Top 30 Most Common Most Asked Sql Interview Questions You Should Prepare For

Landing a role that involves data or database management often requires demonstrating strong SQL proficiency. SQL, or Structured Query Language, is the universal language for interacting with relational databases. Excelling in a technical interview means being prepared for common queries, concepts, and scenarios. This guide breaks down 30 of the most frequently encountered most asked sql interview questions, providing clear, concise answers to help you confidently navigate your next interview. Preparing thoroughly for these core concepts will not only boost your confidence but also showcase your fundamental understanding of database principles and practical data manipulation skills, crucial elements recruiters look for. These most asked sql interview questions cover everything from basic syntax and data types to complex joins, indexing, and performance optimization techniques. Mastering these topics is key to unlocking your potential in data-centric roles. Whether you're a beginner or looking to refresh your knowledge, this compilation of most asked sql interview questions is designed to be your go-to resource for interview preparation.

What Are most asked sql interview questions?

most asked sql interview questions are those fundamental and frequently recurring questions about Structured Query Language that candidates are asked during technical interviews for roles like database administrators, data analysts, data scientists, software engineers, and business intelligence developers. These questions test a candidate's understanding of SQL syntax, database concepts, data manipulation, querying, optimization, and design principles. They cover core commands (SELECT, INSERT, UPDATE, DELETE), different types of joins, subqueries, window functions, constraints, indexing, normalization, and transaction management. Interviewers use these questions to gauge a candidate's practical skills, problem-solving abilities using SQL, and theoretical knowledge of how databases work efficiently. Preparing for these specific most asked sql interview questions is essential for anyone applying for positions that require database interaction.

Why Do Interviewers Ask most asked sql interview questions?

Interviewers ask most asked sql interview questions to evaluate a candidate's foundational knowledge and practical ability to work with databases. SQL is a critical skill across many tech roles, and these questions help determine if you can effectively retrieve, manage, and manipulate data. They assess your understanding of relational database principles, ensuring you know how data is structured and related. Specific most asked sql interview questions reveal your problem-solving approach when dealing with data challenges, how you optimize queries for performance, and your grasp of data integrity and security through constraints and permissions. Your ability to answer these most asked sql interview questions demonstrates readiness for real-world tasks involving database interaction and management.

1. What is SQL?

2. What are the different types of SQL statements?

3. What is a database?

4. What are SQL dialects?

5. What are SQL constraints?

6. What are SQL indexes and why are they important?

7. What is normalization in database design?

8. What is denormalization?

9. How do you insert, update, and delete data in a database?

10. What is the difference between TRUNCATE and DELETE?

11. How do you write a basic SELECT query?

12. What are the different types of joins?

13. What is a subquery?

14. What is a Common Table Expression (CTE)?

15. What are window functions?

16. How do you use GROUP BY and HAVING?

17. What are the common SQL data types?

18. How do you manipulate dates in SQL?

19. What is the main advantage of a clustered index over a non-clustered index?

20. How do you optimize SQL queries?

21. Write a query to select all records with "bar" in their name.

22. How do you fetch unique values from a field?

23. What is the difference between a view and a table?

24. How do you create a view?

25. What is the purpose of a PRIMARY KEY constraint?

26. What is the role of a FOREIGN KEY constraint?

27. What is the ORDER BY statement used for?

28. How does the ORDER BY statement work if no order is specified?

29. What languages are supported by PostgreSQL?

30. How do you restore a PostgreSQL backup created using pg_dump?

31. Preview List

1. What is SQL?

Why you might get asked this:

Tests your fundamental understanding of what SQL is and its primary purpose in interacting with relational databases. A foundational most asked sql interview question.

How to answer:

Define SQL, mention it's a language for managing relational databases, and list its core functions (querying, updating, manipulating data).

Example answer:

SQL stands for Structured Query Language. It's the standard language used to communicate with and manage relational databases. It allows users to retrieve, insert, update, and delete data, as well as manage database schema and access.

