Demonstrating SQL mastery during your interview can be the one skill that opens doors to diverse tech roles

You've applied for one of the many jobs that require an understanding of SQL and got an interview. Congratulations! Now, you have some major work ahead of you to get ready for that interview. 

You probably already know that SQL is the backbone of data management and is one of those skills that's highly coveted by employers in the tech world. In this article, you'll learn how to answer 27 SQL interview questions for positions ranging from Database Administrator to QA Tester. What's more, the great answers that we'll provide will show you how to discuss the concepts in language that doesn't rely on a bunch of jargon. 

Grab a pen and paper (or open Notepad), get ready to take some notes, and let's go!

The basics of SQL

At its core, SQL, or Structured Query Language, is a way to manage data. By putting in a series of commands or standard syntax, you can communicate, interact with, update, and maintain information in relational database management systems (RDBMS). 

Of course, businesses run on databases. Having a database is a great way to store and organize a truckload of information. When you can prove to hiring managers that you've mastered SQL, you showcase that you can help them with one of the most critical components of running their business – selecting, inserting, updating, and deleting relevant information. 

Two main types of databases

Depending on the nature of the data and the requirements of the applications being used, you will run into one of two types of databases – one that is structured or one that is flexible and scalable. 

  1. Relational databases are the structured ones that organize data into tables with rows and columns – much like a spreadsheet – where there are specific datasets related to particular fields that help the user to identify information. Examples of relational databases include MySQL, Oracle, and Microsoft SQL Server.

  2. Non-relational databases, also known as NoSQL databases, are more flexible. These are the databases that handle big volumes of unstructured or semi-structured data and are used to prioritize performance because they let you retrieve and store things quickly. Examples of NoSQL databases include MongoDB, Cassandra, Redis, and Neo4j. 

While most of these basics, plus some less-than-basic details, are probably quite prevalent in your resume, understanding them lays the groundwork for your success during the interview.

Common SQL interview formats

Most people who get an offer for an interview understand that they'll likely be going to some location to sit across the desk from a hiring manager to answer some questions. SQL interviews are a bit different. You could find yourself having a one-on-one interview with a hiring manager, performing an assessment (whether it's an on-site or take-home assessment), or you could be sitting in front of a panel of people who will assess your candidacy. 

HINT: It's because of the panel interview that you want to be able to describe your skills in lay terms, as some members of the panel may not be tech-savvy. 

Your one-on-one interview

This is the type of interview format that everyone immediately thinks of when they get an invitation to interview for a job. It's where you go to a company's office and sit down with the hiring manager to have a conversation. 

EXPERT TIP: It is pretty critical that your interview be an actual conversation, rather than a question-and-answer session. To help you along with that, check out some of these “Great Questions to Ask During an Interview.” They may also throw in some fun interview questions.

Since this type of interview is highly personalized and in-depth, you can expect to have to go through some conceptual understanding of SQL interview questions. There may even be some scenario-based questions, and there are definitely going to be some behavioral questions to see how you handle challenges. 

The one-on-one interview is a great place to showcase not only your technical expertise but also your ability to communicate effectively. Let's face it, one of the components of your job will probably be to articulate complex information to both technical and non-technical audiences. 

Your SQL panel interview

This is a bit different than a one-on-one interview, because instead of talking with a single hiring manager, you'll be faced with discussing your skills, knowledge, and career achievements with two or more people at the same time. 

The most difficult part about this type of interview is that you can expect to encounter people from different roles within the organization. This means that the people assessing your skills may not know SQL from a hole in the ground. So, practice explaining things in a way that will resonate with a diverse audience. 

The reason companies like panel interviews is that varying perspectives can be leveraged, to make sure that you not only fit within the role but also within the company culture. You'll come across a wide range of questions regarding your skills with SQL and how you handle different situations. 

