Top 30 Most Common Sql Interview Questions For 3 Years Experience You Should Prepare For
Written by
James Miller, Career Coach
Preparing for sql interview questions for 3 years experience requires focusing on both foundational knowledge and practical application. At this stage in your career, interviewers expect you to move beyond basic syntax and demonstrate an understanding of query optimization, data integrity, transaction management, and how SQL fits into larger application architectures. You should be comfortable discussing different database objects like views, stored procedures, and indexes, and explaining their purpose and benefits. This list of 30 common sql interview questions for 3 years experience is designed to help you structure your preparation, ensuring you cover the essential topics that frequently appear in interviews for mid-level SQL roles. Mastery of these concepts will not only help you answer questions confidently but also demonstrate your ability to write efficient, reliable, and maintainable SQL code, crucial for any database-driven project. Dive into these topics, practice writing queries, and be ready to articulate your thought process behind your SQL solutions.
What Are sql interview questions for 3 years experience?
sql interview questions for 3 years experience are designed to assess a candidate's solid understanding of SQL principles and practical database management skills accumulated over several years in a professional environment. They go beyond entry-level syntax checks and delve into more nuanced areas. Expect questions on joins, subqueries, window functions, transaction properties (ACID), database normalization and denormalization, indexing strategies, and performance tuning techniques. You'll also likely encounter questions about database objects like views, stored procedures, and triggers, and their appropriate use cases. Scenario-based questions requiring you to write or debug queries to solve specific problems are common. These questions evaluate your ability to apply SQL knowledge to real-world challenges and your readiness for roles requiring more independent database work.
Why Do Interviewers Ask sql interview questions for 3 years experience?
Interviewers ask sql interview questions for 3 years experience to gauge a candidate's practical competence and depth of understanding beyond basic select statements. After three years, candidates are expected to have encountered various database challenges, including optimizing slow queries, designing database schemas (or at least understanding existing ones), ensuring data consistency, and using advanced SQL features. The questions help determine if the candidate can write efficient, maintainable, and secure SQL code, troubleshoot database issues, and make informed decisions about database design and performance. They also assess familiarity with common database systems and best practices. Demonstrating a strong grasp of these concepts assures interviewers that you can handle more complex tasks and contribute effectively to data-driven projects within their organization, reducing the need for constant supervision on database-related tasks.
Preview List
What is SQL?
What are the different types of SQL commands?
What is a primary key?
What are the differences between INNER JOIN and OUTER JOIN?
What is a correlated subquery?
Difference between WHERE and HAVING clause?
How do you find the Nth highest salary from a table?
What is normalization? Explain normal forms.
Explain the difference between UNION and UNION ALL.
What is the difference between TRUNCATE, DELETE, and DROP?
What is an index? What are its types?
What are constraints and name different types?
What is a view and why use it?
What is a transaction? What are ACID properties?
How do you optimize SQL queries?
What is a stored procedure?
What are triggers?
Difference between COALESCE() and ISNULL()?
What is a foreign key?
Explain denormalization.
What are transaction isolation levels?
How do you handle null values in SQL?
What is the difference between char and varchar?
How to delete duplicate rows from a table?
How do you perform pagination in SQL?
Explain the difference between scalar and table-valued functions.
What is a temp table?
What are set operators in SQL?
How to find employees who do not have a manager?
Difference between ROWNUMBER(), RANK(), and DENSERANK()?
1. What is SQL?
Why you might get asked this:
This foundational question confirms your basic understanding of what SQL is and its primary purpose, ensuring you know you're working with a structured language for databases.
How to answer:
Define SQL, state its full name, and explain its role in managing and manipulating relational databases using standard commands.
Example answer:
SQL stands for Structured Query Language. It's the standard language used to interact with relational database management systems (RDBMS). Its main functions are querying data and managing database schemas and data.
2. What are the different types of SQL commands?
Why you might get asked this:
Tests your knowledge of SQL's different functional categories, showing you understand the various operations performed (defining data, manipulating data, controlling access, managing transactions).
How to answer:
List and briefly explain the main categories: DDL, DML, DCL, and TCL, providing examples of commands within each.