2. What are the different types of SQL statements?

Why you might get asked this:

Evaluates your knowledge of SQL command categories, showing you understand the different operations performed in databases.

How to answer:

List and briefly describe the main categories: DDL, DML, DCL, and DQL, noting their purpose.

Example answer:

SQL statements are categorized into: DDL (Data Definition Language - CREATE, ALTER, DROP), DML (Data Manipulation Language - INSERT, UPDATE, DELETE), DCL (Data Control Language - GRANT, REVOKE), and DQL (Data Query Language - SELECT).

3. What is a database?

Why you might get asked this:

Checks your basic understanding of the core concept you'll be working with. It's a foundational most asked sql interview question.

How to answer:

Define a database as an organized collection of data stored and accessed electronically, structured for efficient retrieval and manipulation.

Example answer:

A database is a structured collection of data, typically stored electronically in a computer system. It's organized in a way that makes it easy to manage, update, retrieve, and query data. Relational databases use tables with rows and columns.

4. What are SQL dialects?

Why you might get asked this:

Shows awareness that while SQL is standard, implementations vary across different database systems.

How to answer:

Explain that dialects are variations or extensions of standard SQL used by specific database systems like MySQL, PostgreSQL, or Oracle.

Example answer:

SQL dialects are different versions or extensions of the standard SQL language used by various database management systems (DBMS). While core syntax is similar, functions, features, and specific commands can differ between systems like MySQL, PostgreSQL, SQL Server, or Oracle.

5. What are SQL constraints?

Why you might get asked this:

Tests your understanding of rules that maintain data integrity and reliability within tables.

How to answer:

Define constraints as rules applied to columns or tables to limit data types and ensure data accuracy and integrity. Give examples.

Example answer:

SQL constraints are rules enforced on data columns to limit the type of data that can be inserted or updated. They maintain data integrity and accuracy. Common examples include PRIMARY KEY, FOREIGN KEY, NOT NULL, UNIQUE, CHECK, and DEFAULT.

6. What are SQL indexes and why are they important?

Why you might get asked this:

Assesses your knowledge of performance optimization techniques in databases. A key most asked sql interview question for performance.

How to answer:

Explain indexes as data structures that speed up data retrieval and why this is crucial for query performance.

Example answer:

SQL indexes are special lookup tables that the database search engine uses to speed up data retrieval. They work much like an index in a book, allowing the database to quickly find data without scanning the entire table. They are crucial for efficient query performance.

7. What is normalization in database design?

Why you might get asked this:

Evaluates your understanding of database design principles focused on reducing redundancy and improving data integrity.

How to answer:

Define normalization as organizing database tables to minimize redundancy and dependency, typically through breaking down large tables.

Example answer:

Normalization is a database design technique that reduces data redundancy and improves data integrity by organizing columns and tables. It involves breaking down large tables into smaller, more manageable ones and defining relationships between them, following principles like First, Second, and Third Normal Forms.

8. What is denormalization?

Why you might get asked this:

Checks your awareness of trade-offs in database design, sometimes prioritizing read performance over strict normalization.

How to answer:

Explain denormalization as intentionally adding redundancy to a normalized database to improve query performance, often by adding derived or redundant data.

Example answer:

Denormalization is the process of strategically adding redundant data to a normalized database, often by combining tables or adding summary data. This is done to improve query performance for read-heavy applications, sacrificing some data integrity benefits of strict normalization.

9. How do you insert, update, and delete data in a database?

Why you might get asked this:

Tests your practical knowledge of fundamental DML commands, core to daily database tasks.

How to answer:

Provide the basic syntax for INSERT, UPDATE, and DELETE statements, including necessary clauses.

Example answer:

INSERT adds new rows: INSERT INTO tablename (col1, col2) VALUES (val1, val2);. UPDATE modifies existing data: UPDATE tablename SET col1 = newval WHERE condition;. DELETE removes rows: DELETE FROM tablename WHERE condition;.

10. What is the difference between TRUNCATE and DELETE?

Why you might get asked this:

Assesses your understanding of different methods for removing data and their performance/logging implications.