Things panel interviewers are trying to determine include:

  • Your technical knowledge

  • How well you can communicate with people 

  • Whether you can collaborate in a team setting

  • If you are good at overcoming SQL-related challenges within a group

  • Your leadership skills

  • Your level of curiosity and how well you adapt to change

  • How you do in situations where you have to think strategically

SQL assessment interviews

The hiring manager may decide that they want you to perform a test or complete an assignment to prove your skills. It gives you a great opportunity to show off your skills and problem-solving abilities in a hands-on setting. 

There's no hard and fast rule as to whether these assessments will take place during the first round of interviews or if they'll be set aside for a second round. So, you have to be prepared for an assessment at any time. Some companies even give you assessments to take home or perform them as a pre-screening tool. The bottom line is – be ready. 

Some types of exercises you may encounter include:

  • Hands-on coding exercises where you write SQL scripts to solve problems

  • Database design tasks where you could be doing anything from normalizing data and defining relationships to restructuring tables

  • Optimizing SQL queries for performance

  • Real-world scenarios that allow you to solve business-related challenges and offer practical solutions

You'll want to be vigilant when it comes to SQL assessment interviews – you don't want to unknowingly work for free during an interview. This is where a bit of research into the company comes in handy. If you know what types of projects they have in the queue, you can better protect yourself. Here are a few things to watch out for:

  • Be sure the tasks are clearly defined and are directly related to assessing your skills

  • Be cognizant of how long the assessment will take you – if you're applying for an entry-level position, the assessment may take an hour; if you're applying for a senior role, the assessment may take several hours

  • Ask questions about the purpose of the task and be discerning about how the interviewer answers – you can usually tell if someone is trying to skirt around an issue

  • Don't be afraid to set some boundaries and communicate concerns if the task seems to go beyond what would be required to assess your skills

  • The big one: trust your instincts. If your gut is telling you something isn't right, then listen

EXPERT TIP: All interviews are a two-way evaluation process. You should be evaluating the company and how they interact with you just as much as they're evaluating you. 

How to answer SQL interview questions

Who's ready to answer some questions?! Here are some great answers to 27 SQL interview questions. We've broken the questions down into sections to make it easier to navigate them. You'll find answers to

  • Basic SQL questions

  • SQL Query questions

  • Database design questions

  • Advanced SQL concept questions

Basic SQL interview questions

1. What is SQL, and why is it important for databases?

“SQL, which stands for structured query language, is a standard language that's used to communicate with databases. By using this language, the user can communicate, interact with, and manage the information in the database in a way that makes data management efficient and effective.”

2. What is the difference between SQL and MySQL?

“The basic difference is that SQL is the programming language, and MySQL is the system that implements the capabilities of SQL.”

EXPERT TIP: You don't have to answer each question with a lengthy monologue. Short answers are okay as long as they provide a full explanation of what you want to say.

3. What is the difference between a primary key and a foreign key?

“These keys are a way to establish relationships between tables of information. The primary key is a unique identifier within a database that gives each record an identity. On the other hand, the foreign key is a field in a table that establishes a connection to the primary key in another table.”

4. What are indexes in SQL, and why are they used?

“Indexes in SQL are data structures that speed up data retrieval and serve as a roadmap for quick reference – sort of like an index in a book. Indexes are used to improve query performance, reducing the amount of time it takes to scan the data, but you have to be careful because indexes can impact write operations and take up storage space.” 

5. Describe normalization in databases

“Normalization is all about improving data integrity and eliminating redundancy. It's a way to organize data, breaking it down into smaller, interconnected tables and establishing relationships. Normalization usually involves creating tables up to the third normal form (3NF) and is critical in maintaining a well-structured database.”

6. What are SQL transactions, and why are they important?

“SQL transactions are groups of SQL statements that are treated as a single unit of work and follow ACID (atomicity, consistency, isolation, and durability) principles to guarantee reliability. The key to maintaining the integrity of the data is that all changes within a transaction are applied, or none are applied at all. When SQL transactions are properly used, databases don't get left in an unstable state.”

SQL query interview questions

You'll notice that some of these questions involve you writing out some code – those are the types of questions you can expect in an assessment interview.

