50+ Best SQL Interview Questions to Crack Your Next Interview Round

Coursesity Team

Nov 20, 2024 • 6 min read

SQL is a critical skill for anyone looking to excel in data-related roles. Whether you're a beginner or an advanced learner, preparing thoroughly for SQL interviews is essential. In this article, you'll find 60 SQL interview questions and answers categorized into beginner and advanced levels.

First, we will start with questions related to beginners, and then we will move on to the more advanced questions. So, the following are the top SQL interview questions and answers that interviewers commonly ask:

Beginner-Level SQL Questions

1. What are DBMS and RDBMS?

DBMS: Software for creating and managing databases.
RDBMS: A DBMS that organizes data into tables and establishes relationships between them.

2. What is SQL?

SQL (Structured Query Language) is the standard language for managing and manipulating relational databases, used in SQL coding interviews.

3. Explain SQL Constraints.

SQL constraints enforce rules for data in tables:

NOT NULL: Prevents NULL values.
PRIMARY KEY: Uniquely identifies rows.
FOREIGN KEY: Establishes a relationship between two tables.
UNIQUE: Ensures unique values.
CHECK: Verifies data against a condition.

4. What is a Trigger in SQL?

A trigger is a stored procedure that executes automatically in response to specific database events.

5. What are the Subsets of SQL?

DDL (Data Definition Language): CREATE, ALTER, DROP.
DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE.
DCL (Data Control Language): GRANT, REVOKE.

6. What is the Difference Between SQL and MySQL?

SQL: A standard language for database queries.
MySQL: A relational database management system that implements SQL.

7. What are Scalar Functions in SQL?

Scalar functions return single values based on input:

UCASE(): Converts text to uppercase.
LCASE(): Converts text to lowercase.

8. What is a Deadlock in SQL?

A deadlock occurs when two or more transactions block each other by waiting for the other to release resources.

9. What are ACID Properties in SQL?

ACID properties ensure reliable database transactions:

Atomicity: Transactions are all-or-nothing.
Consistency: Data integrity is maintained.
Isolation: Concurrent transactions do not affect each other.
Durability: Once committed, transactions remain so.

10. What is a System Privilege in SQL?

Permissions that allow users to perform specific database operations, such as creating tables or users.

11. What are Object Privileges in SQL?

Permissions that allow users to perform actions on database objects like SELECT, INSERT, or UPDATE.

12. What is a Data Warehouse?

A central repository of integrated data from multiple sources, optimized for querying and analysis.

Intermediate-Level SQL Questions

13. What is the Difference Between NVL and NVL2 Functions?

NVL(expr1, expr2): Returns expr2 if expr1 is NULL; otherwise, expr1.
NVL2(expr1, expr2, expr3): Returns expr2 if expr1 is not NULL; otherwise, expr3.

14. What are Nested Triggers?

Triggers that fire other triggers as part of their execution.

15. What are the Advantages of PL/SQL Functions?

Reusability
Improved Performance
Error Handling

16. What is the Need for a MERGE Statement?

The MERGE statement allows conditional INSERT, UPDATE, or DELETE operations in one step, useful in SQL coding interviews.

17. Are NULL Values the Same as Zero or Blank Space?

No. NULL indicates missing data, zero is a numeric value, and a blank space is a character.

18. Why Do We Use Commit and Rollback Commands?

COMMIT: Saves all changes made to the database.
ROLLBACK: Reverts uncommitted changes.

19. What is the DISTINCT Statement in SQL?

DISTINCT eliminates duplicate rows in query results.

20. What is the Difference Between DELETE and TRUNCATE in SQL?

DELETE: Removes specific rows; can be rolled back.
TRUNCATE: Removes all rows; cannot be rolled back.

21. How Do You Rename a Column in SQL?

ALTER TABLE table_name RENAME COLUMN old_name TO new_name;

22. What is a SQL Operator?

Symbols or keywords used to perform operations in SQL, such as arithmetic (+, -), comparison (=, >), and logical (AND, OR).

23. What is a View in SQL?

A virtual table derived from SQL query results, used in SQL technical interviews.

24. How Do You Fetch Alternate Records in SQL?

SELECT employee_id FROM employees WHERE MOD(rownum, 2) = 0; -- Even rows

Advanced SQL Questions

25. Explain Currying in SQL.

Currying is a functional programming technique where a function is broken into smaller functions, each taking a single argument.

26. Explain Closures in SQL Procedures.

Closures are nested SQL procedures that retain access to variables in their parent scope.

27. What are SQL Joins?

INNER JOIN: Combines rows with matching values.
LEFT JOIN: Includes unmatched rows from the left table.
RIGHT JOIN: Includes unmatched rows from the right table.
FULL JOIN: Combines all rows from both tables.

28. What is the Difference Between Attributes and Properties?

Attributes: Define metadata or settings.
Properties: Store actual values.

29. Explain Higher-Order Functions in SQL.

Higher-order functions take other functions as arguments or return functions, commonly used with aggregates like SUM or AVG.

30. What are Object Prototypes in SQL Procedures?

Prototypes define templates for reusable procedures.

31. How Do You Optimize SQL Queries?

Use indexes.
Avoid SELECT *.
Minimize joins and subqueries where possible.

