50+ Advanced SQL Interview Questions and Answers to Master SQL

Going through SQL advanced interview questions is essential to prepare yourself for those tough interview scenarios. You can do a self-assessment to know if you possess a deep understanding of database management and complex data operations. 

Advanced questions help identify individuals who can optimize queries, enhance performance, and solve real-world problems efficiently.

These advanced SQL interview questions cover Indexes, data storage optimization, auto-increment fields, unique identifiers, types of variables, recursive procedures, SQL and NoSQL, Joins, and Keys. Additionally, you will also find questions about Dynamic SQL, Stored Procedures, Collation, and more. 

We've compiled this list of advanced SQL interview questions to help you prepare for your upcoming technical interview. Understanding the answers to these questions is essential for acing software engineering and programming interviews.

Most Commonly Asked SQL Advanced Interview Questions

In this section, we’ll look at some popular SQL advanced interview questions and answers. We’ll also look at some additional practice SQL interview questions to help you prepare for your upcoming technical interview.

Q1. What do you Understand About Indexes in SQL?

Indexes in SQL are used to help retrieve data smoothly and efficiently. There are three main types of indexes in SQL - Unique, Clustered, and Non-Clustered. In SQL, tables can have many non-clustered indexes but only one clustered index. Its performance can be significantly enhanced by increasing indexing, but excessive indexing severely degrades the performance at the point of writing.

• Clustered Index: This alters the physical order of records in a table. The table can hold only one clustered index.
• Non-clustered index It doesn't alter the physical order of the data; it rather gives logical order.
• Unique Index: For all values in the column(s) of an indexed column, there must be uniqueness.

Q2. What do you do When Data Gets Overloaded in Your Relational Database?

When relational database data becomes too voluminous, a number of optimizations are possible. Partitioning: Breaking up large tables into smaller, more manageable ones, based on a key.

• Indexing: The optimizer should ensure the addition of proper indexes to optimize queries.
• Caching: Use query results caching to avoid multiple hits to the same database.
• Database Sharding: Dividing the database into smaller, faster databases known as shards. Even offloading data by a soft link can reduce loads by using the facility of external storage.

Q3. What do you Understand About Auto Increment in SQL?

This is one of the most commonly asked advanced SQL interview questions in technical interviews. Auto Increment is a feature that automatically increases numbers when you insert a new record into the table.  This can be done in SQL by leveraging AUTO_INCREMENT for MySQL or IDENTITY property in SQL Server. It helps us mainly in creating primary keys, and we need not provide unique values to insert.

Q4. Which Command is Used to Select a Unique Entry or Record in a Table?

To answer this advanced SQL interview question you can say the command used to select a unique entry from the table is - “distinct.” The command used to select a unique entry from the table is - “distinct.”

The SQL DISTINCT keyword is used to return only distinct (different) values from a column or set of columns.

For example: SELECT DISTINCT column_name FROM table_name;

This function removes duplicates and returns only unique records.

Q5. What are the Two Types of Variables in SQL?

There are mainly two types of variables in SQL - Local and Global. Local variables are those that exist in a single function. Global variables exist in multiple functions and can be present throughout the program.

Local Variables are declared inside a function or procedure and have a very limited scope to that function. Global variables are available throughout a whole session or program and retain the state between functions.

Q6. What do you Understand by Recursive Stored Procedure?

The Recursive Stored Procedure is a common procedure through which developers can use the same code snippet multiple times in a program. Through this procedure, code snippets call themselves until a specific boundary condition is reached.

A recursive stored procedure is a stored procedure that calls itself in a repeated way and continues until a certain termination condition is met. Recursion becomes necessary for those kinds of operations that could be iteratively performed on data with a hierarchical nature, like an organizational chart, or file directory. 

Here is a simple example:

CREATE PROCEDURE Factorial (@n INT)

AS

BEGIN

    IF @n > 1

        RETURN @n * dbo.Factorial(@n - 1);

    ELSE

        RETURN 1;

END

Q7. What is the Main Use of the NVL() Function in SQL?

