sorts and stores data in order.
- Non-Clustered Index – Creates a separate structure for searching.
- Unique Index – Prevents duplicate values in a column.
17. What is a Cursor?
A pointer that enables row-by-row processing of query results.
18. What are the types of Database Relationships?
- One-to-One
- One-to-Many
- Many-to-Many
- Self-Referencing
19. What is a Query?
A request to retrieve, update, delete, or insert data in a database.
20. What is a Subquery?
A query within another query that is executed first to provide results for the main query.
SQL Interview Questions for 5+ Years Experience
50 Essential SQL Interview Questions:
21. What are the types of Subqueries?
- Correlated Subquery – Uses values from the outer query.
- Non-Correlated Subquery – Executes independently of the outer query.
22. What is a Stored Procedure?
A set of SQL statements stored in the database that can be executed as a unit.
23. What is a Trigger?
A database object that automatically executes a response to specific events on a table or view.
24. Difference between DELETE and TRUNCATE?
- DELETE – Removes specific rows based on a condition (can be rolled back).
- TRUNCATE – Removes all rows from a table (cannot be rolled back).
25. What are Local and Global Variables?
- Local Variables – Exist within a function and cannot be accessed outside.
- Global Variables – Exist throughout the database session.
26. What is a Constraint?
A rule applied to a column to enforce data integrity (e.g., NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY).
27. What is Data Integrity?
Ensuring data accuracy, consistency, and reliability in a database.
28. What is Auto Increment?
A column property that generates unique numbers automatically for each new row.
29. Difference between Clustered and Non-Clustered Index?
- Clustered Index – Alters the physical storage of data.
- Non-Clustered Index – Creates a separate lookup table for searching.
50 Essential SQL Interview Questions::
30. What is Data Warehousing?
A centralized repository of data used for business intelligence and analytics.
SQL Interview Questions for 10+ Years Experience
50 Essential SQL Interview Questions:
41. What is UNION, MINUS, and INTERSECT?
- UNION – Combines results of two queries, removing duplicates.
- MINUS – Returns records in the first query but not in the second.
- INTERSECT – Returns common records between both queries.
42. What is an Alias?
A temporary name assigned to a table or column in a query.
43. Difference between TRUNCATE and DROP?
- TRUNCATE – Deletes all rows but keeps the structure.
- DROP – Deletes the table and its structure.
44. What are Aggregate and Scalar Functions?
- Aggregate Functions – Perform calculations on multiple rows (e.g., COUNT, SUM).
- Scalar Functions – Operate on a single value (e.g., UCASE, LCASE).
50 Essential SQL Interview Questions:
45. How to Create an Empty Table from an Existing Table?
sql SELECT * INTO new_table FROM old_table WHERE 1=2;
46. How to Fetch Common Records from Two Tables?
sql SELECT column_name FROM table1
INTERSECT
SELECT column_name FROM table2;
47. How to Select Unique Records from a Table?
sql SELECT DISTINCT column_name FROM table_name;
48. How to Fetch the First 5 Characters of a String?
sql SELECT LEFT(column_name, 5) FROM table_name;
49. Which Operator is Used for Pattern Matching?
The LIKE operator:
sql SELECT * FROM table_name WHERE column_name LIKE 'A%';
50. What is Recursive Stored Procedure?
A stored procedure that calls itself until a termination condition is met.
Final Thoughts
50 Essential SQL Interview Questions: Mastering these SQL questions and answers will significantly boost your chances of success in SQL interviews. Whether you’re a fresher or an experienced professional, these concepts are essential for database management and optimization.
Want to crack your SQL interview? Start practicing these questions today!