7. Write a basic SELECT statement to retrieve data from a table

“A basic SELECT statement in SQL is used to retrieve data and would look like this:

SELECT column1, column2

FROM tableName;

Here, you would be able to view selected columns from the specified table. Of course, we'd specify the names of the columns and the name of the table in real life.”

8. Explain the difference between WHERE and HAVING clauses.

“The WHERE clause allows the SQL user to filter rows before they are grouped. WHERE is used as part of a SELECT statement. Here's what it would look like:

SELECT column1, …

FROM tableName;

WHERE condition;

GROUP BY column name

You can also use a WHERE clause if you want to update or delete information. 

Conversely, HAVING is used after the information is grouped, so that aggregated results can be filtered. HAVING was added to SQL because WHERE cannot be used with aggregated data. Here's what a HAVING clause would look like:

SELECT CustomerID, Country

FROM tableName;

GROUP BY Country

HAVING COUNT >5;

Let's say you had a table containing customers, their contact information, and their country of origin. This SQL statement would list the number of customers in each country, but it would only include countries with more than five customers.”

Source: https://www.w3schools.com/sql/sql_having.asp 

9. How do you JOIN tables in SQL? Provide an example.

“JOIN is cool because it lets you combine rows from several tables, based on the relationship between the information. There are several JOIN clauses, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN. The statement you'd write would involve choosing the information you want to JOIN and from where. This is what it looks like:

SELECT orders.order_id, customers.customer_name

FROM orders

INNER JOIN customers ON orders.customer_id = customers.customer_id;

With this query, you'd be able to get the order ID and customer name by matching the 'customer_id' column in the 'orders' and 'customer' tables.”

10. Describe the difference between INNER JOIN and OUTER JOIN

“The biggest difference is how the queries handle unmatched rows of information. INNER JOIN chooses rows with matching values in specific columns, while OUTER JOIN includes rows from one table and matches them with rows from the other. With OUTER JOIN, it doesn't matter if you're using LEFT, RIGHT, or FULL; you'll get the information from all rows in one table to match with rows from the other.”

11. What is the purpose of GROUP BY in SQL?

“As you may guess, the GROUP BY clause in SQL allows you to sort and group rows of information together based on them having the same value. It's a great tool to use in combination with aggregate functions to perform calculations on a specific set of data within the database.”

12. Write a query to find the second-highest salary in a table

“Sure! You'd want to use the MAX function to identify the maximum salary, then write a statement that identifies the second-highest salary by nesting the MAX into the query. Here's what it would look like:

SELECT MAX (salary)

FROM tableName;

This first step provides the output for who makes the most money. Then, you'd query:

SELECT MAX (salary)

FROM tableName;

WHERE SALARY < (SELECT MAX (salary) FROM tableName)

By embedding the response from the first query into the second query, you get the person with the second-highest income.”

13. Explain the use of the DISTINCT keyword in SQL

“DISTINCT in SQL is used for getting rid of duplicate records that are returned as output by a SQL query. When you use it together with a SELECT statement, you get unique records that improve the readability and make for more efficient data analysis and reporting. It basically streamlines information in a way that's easy to navigate.”

14. How can you prevent SQL injection attacks in your queries?

“You have to use parameterized queries or prepared statements, using placeholders for input values, so that the SQL engine treats them as data rather than executable code. When you do this, you prevent malicious input from altering the intended query structure. You'd also need input validation and the principle of least privilege for security to safeguard against SQL injection vulnerabilities.”

15. Write a query to calculate the average of a column.

“You'd have to use the AVG() aggregate function to calculate the average of numeric values. The code looks like this:

SELECT AVG (column_name) as average_value

FROM tableName;

Of course, you'd have to replace 'column_name' with the actual name of the column you want averaged. The great thing about this particular query is that you can build upon it to GROUP information by a particular piece of data, too.”

16. What is the purpose of the ORDER BY clause in SQL?

“The ORDER BY clause in SQL is used to sort and arrange the output based on columns of information so that data is presented in an organized way. This helps the information that's retrieved be more meaningful and easier to interpret.”