To answer this advanced SQL interview question you can say that the NVL() function in SQL is essentially used to replace null entries with default values. If the first parameter has a null value, the function returns the next parameter. 

The function NVL() is used to replace a null value with another value. It's often used in Oracle databases. If the first argument is null, NVL() returns the second argument.

Q8. What is the Unique Constraint in SQL?

The Unique Constraint in SQL ensures that every value in a particular record is unique. This means, no two rows can have the same value in this column. An important variation here between the unique and the primary key is that a table can have more than one unique constraint, but just one primary key. It prevents identical or similar values in a column from appearing in two different records.

Q9. What are Some Major Differences Between SQL and NoSQL?

This is a common SQL interview question asked in technical interviews.

• SQL is a language used to manage data in relational databases, whereas NoSQL applies to non-relational databases.
• SQL is a language used to store, manage and retrieve structured data, whereas NoSQL is used for unstructured data.
• NoSQL databases scale horizontally while SQL databases scale vertically.
• SQL databases contain data in the form of tables, while NoSQL databases contain data in the form of documents, records, key-values, or graphs.

Q10. What do you Understand About SQL Sandbox?

SQL Sandbox is an environment within the SQL databases where untrusted untested programs can be tested and run without risking data integrity. There are primarily three types of SQL Sandboxes:

• Sandbox for External Access: Allows access to data only within certain bounds.
• Safe Access Sandbox: Allows unrestricted access.
• Unsafe Access Sandbox: Allows executing external system processes.

Q11. What are the Differences Between CHAR and VARCHAR Data Types in SQL?

Both CHAR and VARCHAR are used to store character string data, but they work in different ways. CHAR is a fixed-length string. If the input is shorter, it pads spaces.

VARCHAR is a variable-length string. It stores only the number of characters given to it. It is more efficient for storing data of known length, whereas VARCHAR is more flexible with data of unpredictable length.

Q12. What is a Trigger in SQL?

SQL Triggers are the kind of stored procedure that automatically fires in response to certain events, either on a table or view, which may include the occurrence of an INSERT, UPDATE, or DELETE. 

Triggers enforce business rules as well as audit changes and maintain data integrity. They can be defined to trigger either before the event occurs or after the event occurs so it helps in logging changes or preventing unauthorized actions. However, overuse of these triggers may degrade the performance of databases.

Q13. What is SQL Injection and How Can You Prevent It?

This is one of the most commonly asked SQL advanced interview questions. Injection is a vulnerability in which attackers insert malicious SQL code into the query to hack the database for unethical access. It mainly happens when the user input is not sanitized in the case of dynamic SQL queries. 

In order to avoid SQL Injection, one can apply prepared statements or parameterized queries that properly handle user inputs by validating and sanitizing data and limiting the database permissions to their minimum requirement.

Q18. What is a Common Table Expression (CTE) in SQL?

A Common Table Expression is a temporary named result set by a user that can be referenced in a SELECT, INSERT, UPDATE, or DELETE statement. CTEs make complex queries less complicated, hence more readable and maintainable.

Recursive CTEs are specially useful for traversing hierarchical data like organizational charts or tree structures. CTEs are quite like subqueries but offer much better structuring of complex query logic, so it might be easier to understand.

Q19. What is Collation in SQL?

SQL collation refers to the way strings are sorted and compared when stored in a database. It specifies the character set, along with the behavior for case sensitivity and accent sensitivity. So different collations help in making the SQL database suitable for the language or region. 

For example, case-insensitive collation considers 'A' as equal to 'a'. Making a proper choice of the correct collation is crucial to ensure that sorting and comparison operations work right, particularly in multi-language applications.

Q20. Explain Query Optimization in SQL.

The query optimization in SQL can help make the queries cost-effective, that is the time taken by the query to execute must be a minimum, and resources also. Optimization would involve indexes, rewriting queries to remove unnecessary subqueries, partitioning large tables, and eliminating full table scans Query Optimizers in SQL Databases generally generate executable plans based on the structure of the query which is way much easier to have higher performance.

