Interview Questions for SQL Developer: Answers, and Explanations

Short for Structured Query Language, SQL lets developers access and manipulate databases. It's especially useful for handling structured data — data incorporating relations among variables and entities.

Unfortunately, as with all things, not every SQL developer is suitable for your company. Even if they have all ample experience and skills, they might have a personality that makes it difficult for them to collaborate with their coworkers. They may also lack the communication, mentoring, and leadership skills to succeed.

Fortunately, a great way to find and hire SQL developers is by asking them the right interview questions. Read this guide to learn about the best  interview questions for SQL developer and answers.

Web Core Concepts and Basic Interview Questions

To hire the right SQL dev, you must understand their technical background, experiences, and education. You also need to test their understanding of their basic SQL concepts.

Basic Interview Questions for SQL Developer

Before testing applicants' practical knowledge, ask some basic interview questions to learn more about their education, experiences, and motivations. These include:

• Tell me about yourself. This prompts the applicant to tell you more about their skills and areas of expertise. Ask about their SQL work experience, how they learned SQL, and why they enjoy working as an SQL developer.
• How did you learn SQL? This encourages the job seeker to share their motivations for learning SQL. You should also be able to find out why they were driven to learn SQL and what kind of projects they have worked on.
• What do you like the most about SQL? This question reveals why an applicant enjoys using SQL. Look for someone passionate about SQL and SQL trends.
• Have you worked with NoSQL databases before? If so, what kind of NoSQL databases have you worked with? Ask this question if you need an SQL developer to work on NoSQL or non-relational databases or help others with NoSQL databases. If your applicant knows how to use NoSQL databases, ask them what types of NoSQL databases they're familiar with. There are four major types of NoSQL databases:

• Document databases store data in documents like JavaScript Object Notation (JSON) objects. Each document contains pairs of values and fields.
• Graph databases store data in edges and nodes. Edges store data about the relationships between the nodes, and nodes usually store information about places, people, and things.
• Key-value databases are a simpler kind of database where each item contains values and keys.
• Wide-column stores, which store data in a set of rows, tables, and columns.

• Walk me through your portfolio. Now that you have a clearer idea about how the applicant learned SQL, ask them to walk you through their portfolio. Zoom in on projects that are similar to the ones you'll be hiring them to do.
• What relevant certifications do you have? This question reveals the applicant's educational background. Although most companies prefer hiring SQL developers with bachelor's or master's degrees, you should consider expanding your potential applicant pool to include SQL professionals without degrees. Many SQL professionals without degrees boast impressive experience and skills. Some also have top-notch certifications, including:

• Microsoft Certified: Azure Data Fundamentals: This entry-level certification validate's a job seeker's knowledge of data concepts using Microsoft Azure data services. It also demonstrates their understanding of how databases work with MySQL and PostgreSQL.
• Oracle Database SQL Certified Associate Certification: This certification demonstrates a developer's knowledge of basic SQL concepts. It also demonstrates the developer's ability to use SQL with the Oracle Database server.
• IBM Certified Database Associate: IBM offers three Certified Database Associate certifications for professionals who want to demonstrate their understanding of the Db2 or Informix database management systems.

• What SQL editor tools are you familiar with? Ideally, your hire should be extremely familiar with your company's preferred SQL tools. However, if they're not, you can offer to train them by pairing them with an expert in the tool. Common examples of SQL tools include:

• MySQL
• RazorSQL
• Microsoft SQL Server Management Studio
• DbVisualizer
• MySQL Workbench
• PopSQL
• dbForge Studio
• TablePlus
• WinSQL
• HeidiSQL

• Besides SQL, what other programming languages do you know? Consider asking applicants this question if you need them to work on tasks that require knowledge of other languages. Here are some languages you should look for:

• Rust
• Python
• Java
• JavaScript
• PHP
• C++
• .NET

Common SQL Developer Interview Questions

Now that you know more about your applicant, it's time to test their technical knowledge. Here are some common SQL developer practical questions and answers:

What Is the Difference Between NoSQL and SQL? When Should You Use Each?

When choosing a database, developers often have to choose between a NoSQL or SQL data structure.