How to answer:

Explain that TRUNCATE removes all rows quickly without logging, while DELETE removes rows based on a condition and logs changes.

Example answer:

DELETE is a DML command that removes rows based on a condition and logs each row deletion, allowing rollback. TRUNCATE is a DDL command that removes all rows by deallocating the table pages, is faster, doesn't log individual rows, and cannot be easily rolled back.

11. How do you write a basic SELECT query?

Why you might get asked this:

Confirms your ability to perform the most common database operation: data retrieval. A fundamental most asked sql interview question.

How to answer:

Provide the standard syntax for selecting specific columns or all columns from a table, optionally with a WHERE clause.

Example answer:

A basic SELECT query retrieves data from a table. The syntax is SELECT column1, column2 FROM tablename;. To retrieve all columns: SELECT * FROM tablename;. You can add WHERE condition; to filter rows.

12. What are the different types of joins?

Why you might get asked this:

Crucial for data analysis, joins are fundamental to combining data from multiple tables. A very common most asked sql interview question.

How to answer:

List and briefly describe the main join types: INNER, LEFT (OUTER), RIGHT (OUTER), and FULL (OUTER), explaining which rows are returned for each.

Example answer:

The main join types are: INNER JOIN (matching rows), LEFT JOIN (all left rows + matching right), RIGHT JOIN (all right rows + matching left), and FULL OUTER JOIN (all rows when there's a match in either table).

13. What is a subquery?

Why you might get asked this:

Tests your ability to write more complex queries involving nested logic and multiple steps.

How to answer:

Define a subquery as a query nested inside another SQL query, explaining where it can be used (WHERE, FROM, SELECT clauses).

Example answer:

A subquery (or inner query/nested query) is a query embedded inside another SQL query. It can be used in the WHERE clause to filter results, in the FROM clause as a derived table, or in the SELECT clause to return a single value.

14. What is a Common Table Expression (CTE)?

Why you might get asked this:

Evaluates your knowledge of structuring complex queries for readability and reusability within a single statement.

How to answer:

Define a CTE as a temporary named result set defined within a single statement that can be referenced by subsequent subqueries or other CTEs.

Example answer:

A Common Table Expression (CTE) is a temporary, named result set that you can reference within a single SELECT, INSERT, UPDATE, or DELETE statement. They improve query readability and make complex hierarchical queries easier to manage. Defined using WITH.

15. What are window functions?

Why you might get asked this:

Tests your ability to perform calculations across a set of table rows related to the current row without grouping. A more advanced most asked sql interview question.

How to answer:

Explain that window functions perform calculations across a "window" or set of rows related to the current row, often with OVER clause. Give examples like RANK, ROW_NUMBER.

Example answer:

Window functions perform calculations across a set of table rows that are related to the current row, unlike aggregate functions which group rows. They use the OVER() clause and are useful for ranking, numbering rows, or calculating running totals without collapsing rows. Examples: ROW_NUMBER(), RANK().

16. How do you use GROUP BY and HAVING?

Why you might get asked this:

Assesses your ability to aggregate data and filter those aggregated results.

How to answer:

Explain that GROUP BY groups rows based on columns for aggregate functions, and HAVING filters those grouped results based on aggregate conditions.

Example answer:

GROUP BY groups rows with the same values into summary rows (like "find the number of customers in each city"). HAVING filters these groups based on a condition that typically involves an aggregate function (like "show only cities with more than 100 customers").

17. What are the common SQL data types?

Why you might get asked this:

Checks your basic understanding of how different types of information are stored in database columns.

How to answer:

List common data types used for numbers, text, dates, and mention variations between database systems.

Example answer:

Common SQL data types include: INTEGER (whole numbers), VARCHAR (variable-length strings), CHAR (fixed-length strings), TEXT (long strings), DATE (date values), TIME (time values), TIMESTAMP (date and time), BOOLEAN (true/false).