Example answer:
SQL commands are categorized into DDL (Data Definition Language like CREATE, ALTER), DML (Data Manipulation Language like SELECT, INSERT), DCL (Data Control Language like GRANT, REVOKE), and TCL (Transaction Control Language like COMMIT, ROLLBACK).
3. What is a primary key?
Why you might get asked this:
Assesses your understanding of fundamental database design concepts crucial for ensuring data integrity and unique record identification.
How to answer:
Define a primary key as a unique, non-null identifier for a table record and explain its purpose in ensuring uniqueness and serving as a target for foreign keys.
Example answer:
A primary key is a column or set of columns that uniquely identifies each row in a table. It must contain unique values and cannot contain NULL values, ensuring data integrity and unique record identification.
4. What are the differences between INNER JOIN and OUTER JOIN?
Why you might get asked this:
A core question for 3 years of experience, testing your ability to combine data from multiple tables effectively based on matching conditions.
How to answer:
Explain that INNER JOIN returns only rows with matches in both tables, while OUTER JOIN (LEFT, RIGHT, FULL) includes matching rows plus non-matching rows from one or both tables.
Example answer:
INNER JOIN returns only rows where the join condition is met in both tables. OUTER JOIN includes matching rows plus non-matching rows from the left table (LEFT JOIN), right table (RIGHT JOIN), or both (FULL OUTER JOIN).
5. What is a correlated subquery?
Why you might get asked this:
Tests your understanding of more complex subquery types and their execution behavior, relevant for specific filtering scenarios.
How to answer:
Define a correlated subquery as one that depends on the outer query and executes row-by-row, contrasting it with a non-correlated subquery.
Example answer:
A correlated subquery is a subquery that refers to a column from the outer query. It is executed once for each row processed by the outer query, making it dependent on the outer query's evaluation.
6. Difference between WHERE and HAVING clause?
Why you might get asked this:
A classic question to check if you understand the order of SQL operations and how to filter data before vs. after aggregation.
How to answer:
Explain that WHERE filters individual rows before grouping and aggregation, while HAVING filters groups after grouping and aggregation, typically used with aggregate functions.
Example answer:
The WHERE clause is used to filter individual rows before they are grouped. The HAVING clause is used to filter groups created by the GROUP BY clause after aggregation has occurred, often using aggregate functions.
7. How do you find the Nth highest salary from a table?
Why you might get asked this:
A common practical coding question testing your ability to apply ranking or offset techniques to retrieve specific rows based on order.
How to answer:
Describe using techniques like ORDER BY with LIMIT/OFFSET (MySQL/PostgreSQL/SQL Server 2012+) or window functions like ROWNUMBER()/RANK()/DENSERANK() with a subquery or CTE.
Example answer:
You can use ORDER BY salary DESC and then OFFSET (N-1) ROWS FETCH NEXT 1 ROW ONLY (SQL Server/PostgreSQL). Alternatively, use SELECT DISTINCT salary ... ORDER BY salary DESC LIMIT 1 OFFSET (N-1) (MySQL).
8. What is normalization? Explain normal forms.
Why you might get asked this:
Evaluates your grasp of database design principles aimed at reducing redundancy and dependency, crucial for maintainable schemas.
How to answer:
Define normalization and briefly explain the goals (reduce redundancy, improve integrity). Describe 1NF, 2NF, and 3NF simply.
Example answer:
Normalization is a database design technique to reduce data redundancy and improve data integrity. 1NF means atomic values; 2NF requires no partial dependencies; 3NF requires no transitive dependencies on the primary key.
9. Explain the difference between UNION and UNION ALL.
Why you might get asked this:
Checks your knowledge of combining result sets and whether you understand the performance implications of removing duplicates.
How to answer:
State that UNION combines result sets and removes duplicate rows, while UNION ALL combines result sets without removing duplicates. Note UNION ALL is usually faster.
Example answer:
UNION combines the result sets of two or more SELECT statements and removes duplicate rows. UNION ALL combines results but includes all rows from each SELECT statement, including duplicates. UNION ALL is generally faster.
10. What is the difference between TRUNCATE, DELETE, and DROP?
Why you might get asked this:
Tests your understanding of commands that remove data or objects and their fundamental differences regarding logging, rollback, and scope.
How to answer:
Explain DELETE removes rows (logged, rollback possible), TRUNCATE removes all rows quickly (less logged, often not rollbackable), and DROP removes the entire object (table structure and data).