There are five main differences between NoSQL and SQL data structures:

1. NoSQL databases are non-relational, while SQL databases are relational.
2. SQL databases use structured query language and predefined schemas, while NoSQL databases use dynamic schemas for unstructured data.
3. SQL databases are scaled vertically, while NoSQL databases scale horizontally.
4. SQL databases are table-based, while NoSQL databases are key-value, document, wide-column, and graph stores.
5. SQL databases are best for multi-row transactions and legacy systems built around relational structures, while NoSQL is best-suited for unstructured data like JSON or documents.

How Do You Find Duplicate Records in SQL?

You can find duplicate records in SQL by using the following functions:

• RANK: The RANK function returns a unique rank number for every unique row to a distinct column value.
• GROUP BY: This function arranges identical data into groups via some functions. For instance, if a certain column has the same values in different rows, GROUP BY will arrange these rows in a group.

What Is the Difference Between Static and Dynamic SQL?

Embedded or static SQL are SQL statements that don't change at runtime. As such, devs can hard-code them into the application.

Meanwhile, dynamic SQL refers to SQL statements that are generated dynamically based on users' input. Dynamic SQL is often used to create flexible applications. It typically requires more security handling since malicious threat actors can use it to create dangerous code.

What Is an SQL Join and What Are the Various Types of SQL Joins?

An SQL Join statement combines rows or data from two or more tables based on a shared field between them. There are several types of SQL Joins:

• Inner Join: This picks and returns records with matching values in both tables.
• Right Join: This concatenates all right table rows and the corresponding left table rows.
• Left Join: This concatenates all left table rows and the matching right table rows.
• Self Join: This joins a table to itself.
• Full Join: This returns all records of both tables if there's a matching record in either the right or the left table.
• Cartesian Join: Also called a cross join, this returns the rows in the first table by multiplying them with the second table rows.

What Are the Different Types of SQL Sandbox?

There are three different types of SQL sandboxes:

1. External Access Sandboxes: These sandboxes can access files without having permission to manipulate memory allocation.
2. Safe Access Sandbox: These sandboxes empower users to perform SQL operations such as creating triggers and stored procedures without having access to the memory. Users also don't have the right to create files.
3. Unsafe Access Sandbox: These sandboxes give users access to threads and memory management.

How Do You Perform a Pattern Match?

You can perform a pattern match by using the REGEXP operator and a regular expression pattern. Alternatively, you can use an SQL pattern.

What Is a View in SQL?

A view is a virtual table based on an SQL statement's result set. Like a real table, a view contains columns and rows. A view's fields are from one or more tables in the database.

Junior SQL Developer Interview Questions

Once you've asked the basic questions above, you should ask job seekers role-specific interview questions. For example, if you're hiring junior or entry-level SQL developers, you should ask questions specifically tailored to that skill level.

As their name indicates, entry-level or junior SQL developers have little to no experience under their belt. Accordingly, you shouldn't have extremely high standards for their experiences and portfolios. Instead, just focus on their basic competencies and personality traits, such as communication and collaboration skills. If they're a good fit for your team, invest in learning and development opportunities so that they can continue expanding and strengthening their skill sets.

Without further ado, here are the best junior and entry-level interview questions and answers for SQL developers:

How Do You Write a Relational Database Management System (RDMS)?

This technical question reveals whether a junior or entry-level SQL dev understands relational database management systems (RDMS). Although junior or entry-level SQL devs aren't required to have a deep understanding of RDMS, they should know what RDBMS are and how to create them. A good answer would look something like this:

An RDBMS is a collection of capabilities and programs that empower information technology (IT) teams and others to build, update, administer, and interact with a relational database. RDBMS typically use SQL to store data in tables.

You can write an RDMS by:

1. Defining the purpose of the database.
2. Gathering the data that will be stored in the database and dividing such data into subject-based tables.
3. Creating relationships between tables. The types of relationships include one-to-one, one-tomany, and many-to-many.
4. Refining the design by applying normalization rules. These rules check whether a database is structurally optimal or correct. You can also add more columns, split a large table into smaller tables, and create new tables for optional data.

Which Operation Is Used in Query For Pattern Matching?

This technical question reveals whether the applicant understands pattern matching. Here's what an ideal answer looks like:

You can use the LIKE operator for pattern matching. There are two wildcards you can use with the LIKE operator:

• The underscore sign (_) represents a single character
• The percent sign (%) represents zero or more characters

What Is Your Approach To Security?