18. How do you manipulate dates in SQL?

Why you might get asked this:

Tests your practical skills with date/time operations, common in many data tasks.

How to answer:

Mention that SQL provides functions for date extraction, formatting, calculation, and retrieving current dates/times, though syntax varies by DBMS.

Example answer:

SQL provides functions for date manipulation. Examples include extracting parts (YEAR, MONTH), formatting dates (DATEFORMAT in MySQL, TOCHAR in PostgreSQL/Oracle), performing calculations (adding/subtracting intervals), and getting the current date/time (NOW(), CURRENT_DATE).

19. What is the main advantage of a clustered index over a non-clustered index?

Why you might get asked this:

Evaluates your deeper understanding of indexing and its impact on physical data storage and retrieval performance. A key most asked sql interview question for optimization.

How to answer:

Explain that a clustered index dictates the physical order of data rows in the table, making range scans or sequential access very fast.

Example answer:

A clustered index determines the physical storage order of the data rows in the table itself. There can only be one per table. Its main advantage is highly efficient retrieval for data that is accessed sequentially or within a range, as related rows are physically stored together.

20. How do you optimize SQL queries?

Why you might get asked this:

A critical question assessing your ability to write efficient code that performs well on large datasets. A high-level most asked sql interview question.

How to answer:

Discuss using indexes, avoiding SELECT *, minimizing subqueries/joins where possible, using EXPLAIN or EXPLAIN PLAN, and proper database design.

Example answer:

Query optimization involves several techniques: ensuring appropriate indexes exist, avoiding SELECT * by selecting only needed columns, using joins efficiently, minimizing complex subqueries, analyzing the query execution plan (EXPLAIN), and ensuring database schema is well-designed.

21. Write a query to select all records with "bar" in their name.

Why you might get asked this:

A practical test of using the LIKE operator for pattern matching.

How to answer:

Use the SELECT statement with a WHERE clause and the LIKE operator with wildcards (%).

Example answer:

SELECT *
FROM your_table_name
WHERE name LIKE '%bar%';

To select all records where the 'name' column contains "bar":

22. How do you fetch unique values from a field?

Why you might get asked this:

Tests your knowledge of basic data de-duplication using SQL.

How to answer:

Explain the use of the DISTINCT keyword in the SELECT statement.

Example answer:

SELECT DISTINCT column_name
FROM your_table_name;

Use the DISTINCT keyword before the column name in your SELECT statement:

23. What is the difference between a view and a table?

Why you might get asked this:

Assesses your understanding of database objects and how they store/represent data differently.

How to answer:

Explain that a table physically stores data, while a view is a virtual table representing the result set of a stored query, accessing data from underlying tables.

Example answer:

A table is a base object that physically stores data in rows and columns. A view, on the other hand, is a virtual table or a stored query result set. It doesn't store data itself but retrieves it from the underlying tables whenever it's accessed.

24. How do you create a view?

Why you might get asked this:

Tests your knowledge of DDL commands for creating virtual database objects.

How to answer:

Provide the basic CREATE VIEW syntax, showing it wraps a SELECT statement.

Example answer:

CREATE VIEW view_name AS
SELECT column1, column2
FROM table_name
WHERE condition;

You create a view using the CREATE VIEW statement followed by the view name and the AS keyword, which contains the SELECT query defining the view's data:

25. What is the purpose of a PRIMARY KEY constraint?

Why you might get asked this:

Fundamental database design concept. Tests your understanding of how uniqueness and identification are enforced. A core most asked sql interview question.

How to answer:

Explain that a primary key uniquely identifies each row in a table and cannot contain NULL values.

Example answer:

A PRIMARY KEY constraint uniquely identifies each record in a table. It must contain unique values for each row and cannot contain NULL values. A table can have only one primary key, which can consist of one or more columns.

26. What is the role of a FOREIGN KEY constraint?

Why you might get asked this:

Fundamental database design concept. Tests your understanding of how relationships between tables are enforced and data consistency maintained.

How to answer:

Explain that a foreign key establishes a link between two tables by referencing the primary key of another table, ensuring referential integrity.

