25 advanced SQL interview questions to ask developers

If you’re aiming to hire a SQL developer with data-focused abilities, using skills assessments and conducting interviews is crucial.

Skills assessments that include a SQLite (Coding): Intermediate-Level Querying test will show you which applicants have the best skills. The interview process will help you discuss the perks of the role, keep your candidates engaged, and learn if they have the senior-level SQL experience you require.

You need a range of insightful interview questions to complete an interview with candidates. But how do you select the correct ones? Ask your candidates the questions in this article to evaluate their experience and abilities.

25 advanced SQL interview questions to ask senior SQL applicants

This list of 25 advanced SQL interview questions tests applicants’ knowledge of commands, processes, and databases. 

1. What do you understand about normalization?

Normalization is a method developers use to organize data in a database to minimize data redundancy and improve its integrity. It involves restructuring the database to follow a specific normal form.

The best answers will also describe some of the most widely used normal forms, such as the following:

• 1NF: Every column in the table contains only atomic values (meaning they cannot be divided). Each column has only one value for each row in the table.

• 2NF: The database satisfies the conditions of 1NF, plus all non-key attributes are functionally dependent on the primary key.

• 3NF: Relations are in 3NF if they are in 2NF and have no transitive dependencies (meaning no non-key columns are dependent on other non-key columns).

• BCNF: This normal form is also known as Boyce-Codd normal form. It extends 3NF such that functional dependencies must be candidate keys or super keys.

2. Can you give us a definition of DDL (data definition language)?

Applicants with advanced knowledge of SQL will understand that DDL stands for data definition language. They should know that this language features the SQL commands developers use to define the database schema and creates, deletes, and alters database objects.

Skilled applicants will be able to name some of the objects they can alter with DDL, such as the following:

• Tables

• Catalogs

• Variables

• Indexes

They will also know some examples of DDL commands, including:

• CREATE****: Developers use this command to create database objects

• DROP****: SQL engineers use the drop command to delete database objects

• ALTER****: Developers use this command to change the database structure

• TRUNCATE****: SQL developers use this command to remove all records from the table

3. Could you give us a definition of DML (data manipulation language)?

DML means data manipulation language. This language includes the commands developers use to manipulate data in a database. Below are some examples of DML command components that developers can use to control access to a database:

• INSERT****: inserts data into a table

• UPDATE****: updates data in a table

• DELETE****: deletes database table records

4. Why is database knowledge crucial for SQL developers?

Database knowledge is critical for SQL developers since they use this language to manage data within databases. Consider if your applicants can use systems like MySQL or PostgreSQL with the SQL programming language to perform complex database operations.

You can test your applicants’ database knowledge with our MySQL skills test or PostgreSQL online test.

5. Could you explain the difference between DDL and DML?

Here are some of the main differences between DDL and DML your applicants may mention:

• DDL statements define a database’s structure and organization, whereas DML is used to add and modify the data within the database

• DDL commands affect the whole database or specific database objects, whereas DML statements affect specific rows or sets of data within database tables

• The WHERE clause is used more commonly in DML statements than in DDL statements

Consider if applicants can also name a few examples of DDL and DML commands to prove their SQL expertise.

6. What do you understand about ACID properties in SQL?

ACID is an acronym for atomicity, consistency, isolation, and durability. These properties describe transaction processing features and apply to any modern relational database’s architecture. Developers should understand what ACID is to ensure the highest possible data integrity and consistency.

7. Which query would you write to find every duplicate in a table?

To show their SQL experience, applicants should mention a couple of methods developers can use to find every duplicate in a table, such as the following:

• Using the GROUP BY command to group the table’s rows by target columns and check for duplicates

• Using the HAVING command and COUNT function to determine if any group has multiple entries

8. Could you explain what a clustered index is?

Developers use clustered indexes to define the order in which they store data in a table. Candidates should understand that there is only one clustered index per table because there is only one way to store the records.

Consider whether your applicants know that developers create clustered indexes on a primary key column and that the primary key enables them to create the index based on that particular column.

9. Could you explain why basic SQL syntax skills are important for developers?

Developers require SQL syntax knowledge to manipulate data in a database using the correct commands while avoiding making errors. It’s important to check if applicants understand commands like SELECT and clauses like FROM. They should also know how to avoid keyword errors and incorrect use of quotation marks.

Remember to ask follow-up questions about syntax and queries to further assess their SQL syntax knowledge.

10. Could you give us a definition of a non-clustered index?

A non-clustered index is a type of index that does not physically sort data rows in a table. A non-clustered index’s structure is separate from the data rows. It uses a key-value structure in which the key contains the column values.

This type of index can help developers find data more efficiently by speeding up data retrieval.

11. Could you explain the difference between clustered and non-clustered indexes?

Applicants need to know the difference between clustered and non-clustered indexes. They should mention that developers use clustered indexes to sort tables or arrange data alphabetically, whereas non-clustered index records don’t match the physical order of the table’s rows. 

If they have used indexes frequently, applicants will be able to explain a few other differences, such as the following points:

• Clustered indexes are faster than non-clustered indexes

• Clustered indexes require less memory than non-clustered ones

• Tables can only have only one clustered index but multiple non-clustered ones

• Clustered indexes define the order of data within a table, whereas non-clustered indexes define the order of data within an index

12. What do you understand about denormalization?

Denormalization is when developers intentionally add redundancy to normalized relational databases. The purpose of denormalization is to improve the database’s read performance in cases in which query speed is more important than a consistent data structure or space optimization.