Example answer:
DELETE removes rows, logs each deletion, and can be rolled back. TRUNCATE removes all rows faster, typically without logging individual deletions, and is often not rollbackable. DROP removes the entire table structure and its data permanently.
11. What is an index? What are its types?
Why you might get asked this:
A key topic for performance tuning. Assesses your understanding of how indexes work and the different types available to speed up queries.
How to answer:
Define an index as a database structure that speeds up data retrieval. Mention common types like clustered (physical order) and non-clustered (separate structure pointing to data).
Example answer:
An index is a performance tuning method that allows for faster retrieval of records. It works by creating pointers to data. Common types include clustered indexes (determining physical row order) and non-clustered indexes.
12. What are constraints and name different types?
Why you might get asked this:
Tests your understanding of rules enforced on data columns to maintain database integrity and accuracy.
How to answer:
Define constraints as rules that enforce data integrity. Name common types like PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK.
Example answer:
Constraints are rules applied to columns or tables to limit the type of data inserted, ensuring accuracy and integrity. Types include PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK constraints.
13. What is a view and why use it?
Why you might get asked this:
Evaluates your knowledge of database objects that simplify complex queries, enhance security, and provide abstraction.
How to answer:
Define a view as a virtual table based on a query. Explain its benefits like simplifying complex joins, providing a security layer, and abstracting underlying table structures.
Example answer:
A view is a virtual table derived from the result set of a SQL query. It doesn't store data itself. Views are used to simplify complex queries, restrict access to specific data, and provide data independence.
14. What is a transaction? What are ACID properties?
Why you might get asked this:
Crucial for understanding data consistency and reliability in database operations, especially in multi-user environments.
How to answer:
Define a transaction as a sequence of operations treated as a single unit. Explain the ACID properties: Atomicity, Consistency, Isolation, Durability.
Example answer:
A transaction is a single logical unit of work in a database. ACID properties ensure reliable transaction processing: Atomicity (all or nothing), Consistency (valid state transitions), Isolation (transactions don't interfere), Durability (changes are permanent).
15. How do you optimize SQL queries?
Why you might get asked this:
A core skill for someone with 3 years experience. Tests your practical knowledge of improving query performance.
How to answer:
Discuss key strategies like using indexes, avoiding SELECT *, limiting results with WHERE, using appropriate joins, analyzing execution plans, and minimizing subqueries/loops where possible.
Example answer:
Optimization involves analyzing query execution plans, using appropriate indexes, avoiding SELECT * in favor of specific columns, filtering data early with WHERE, minimizing joins where possible, and structuring queries efficiently.
16. What is a stored procedure?
Why you might get asked this:
Assesses your familiarity with executable code stored within the database, used for reusability, performance, and security.
How to answer:
Define a stored procedure as precompiled SQL code stored in the database. Mention benefits like improved performance, reduced network traffic, reusability, and enhanced security.
Example answer:
A stored procedure is a prepared SQL code block that is stored in the database. It can be executed repeatedly and offers benefits like improved performance (due to compilation), modularity, and enhanced security control.
17. What are triggers?
Why you might get asked this:
Tests your knowledge of automated database actions that respond to specific data modification events.
How to answer:
Define triggers as special stored procedures that execute automatically in response to INSERT, UPDATE, or DELETE events on a table.
Example answer:
Triggers are database objects that automatically execute a specified SQL code block when certain events occur on a table, such as INSERT, UPDATE, or DELETE operations. They are used to enforce complex business rules.
18. Difference between COALESCE() and ISNULL()?
Why you might get asked this:
Checks your knowledge of functions for handling NULL values and understanding platform-specific differences (ISNULL is primarily SQL Server).
How to answer:
Explain that COALESCE returns the first non-NULL expression in a list of arguments and is standard SQL, while ISNULL checks if an expression is NULL and returns a specified replacement, often specific to SQL Server and taking only two arguments.
Example answer:
COALESCE() is a standard SQL function that returns the first non-null value in a list of arguments. ISNULL() is a SQL Server function that checks if the first argument is null and returns the second argument if it is. COALESCE is more versatile.
19. What is a foreign key?
Why you might get asked this:
Fundamental to relational database design, ensures referential integrity between related tables.