The optimizations are imperative because they reduce the execution time; greatly important in large datasets, making our searches/projections with lower response times that are intended for more speed and efficiency in this retrieval of data.

Additional Advanced SQL Interview Questions for Practice‍

In this section, we’ll look at some additional Advanced SQL interview questions that often appear in technical interviews. You can use these questions to practice for the interview.

21. Where is the MyISam table stored in the SQL database?

22. What do you understand about Joins in SQL?

23. What is the fundamental difference between CVarChar2 and Char datatype in SQL?

24. What is the difference between Unique Key and Foreign Key in SQL?

25. What do you understand about Data Integrity, and how do you ensure it in SQL?

26. Explain the differences between the different types of Indexes in SQL

27. Explain the process of Query Optimization in SQL

28. What are the advantages of Normalization in SQL?

29. What is the difference between Truncate and Drop commands in SQL?

30. What do you understand about SQL Injection?

31. What is a Trigger in SQL?

32. Which are the different Operators that are available in SQL?

33. What do you understand about SubQuery in SQL?

34. What are Relationships in SQL? Which are the different types of Relationships in SQL?

35. What do you understand about Clauses in SQL?

36. Which are the ways by which Dynamic SQL can be deployed?

37. Which are the different types of Set Operators available in SQL?

38. What are Scalar and Aggregate functions in SQL?

39. What are the advantages of Stored Procedures in SQL?

40. What do you understand about Collation in SQL?

41.  Write a query to find the average order amount for each customer, but also include each order's deviation from this average. Use the Orders table, which includes OrderID, CustomerID, and OrderAmount.

42. Explain what an indexed view is in SQL Server and provide an example of how to create one. Discuss the benefits and potential drawbacks of using indexed views.ludes OrderID, CustomerID, and OrderAmount.

43. Describe the use of Common Table Expressions (CTEs) in SQL Server. Provide an example of a recursive CTE to generate a hierarchy of categories in a product catalog.

44. Explain how Full-Text Search works in SQL Server and provide an example of how to set it up and use it to search for a term within a table of documents.

45. Explain the difference between INNER JOIN and OUTER JOIN.

46. What is a Self Join? Provide an example.

47. What are Views in SQL, and what are their advantages?

48. How do you optimize JOIN queries in SQL?

49. What is a Cross Join?

50. How do you implement data encryption in SQL Server?

As a developer, your knowledge of SQL is extensively tested in technical interviews. These 50 advanced SQL interview questions will help you prepare for your upcoming software engineering interview.

Learn to Crack Your Next SQL Interview with IK

Exploring advanced SQL interview questions is just the first step toward Data Engineering interviews. You have to enhance your skills and dive deeper into SQL’s concepts. In addition to just preparing for the interviews, you should also look for programs to gain hands-on experience so that you learn through practicals.

Our Data Engineering course has been designed to equip you with the essential skills and knowledge needed to excel in this high-demand field. Our experienced instructors help you learn through hands-on projects and real-world applications so that you learn to handle complex data challenges. 

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FAQs: Advanced SQL Interview Questions

1. Is your Knowledge of SQL Tested in Coding and Design Interviews at FAANG+ companies?

Your knowledge of SQL is primarily tested in systems design interviews at FAANG+ companies. Design interviews are a key part of the technical interview process.

2. What do you Understand About SQL Limit?

The SQL Limit is a feature in SQL that limits the number of entries or records returned from a Query. This helps in the consistency and seamless retrieval of information from the database.

3. What are the Different Types of User-Defined Functions in SQL?

There are three main types of user-defined functions in SQL - Inline Table-Valued Functions, Multi-Statement Valued Functions, and Scalar Functions.

4. What are your Main Responsibilities as an SQL Developer?

As an SQL developer, you’re responsible for how data is managed and stored in relational databases. You’ll work closely with database administrators to ensure that the database is secure, responsive, and functions flawlessly. 

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