Some actions developers might take to conduct denormalization include:

• Using extra attributes in a table

• Adding a new table

• Creating instances of tables

13. What do you understand about collation?

In the context of databases, collation is the set of rules for sorting, case sensitivity properties, and accent sensitivity properties for data. Developers can use collation settings to indicate how the database engine should manage character data.

Consider if your applicants also know that developers can access the list of collations by using the following command:

SELECT * FROM sys.fn_helpcollations();

14. Could you give us a definition of an inner join in SQL?

To test your applicants’ SQL keyword knowledge, ask them if they understand what the INNER JOIN keyword does. Knowledgeable applicants will know that this keyword selects records with matching values in two tables and joins two tables based on their common column.

15. Can you give us a definition of an outer join in SQL?

Candidates can prove their SQL keyword knowledge by explaining that an outer join retrieves records from two tables, including unmatched rows. They should also explain that this join returns those remaining rows from one table – in other words, it returns values from either table or both.

16. Is there a difference between an outer join and a full outer join in SQL?

See if your candidates can explain that developers can use a full outer join to return all rows from both tables, including both matched and unmatched rows. On the other hand, an outer join only returns the matched rows and the unmatched rows from one table.

17. Is PHP knowledge useful for SQL developers? Could you explain why?

Since SQL developers can use PHP skills to interact with popular SQL database management systems like MySQL, they should have some mastery of this scripting language. With this knowledge, they can create nearly any type of website.

Therefore, it’s important to check if your candidates have PHP knowledge using our PHP (Coding): Intermediate-Level Algorithms test before you choose someone to join your team. 

18. Could you explain why Microsoft SQL Server skills are important for SQL developers?

If your company uses Microsoft SQL Server, consider whether your applicants understand how to use this relational database management system to store and retrieve data. To evaluate their knowledge, ask them follow-up questions such as “What is the ideal way to interact with Microsoft SQL Server?”

It’s worthwhile to assess your candidates’ Microsoft SQL Server skills because they help developers carry out the following tasks:

• Design databases

• Maintain databases

• Validate databases’ stability

To assess your applicants’ Microsoft SQL Server knowledge, use our Microsoft SQL Server test for a data-driven hiring method.

19. Could you name three advantages of having an index?

When responding to this advanced SQL interview question, candidates might mention many advantages of having an index, such as the following:

• Indexes speed up SELECT queries by enabling the database to retrieve the data more quickly

• Indexes can be used to ensure values in a column or multiple columns are unique

• Developers can use indexes to sort data in a specific order

• Full-text indexing enables developers to insert and search for large strings of text, making searching more efficient

20. Can you define a union in SQL?

Ask interviewees this advanced SQL interview question to check if their knowledge matches your expectations. Candidates should understand that developers use unions to combine two different SELECT statements. They should also know that unions eliminate duplicate data from the rows developers select.

21. Could you explain the difference between a join and a union in SQL?

The following are a couple of the main differences between a join and a union:

• A join fetches records that match each other from two tables. Meanwhile, a union combines data from two different SELECT statements.

• A join will not eliminate duplicate data, whereas a union can remove duplicate data.

Consider if your candidates have sufficient experience with joins and unions by asking them follow-up questions such as “Describe a project in which you used a join or a union to manipulate data.”

22. Could you give us a definition of shared locks in SQL?

Developers use shared locks when they execute SELECT statements. Shared locks reserve a resource for reading purposes only and ensure the developer is not updating the record when there is a read-only request.

In this way, shared locks help developers make sure that multiple read-only requests can access the same resource simultaneously without conflicts.

Shared locks also block other transactions from modifying a row until the transaction is complete.

23. Could you give us a definition of exclusive locks in SQL?

Knowledgeable applicants should know that when statements modify data, the transaction holds an exclusive lock on the data. An exclusive lock prevents other transactions from accessing or modifying the data at the same time to prevent conflicts.

Developers cannot remove the lock until the transaction that holds the lock issues a rollback or a commit.

24. What do you understand about transposition in SQL?

Transposition is a process through which developers can change a row or column to a specific format to visualize the data from a different perspective. One transposition method is to change a column to a row or vice versa.

Consider if your applicants know that two common transposition methods are dynamic and joined transposition, and ask if they can explain the advantages of these methods.

25. Why are problem-solving skills crucial for SQL developers?

Developers may run into various problems when working with databases, so they require problem-solving skills to resolve these issues and keep the data secure and efficient. They must use these skills to find solutions to database problems and minimize the problems these issues can create.

Use a Problem Solving test to determine if your applicants can work through problems and make the best decisions.

When should you use advanced SQL interview questions in the hiring process?

The ideal way to use advanced SQL interview questions when hiring is to first invite applicants to complete a SQL assessment. All you need to do is select up to five skills tests to include in the assessment, send it to applicants, wait until you receive their results, and then invite the top-performing candidates to an interview.

Some advantages of using this method include the following:

• It’s an easy way to create a candidate shortlist

• You can minimize unconscious bias when you choose candidates for an interview

• You won’t need to screen resumes or miss top talent

• Applicants’ test results may give you ideas for additional interview questions

To assess a full range of skills, you should select both hard- and soft-skills tests and ask advanced SQL interview questions that also test both soft and hard skills. This method will help you gather sufficient data to evaluate applicants and check if their abilities match your requirements.

Hire a SQL developer with advanced SQL interview questions and skills tests

With prescreening tests and advanced SQL interview questions, you’ll find it easy to hire an applicant who meets your expectations. This data-driven hiring method will help you quickly compare applicants and screen out those who are unqualified.

If you want to find out more about the full selection of pre-employment tests we offer, check out our skills test library or request a quick, free demo.

You’re now ready to hire an expert SQL developer. Find top talent with skills tests and advanced SQL interview questions.