How to answer:
Define a foreign key as a column(s) in one table that references the primary key in another table, used to link tables and maintain referential integrity.
Example answer:
A foreign key is a column or set of columns in a table that refers to the primary key in another table. It establishes a link between tables and enforces referential integrity, preventing actions that would destroy links.
20. Explain denormalization.
Why you might get asked this:
Tests your awareness that normalization isn't always the final step and performance sometimes requires adding redundancy back.
How to answer:
Define denormalization as intentionally introducing redundancy into a database schema, typically by combining tables or duplicating data, to improve read performance.
Example answer:
Denormalization is the process of deliberately adding redundancy to a database schema, often by flattening tables or duplicating data. It's done to improve the performance of read operations, sometimes at the cost of write performance and data integrity risk.
21. What are transaction isolation levels?
Why you might get asked this:
Important for understanding how concurrent transactions interact and potential issues like dirty reads, phantom reads, etc.
How to answer:
Explain that isolation levels define how concurrent transactions are isolated from each other. Name common levels like Read Uncommitted, Read Committed, Repeatable Read, and Serializable and mention the issues they prevent.
Example answer:
Transaction isolation levels control the degree to which one transaction must be isolated from the changes made by other transactions. Levels like Read Committed and Serializable prevent issues such as dirty reads, non-repeatable reads, and phantom reads to varying degrees.
22. How do you handle null values in SQL?
Why you might get asked this:
Tests your practical handling of missing or undefined data, a common task.
How to answer:
Explain using IS NULL and IS NOT NULL in WHERE clauses, and functions like COALESCE(), NVL() (Oracle), or ISNULL() to substitute NULLs with other values.
Example answer:
Null values are handled using IS NULL or IS NOT NULL in filtering conditions. Functions like COALESCE(), ISNULL(), or NVL() are used to replace NULLs with a default value for display or calculation purposes.
23. What is the difference between char and varchar?
Why you might get asked this:
A basic but important question about data types, testing your understanding of storage efficiency and behavior.
How to answer:
Explain that CHAR is a fixed-length string padded with spaces, while VARCHAR is a variable-length string that only uses the required storage space (plus a small overhead).
Example answer:
CHAR(n) stores fixed-length strings up to n characters, padded with spaces. VARCHAR(n) stores variable-length strings up to n characters, using only the needed space (plus length info). VARCHAR is usually more space-efficient for varying data lengths.
24. How to delete duplicate rows from a table?
Why you might get asked this:
A practical query writing task demonstrating your ability to identify and remove redundant data using techniques like window functions or self-joins.
How to answer:
Describe using a Common Table Expression (CTE) with ROWNUMBER() partitioned by the columns that define duplicates, then deleting rows where ROWNUMBER is greater than 1.
Example answer:
Use ROWNUMBER() OVER (PARTITION BY column1, column2 ORDER BY anycolumn). Wrap this in a CTE and delete rows where the row number assigned to the duplicate group is greater than 1.
25. How do you perform pagination in SQL?
Why you might get asked this:
Tests your ability to retrieve a specific subset of rows for display purposes, a common web/application requirement.
How to answer:
Explain using ORDER BY combined with OFFSET and FETCH NEXT (SQL Server 2012+) or LIMIT and OFFSET (MySQL/PostgreSQL) to get a specific "page" of results.
Example answer:
Pagination is done using ORDER BY to define the sort order, then OFFSET x ROWS FETCH NEXT y ROWS ONLY (SQL Server/PostgreSQL) or LIMIT y OFFSET x (MySQL), where x is the number of rows to skip and y is the page size.
26. Explain the difference between scalar and table-valued functions.
Why you might get asked this:
Tests your knowledge of different types of user-defined functions and what they return.
How to answer:
Explain that scalar functions return a single value (like a string or number), while table-valued functions return a result set that can be used like a table in queries.
Example answer:
A scalar function returns a single value, such as an integer, string, or date. A table-valued function returns a result set, which can be selected from or joined with, similar to a table or view.
27. What is a temp table?
Why you might get asked this:
Evaluates your understanding of temporary data storage structures useful for complex queries or procedures.
How to answer:
Define a temporary table as a short-lived table stored in memory or the database's temp space, accessible only within the current session or batch, automatically dropped when the session ends.