32. What is the Role of SQL Functions in Queries?

SQL functions perform operations such as:

String Manipulation (e.g., CONCAT).
Date Calculations (e.g., DATEADD).
Aggregate Calculations (e.g., COUNT).

33. What is the Difference Between UNION and UNION ALL?

UNION: Combines rows from multiple queries and removes duplicates.
UNION ALL: Combines rows without removing duplicates.

34. Explain the Use of Window Functions.

Window functions perform calculations across a set of rows related to the current row, commonly used for SQL scenarios for interviews.

Example:

SELECT employee_id, salary, RANK() OVER (ORDER BY salary DESC) AS rank FROM employees;

35. What is the Purpose of Indexes in SQL?

Indexes improve the speed of data retrieval.

36. How Do You Handle SQL Errors in Stored Procedures?

Use TRY...CATCH blocks to handle exceptions.

37. What are Common Table Expressions (CTEs)?

CTEs provide temporary result sets for SQL queries:

WITH EmployeeCTE AS ( SELECT employee_id, department_id FROM employees ) SELECT * FROM EmployeeCTE WHERE department_id = 1;

38. What is the Role of SQL Variables?

Variables store temporary values for use in SQL queries and procedures.

39. How Do You Implement Transactions in SQL?

Transactions ensure data consistency with:

BEGIN TRANSACTION
COMMIT
ROLLBACK

40. Explain the Concept of Denormalization.

Denormalization adds redundancy to optimize query performance.

41. What are Entities and Relationships?

Entities: Objects that represent real-world data, such as "customers" or "products."
Relationships: Associations between entities, such as "customers purchase products."

42. What is an Index in SQL?

An index is used to improve query performance by allowing faster data retrieval.

43. Explain the Different Types of Indexes.

Clustered Index: Reorders the physical table data.
Non-Clustered Index: Keeps logical order independent of physical order.
Unique Index: Prevents duplicate values.

44. What is Normalization in SQL?

Normalization organizes data to minimize redundancy and dependency by dividing data into tables.

45. Explain the First Three Normal Forms (1NF, 2NF, 3NF).

1NF: No duplicate rows or columns.
2NF: Every non-prime attribute is fully functionally dependent on the primary key.
3NF: No transitive dependencies.

46. What are SQL Window Functions?

Window functions perform calculations across a set of table rows that are related to the current row.

47. What is the Difference Between OLTP and OLAP?

OLTP: Optimized for transactional tasks (e.g., INSERT, DELETE).
OLAP: Used for analytical and complex queries.

48. How Do You Create a Temporary Table in SQL?

CREATE TEMPORARY TABLE temp_table (id INT, name VARCHAR(50));

49. What is Auto Increment in SQL?

Auto Increment automatically generates a unique value for a new row.

50. What is a View and Why is it Used?

A view is a virtual table created from a query, used for security, simplicity, and abstraction.

51. What is the Difference Between UNION and INTERSECT?

UNION: Combines rows from two queries, removing duplicates.
INTERSECT: Returns rows common to both queries.

52. How Do You Fetch Alternate Rows?

SELECT * FROM employees WHERE MOD(employee_id, 2) = 1; -- Odd rows

53. Explain the Use of SQL Joins.

Joins combine rows from two or more tables based on a related column.

54. What is a Composite Key?

A composite key is a primary key made up of two or more columns.

55. What is the Difference Between DELETE and DROP?

DELETE: Removes rows from a table; can be rolled back.
DROP: Removes the table entirely.

56. What is a Stored Procedure?

A stored procedure is a precompiled set of SQL statements.

57. What is a Recursive Query?

A recursive query calls itself until a condition is met, often used for hierarchical data.

58. Explain the Use of SQL CASE Statements.

CASE statements allow conditional logic in queries:

SELECT name, CASE WHEN age < 18 THEN 'Minor' ELSE 'Adult'END AS age_group FROM persons;

59. How Do You Optimize SQL Queries for Performance?

Use indexes.
Avoid **SELECT ***.
Write efficient joins.
Use EXPLAIN PLAN.

60) What is the main difference between SQL and PL/SQL?

SQL is a query language that lets you issue a single query or perform a single insert/update/delete. PL/SQL is Oracle's "Procedural Language" SQL, which allows you to write a full program (loops, variables, etc.) to perform multiple operations like selects/inserts/updates/deletes.

Due to the large and thriving community surrounding PostgreSQL, it is a great asset to learn this amazing technology.

Preparation Tips for SQL Interview

Know the basics of SQL - SELECT, INSERT, UPDATE, DELETE, JOIN, GROUP BY.
Practice advanced queries - subqueries, CTEs, and window functions like ROW_NUMBER().
Database design - Normalization, 1NF, 2NF, 3NF, primary key, foreign keys.
Query optimization: Read execution plans. Optimize queries by indexing and partitioning.
Hands-on Practice: Use websites like LeetCode or SQLZoo for hands-on practice with real-world problems in SQL.
Mock Interviews: Conduct mock interviews in SQL questions with a view toward explanation and optimization of solutions.
Stay Updated: Keep abreast of new features and best practices in SQL toward writing efficient queries.

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