Example answer:

A FOREIGN KEY constraint is used to link two tables together. It's a field (or fields) in one table that refers to the PRIMARY KEY in another table. Its role is to ensure referential integrity, preventing actions that would destroy links between tables.

27. What is the ORDER BY statement used for?

Why you might get asked this:

Tests your ability to sort query results, a common reporting and data presentation requirement.

How to answer:

Explain that ORDER BY is used to sort the result set of a query based on one or more columns.

Example answer:

The ORDER BY statement is used in SQL queries to sort the result set in ascending (ASC) or descending (DESC) order based on one or more columns. It's commonly used to present data in a structured sequence.

28. How does the ORDER BY statement work if no order is specified?

Why you might get asked this:

Tests your knowledge of the default behavior of the ORDER BY clause.

How to answer:

State that the default sorting order is ascending (ASC) if ASC or DESC is not explicitly specified.

Example answer:

If neither ASC (ascending) nor DESC (descending) is specified with the ORDER BY statement, the default sorting order is typically ascending (ASC). The database system will sort the results from lowest to highest value.

29. What languages are supported by PostgreSQL?

Why you might get asked this:

Specific knowledge about a popular database system, assessing familiarity beyond generic SQL.

How to answer:

Mention SQL itself, PL/pgSQL, and the ability to use various procedural languages through extensions.

Example answer:

PostgreSQL natively supports SQL and its own procedural language, PL/pgSQL. It also supports various other procedural languages via extensions, including PL/Python, PL/Java, PL/Perl, and PL/TCL, allowing for more complex logic implementation.

30. How do you restore a PostgreSQL backup created using pg_dump?

Why you might get asked this:

Tests practical database administration skills related to data recovery in a specific common system.

How to answer:

Provide the command-line utility used for restoring plain-text or archive format backups created by pg_dump.

Example answer:

You restore a PostgreSQL backup created with pgdump using the pgrestore command for custom or directory format backups. For plain-text SQL dumps, you can use psql -d databasename -f backupfile.sql. pgrestore -C -d databasename backup_file is common for archive formats.

Other Tips to Prepare for a most asked sql interview questions

Preparing for most asked sql interview questions involves more than just memorizing syntax; it requires understanding the underlying concepts and being able to apply them. Practice writing queries for various scenarios, including complex joins, aggregations, and subqueries. Work with sample databases to get hands-on experience. Consider using a tool like Verve AI Interview Copilot at https://vervecopilot.com to simulate interview scenarios and get feedback on your responses to most asked sql interview questions. As database expert Joe Celko says, "SQL is not a programming language...it's a set-based declarative language." Understanding this perspective can shift how you approach problems.

Another crucial tip for most asked sql interview questions is to understand the 'Why' behind different techniques, like indexing or normalization, not just the 'How'. Be ready to discuss real-world use cases. Mock interviews, particularly using platforms like Verve AI Interview Copilot, can significantly improve your delivery and confidence in answering most asked sql interview questions. They provide a low-pressure environment to practice explaining your logic. Remember, clarity and conciseness are key. As another expert notes, "The best query is often the simplest one that gets the job done efficiently." Use Verve AI Interview Copilot to refine your explanations for those tricky most asked sql interview questions.

Frequently Asked Questions

Q1: What is the difference between WHERE and HAVING clauses?
A1: WHERE filters individual rows before grouping; HAVING filters groups after the GROUP BY clause based on aggregate conditions.

Q2: What is the difference between UNION and UNION ALL?
A2: UNION combines result sets and removes duplicates; UNION ALL combines result sets including duplicates.

Q3: What is a self join?
A3: A self join is a join where a table is joined with itself, useful for comparing rows within the same table.

Q4: What is an alias in SQL?
A4: An alias is a temporary name given to a table or a column within a SQL query, improving readability.

Q5: What is a transaction in SQL?
A5: A transaction is a single unit of work; a sequence of operations performed as a single logical unit, either completing fully or failing fully.