17. Explain the difference between UNION and UNION ALL

“UNION and UNION ALL both work with SELECT statements. UNION is used to remove duplicate rows so that the output contains only unique values, while UNION ALL obtains results regardless of duplication. Of course, that means UNION is a more resource-intensive process because of its need to identify and remove duplicates, and UNION ALL is more efficient in terms of performance – you just have to remove the duplicate information on the backend.”

Database design interview questions

18. Describe the concept of database partitioning. How does it contribute to performance optimization and what would make you consider implementing partitioning in a database?

“Partitioning involves dividing large tables into smaller, more manageable pieces called partitions. These partitions are usually culled out based on particular criteria, like a range or list, and help improve performance and allow for parallel processing. It's also great for efficient data retrieval. The best time to implement partitioning is when you have a metric ton of data that's cumbersome to manage, especially when there are frequent queries on a specific set of data within the database. It's a perfect way to improve efficiency and manage time in analyzing data.”

19. Explain denormalization and when it's appropriate

“Denormalization is great when you need quick access to data and is best used in data warehousing and reporting systems where the information is mostly read-only, with very few write operations. It helps reduce the number of JOINs and simplifies complex queries.”

20. What are the ACID properties of a database?

“Back when we were talking about SQL transactions, I mentioned ACID. Basically, they're a set of properties that define the reliability and consistency of database transactions. The 'A' is Atomicity and goes a long way to ensure that transactions are fully completed or rolled back. The 'C' is for Consistency and makes sure that the transaction brings data from one valid state to another. The 'I' stands for Isolation and prevents interference between concurrent transactions. Finally, the 'D' is for Durability and makes sure that the changes in a transaction are permanent.”

21. How do you create an index on a table column?

“Indexes are great to have, especially as data grows. In fact, performing queries on unindexed data can tax or overwork CPUs. So, to create an index using SQL, you'd use the CREATE INDEX command. Here's what it looks like:

CREATE INDEX index_name 

ON table_name (column_name);

Of course, you'd input the name of the index where it says 'index_name' and the names of the table and column to fill in those placeholders.”

22. What is a trigger in SQL, and when would you use it?

“It's a set of instructions that automatically run a predefined event – meaning it's a thing that's executed in response to an event occurring. Triggers are used to enforce constraints and maintain the integrity of data by automating rules or performing actions like logging changes. For example, you'd use a trigger to update a summary table when certain columns are modified. That way, you get real-time updates in response to an action.”

Advanced SQL interview questions

23. Describe the advantages and disadvantages of using stored procedures

“The short answer is that the advantages have to do with performance, and the disadvantages have to do with increased learning curves and additional maintenance. Stored procedures in SQL are great for boosting performance through precompiled execution plans and increasing security through restricted table access. However, Developers sometimes have a hard time learning the concepts – the learning curve is steep. Even based on increased learning curves and more maintenance, the pros often outweigh the cons, but, of course, that does depend on the needs of the database application.”

24. What is a subquery in SQL? Provide an example

HINT: We've already had an example of a subquery in question 12.

“A subquery is a query embedded into another query that allows you to retrieve data that will be used to expand on details obtained in the main query. For example:

SELECT employee_name

FROM employees

WHERE department_id IN (SELECT department_id FROM departments WHERE location = 'New York');

The subquery in this example would give you department IDs for departments in New York, while the main query chooses employee names from the 'employees' table for those departments. Two of the key components of working with SQL are to interact with and manipulate data, and using a subquery is a great way to filter data based on a specific condition so that you can work with particular information.”

25. How do you handle NULL values in SQL?

“NULL in SQL is the absence of a value of data. It's important to note that doesn't mean there's nothing in the field, but that the data that's present is unknown, unidentified, or not applicable to the query being run. Because getting a NULL is often a thorn in the side of SQL users, it's advantageous to use IS NULL or IS NOT NULL conditions as they address challenges associated with NULL values to effect data filtering.”

26. Discuss the use of common table expressions (CTEs) in SQL

“CTEs let you get temporary result sets within a query for improved readability and data organization. Particularly useful in the context of complex queries, CTEs allow for the modular decomposition of SQL logic. Using the WITH clause, CTEs enable a step-by-step approach to constructing intricate queries, contributing to code modularity and ease of understanding.”