This question reveals the job seeker's approach to security. The right pick for your team should prioritize security and factor it in from the beginning. Otherwise, your databases will be vulnerable to attack. The ideal answer should also touch on ways to secure SQL, including:

• Isolating production database servers from other services and applications
• Removing additional features you don't need to reduce the attack surface
• Protecting against SQL injections
• Keeping SQL programs regularly patched
• Securing your backups
• Getting everything configured and setup securely
• Restricting database processes
• Auditing database logins
• Only allowing SQL traffic to and from trusted IP addresses
• Setting a complex admin password
• Using the principle of least privilege when assigning permissions

SQL Interview Questions for Experienced Developers

Now that you know junior and entry-level interview questions for SQL developers, here are some questions and answers for advanced and senior SQL devs.

What Are the Differences Between Views and Tables?

This question reveals the depth of a senior SQL developer's knowledge about views and tables. Their answer should cover the following:

A view is a virtual table that is extracted from a database and doesn't hold data itself. SQL developers use views to query certain information contained in certain tables. We will usually get queried information in views.

Meanwhile, a table has a limitless number of rows and a set number of columns. It stores data in databases and holds vital client information. It may also contain cases of characterized objects. If a user or dev changes the information in a database table, the information that appears inthe view will also change.

What Is a Stored Procedure? Why Do Devs Use Them? Can You Give an Example of a Stored Procedure?

This question highlights a programmer's understanding of SQL programming tricks and security. The ideal answer should touch on the following:

• The definition of stored procedures: A stored procedure is an SQL code that can be reused and saved. You can consolidate several SQL statements into a stored procedure and use them as needed. Many devs use stored procedures to save time and accelerate execution.
• Code samples: The programmer should be able to code an example of a stored procedure.
• The benefits of stored procedures: Stored procedures accelerate database management. They also provide a critical layer of security between the database and the user interface.

What's the Difference Between OLTP and OLAP?

This question demonstrates the depth of an SQL developer's database knowledge. Here's what an ideal answer looks like:

OLTP stands for online transaction processing. It's a type of data processing that supports transaction-oriented applications via a three-tier architecture. OLTP applications execute many transactions concurrently. Examples of OTLP include online shopping, sending text message, and online banking.

In contrast, OLAP stands for online analytical processing. It's software that performs multi-dimensional analytical queries at high speeds and volumes. This data typically comes from data marts, data warehouses, and other centralized data storages. For OLAP systems, computing efficiency depends heavily on the response time. As such, OLAP systems are generally used for maintaining aggregated historical data, complex analytical calculations, business intelligence, and data mining.

Do You Enjoy Leading and Mentoring Junior SQL Developers?

As previously covered, junior and entry-level staff often require a lot of training and coaching. As such, if you're planning to hire junior and senior SQL developers, you should ask the senior SQL developers if they have the time, expertise, energy, and motivation to lead and mentor junior colleagues.

Recruitment SQL Developer Coding Challenges

Interview questions can reveal a lot about a job seeker's education, experiences, and skills. However, they don't reveal much about candidates' problem-solving skills. As such, you can easily hire someone with in-depth SQL knowledge who has difficulty applying that knowledge.

To recruit SQL developers who can apply their knowledge, consider having applicants complete recruitment SQL developer coding challenges. These are online or in-person tests where you can witness candidates' problem-solving abilities and work ethic. There are several types of recruitment SQL challenges, including:

• Pair programming: Pair programming is when a candidate sits down with one of your current staff to work on a coding problem. Your staff will evaluate the candidate's soft and technical skills as they work through the exercise together.
• Whiteboarding: This requires applicants to solve SQL coding problems on a physical or digital whiteboard in front of the recruiter. These tests evaluate job seekers' technical and soft skills, including problem-solving and communication skills.
• Open coding challenges: These are tech events that invite developers to solve SQL coding challenges and win awards. With the right amount of marketing, these events can help you recruit MAMAA-caliber SQL programmers from around the world.

Ask The Right Questions When Hiring SQL Developers

The best way to recruit MAMAA-caliber SQL coders is to ask them the right SQL developer interview questions. Before you can do that, though, you need to source SQL developers by joining a premier tech talent marketplace like Revelo. As Latin America's leading tech talent marketplace, we provide access to over 300,000 remote IT professionals, including SQL programmers, Unreal Engine developers, mobile app developers, quality assurance testers, and more.

Interested in getting started with Revelo? Fill in this form today to start hiring nearshore SQL developers. In three days, we'll send you a curated list of SQL talent. You can then interview and hire the ones you want.

Further Resource: Alternative In-house Technical Recruiting

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