Example answer:
A temporary table is a short-term storage structure used to hold intermediate results within a single database session or transaction. They are automatically dropped when the session or connection ends.
28. What are set operators in SQL?
Why you might get asked this:
Tests your ability to combine the results of multiple SELECT statements in different ways.
How to answer:
List and briefly explain set operators: UNION (combines, removes duplicates), UNION ALL (combines, keeps duplicates), INTERSECT (returns common rows), and EXCEPT/MINUS (returns rows in the first query not in the second).
Example answer:
Set operators combine the results of two or more SELECT statements. Common ones are UNION (combines, distinct rows), UNION ALL (combines, all rows), INTERSECT (common rows), and EXCEPT/MINUS (rows in first query not in second).
29. How to find employees who do not have a manager?
Why you might get asked this:
A common scenario-based question testing your ability to handle relationships and NULL values or use anti-join techniques.
How to answer:
Describe using a simple WHERE managerid IS NULL clause, assuming a self-referencing managerid column that is NULL for top-level employees. Or, if managers are in another table, use a LEFT JOIN and check for NULLs on the manager table's key.
Example answer:
Assuming the employees table has a managerid column that is NULL for employees without a manager: SELECT * FROM employees WHERE managerid IS NULL;
30. Difference between ROWNUMBER(), RANK(), and DENSERANK()?
Why you might get asked this:
Tests your understanding of window functions for ranking, a key feature for advanced data analysis and retrieval scenarios.
How to answer:
Explain that ROWNUMBER assigns a unique sequential number to each row within a partition, RANK assigns ranks with gaps for ties, and DENSERANK assigns ranks without gaps for ties.
Example answer:
ROWNUMBER() gives sequential numbers. RANK() gives ranks with gaps for ties (e.g., 1, 1, 3). DENSERANK() gives ranks without gaps for ties (e.g., 1, 1, 2).
Other Tips to Prepare for a sql interview questions for 3 years experience
Beyond mastering these sql interview questions for 3 years experience, practical preparation is key. Write actual SQL queries for common scenarios: finding duplicates, calculating running totals, pivoting data, handling complex joins, and implementing pagination. Practice explaining your query logic step-by-step. "The only way to learn SQL is to write SQL," says database expert Joe Celko. Work through online coding platforms focusing on SQL problems. Be ready to discuss your experience with specific database systems (like SQL Server, PostgreSQL, MySQL, Oracle) and their nuances. Understand execution plans – they reveal how your query runs and where bottlenecks might be.
Consider using an interview preparation tool like Verve AI Interview Copilot. Verve Copilot can help you practice answering sql interview questions for 3 years experience in a simulated environment, providing feedback on your clarity and completeness. Using Verve AI Interview Copilot can boost your confidence by letting you rehearse explanations for complex topics. Don't just memorize answers; understand the underlying concepts. As another expert often says, "Understanding the 'why' behind the SQL is as important as knowing the 'how'." Prepare behavioral questions as well, thinking about times you've used SQL to solve a problem, optimized a query, or dealt with a data integrity issue. Verve AI Interview Copilot can assist with structuring your answers to behavioral questions, integrating your technical skills effectively. Check out https://vervecopilot.com to see how Verve AI Interview Copilot can refine your approach to sql interview questions for 3 years experience and other technical topics.
Frequently Asked Questions
Q1: How much SQL coding should I expect?
A1: Expect both theoretical questions and practical coding challenges to write or debug queries.
Q2: Should I prepare for a specific SQL dialect?
A2: Focus on standard SQL but be ready to mention your experience with dialects like T-SQL (SQL Server) or PL/SQL (Oracle).
Q3: What non-query topics are important?
A3: Database design (normalization/denormalization), indexing, transactions (ACID, isolation levels), and database objects (views, procs).
Q4: How do I explain complex queries?
A4: Break it down: explain the goal, then each clause (FROM, JOIN, WHERE, GROUP BY, HAVING, SELECT, ORDER BY) and why you used it.
Q5: Is knowing execution plans necessary?
A5: Yes, for 3 years experience, understanding how to read and interpret execution plans for optimization is highly valued.
Q6: How can I practice coding specific queries?
A6: Use online SQL platforms, set up a local database, or use practice problems focused on common interview scenarios.