27. How do you optimize a slow-performing SQL query?

“There are several strategies that you can use to address performance bottlenecks in SQL, including reviewing query execution plans, indexing, changing SELECT to TOP or LIMIT to reduce data retrieval, and partitioning. On top of that, regular monitoring and profiling are essential to maintaining performance. Slow-performing SQL queries can also be caused by network latency. The first step, though, is to check that the query was written properly in the first place and then to start going down the list of strategies to eliminate them and find the culprit.”

Tips on how to answer SQL interview questions

As you can see, answering SQL interview questions requires a lot of technical knowledge, plus some problem-solving and communication skills. It could be argued that clear communication is among the most critical skills you can possess. With that said, let's explore some tips you can employ to answer these questions and those common interview questions you can expect. 

Be sure you understand the question

This seems like a no-brainer, but some of the questions you come across during a SQL interview could require that you talk about code, while others don't require that at all. If you're unsure, don't be afraid to ask clarifying questions. When you seek clarification, you hold up a big flag that says you don't jump into something without knowing exactly what's expected. A hiring manager will appreciate that.

Think before speaking

Because of the detailed nature of some of the more technical SQL interview questions, you'll want to take a moment to organize your thoughts before you open your mouth. A short pause to think about what you want to say won't be as long as it feels and the person conducting your interview will be happy that you take the time to give clear answers before muddling through a disorganized response. 

It helps if you do a few things here:

  1. Break the question down 

  2. Consider multiple approaches

  3. Adapt your approach if the interviewer makes suggestions

  4. Provide context 

  5. Use structured responses

A lot of being able to do these five things comes from preparation. You have to practice the responses before you step foot into the interview room, so you'll be ready to engage in active dialogue. 

Use paper (or a whiteboard if it's available)

You do not – let's repeat that, you do not – have to rely on relaying the more technical aspects of these questions verbally. If the hiring manager or panel wants you to write out a SQL query, you should do just that – write it out. The best thing you can do is bring a padfolio with you in case there isn't a whiteboard available but, if there is a whiteboard, let them know that you'd like to use it. There is virtually zero chance that you'll be told no. 

Seek feedback

Let's say you talk about a particular SQL transaction or complex query, and you're met with stunned silence. Don't assume you've done something wrong. It's essential that you remember there's often more than one valid approach to a problem. The hiring manager may simply have a different way of doing something or may not know the latest trick. 

Ask if there's anything you can clarify. This demonstrates that you understand alternative methods and that you're in tune with nonverbal communication cues. It's a great approach for opening a collaborative dialogue that will go a long way in proving you can work well with others. 

SQL in different roles

As the world of technology and automation changes, the roles that require knowledge of SQL are expanding rapidly. Data, big data, and advanced analytics are becoming more important to the success of business, which means your ability to understand and use SQL is going to be sought-after. 

Here are a few roles in which you can expect to useSQL and, therefore, should expect to run into some SQL interview questions:

  • Database Administrator (DBA)

  • Data Analyst

  • Data Scientist

  • Business Intelligence Developer

  • Software Engineer

  • Software Developer

  • Data Engineer

  • Systems Analyst

  • IT Manager

These roles also carry weight across a variety of industries. In fact, almost every sector, including finance, healthcare, and e-commerce, relies on databases for storing data. Cloud-based solutions and cloud platforms also use SQL-based solutions for managing data. Basically, if you're applying to a position that involves any amount of data, you should put forth the notion that you know what you're doing with SQL. 

Mastering SQL interviews can get you hired

SQL may be a programming language, but it's becoming one of those foundational skills that almost everyone will need, especially as data gets bigger and starts to rule the world. By harnessing those skills and clearly communicating your abilities, you'll win the day with a much-deserved job offer. 

If you want to boost your confidence in answering SQL interview questions or questions such as where you see yourself in five years, TopInterview's team of career coaches is here to help. Check out the options we have that will help you get your next job.

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