TOP 558 sql interview questions for candidate screening in 2025

Why SQL Interview Questions Matter More Than You Think

Look, I get it. You're probably sitting there thinking "Great, another technical screening to figure out." But here's the thing - sql interview questions aren't just some checkbox exercise. They're your secret weapon for finding candidates who can actually think with data, not just memorize syntax. I've seen too many hiring managers skip proper SQL screening only to end up with developers who crumble the moment they need to join three tables or optimize a slow query. Trust me, getting this right upfront saves you months of headaches later.

The SQL Landscape is Bigger Than You Realize

Here's what most people don't know: SQL was developed by IBM back in the 1970s, and it's still the backbone of virtually every data-driven business today. We're talking about a language that powers everything from your startup's user analytics to Amazon's massive recommendation engine. The numbers don't lie - professionals with advanced SQL skills receive a median annual pay of $117,450, and job growth outlook is 8% in the coming years for database administrators and database architects.

The technical landscape is evolving fast. Modern databases like PostgreSQL (developed by Computer Science Professor Michael Stonebraker's team in 1986) can handle up to 32TB of data with 4 standard isolation levels and 3 types of inbuilt partitioning methods. Your candidates need to understand concepts like ACID properties (Atomicity, Consistency, Isolation, Durability), the difference between clustered and non-clustered indexes, and why UNION ALL is faster than UNION because it doesn't perform duplicate elimination.

What Your Candidates Actually Need to Know

The reality is harsh: SQL is required in most data and tech job listings today. Whether you're hiring for entry-level roles or looking for someone with 10+ years of experience, they need to grasp fundamentals like the two types of SQL queries (select and action), understand that each table can define at most one PRIMARY KEY, and know the difference between TRUNCATE (a DDL command) and DELETE (a DML command).

Your interview process should cover everything from basic joins to complex scenarios involving multiple non-clustered indexes and understanding why PostgreSQL allows multiple NULLs in UNIQUE constraints while SQL Server allows only one NULL unless using filtered indexes.

Ready for the Complete SQL Interview Question Arsenal?

After working with hundreds of hiring managers and analyzing successful technical interviews at companies like Google, Oracle, Amazon, and Microsoft, I've compiled the most comprehensive collection you'll find anywhere. We're about to dive into 558 best practice sql interview questions for 2024 that cover everything from basic SELECT statements for entry-level candidates to advanced optimization techniques for senior developers. These aren't theoretical questions - they're real scenarios your team will face, tested and refined through actual hiring experiences.

Because you're reading this article, you might be interested in the following article as well: Asynchronous Video Interview: Main Characteristics.

🏆 The list of TOP 558 sql interview questions in 2025

What is SQL?

SQL stands for Structured Query Language. It's a widely used programming language designed to manage and manipulate relational databases. With SQL, candidates can create, read, update, and delete records—commonly known as CRUD operations.

Comment:

This is a basic but essential screening question for any role involving data manipulation or database management. The ideal candidate should clearly explain SQL's core purpose—interacting with relational databases. Look for a response that mentions SQL being used to create, retrieve, update, and delete data, and possibly touches on schema design or data security features.

Best Practice: If the role requires daily hands-on database work, dig deeper. Ask them to describe a query they’ve written recently or a database issue they resolved using SQL.

What is a database?

A database is an organized collection of information that is stored electronically and accessed using computer systems. It’s usually structured into tables with rows and columns, making it easy to retrieve, update, and manage data. Most modern databases are managed by a system called a Database Management System (DBMS)—this software handles all interactions between users and the data.

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Why ask this question during a candidate screening? This is a basic yet important question for roles in IT, software development, analytics, and data management. A good answer should cover:

• What a database is (organized data, electronic storage)
• How it's managed (mention of DBMS is a plus)
• Common examples (like SQL, MySQL, or PostgreSQL)

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What to look for in responses: Look for signs that the candidate understands more than just the definition. Good candidates will mention:

• How data is structured (tables, rows, columns)
• Types of databases (relational vs. non-relational)
• Real-life examples or use cases

This shows not only knowledge but actual hands-on experience.

Best practice: For technical roles, follow up with more detailed questions like, “Can you explain the difference between SQL and NoSQL databases?” or “Describe a time you designed or queried a complex database.”

This helps you dig deeper into their practical understanding.

What are the main types of SQL commands?

When interviewing candidates for data or database-related roles, this is a great question to assess their understanding of SQL fundamentals. A strong answer should clearly cover these main categories of SQL commands:

• DDL (Data Definition Language): Commands like `CREATE`, `ALTER`, `DROP`, and `TRUNCATE` used to define or modify database structures.
• DML (Data Manipulation Language): Includes `SELECT`, `INSERT`, `UPDATE`, and `DELETE`—used for managing data within tables.
• DCL (Data Control Language): Commands like `GRANT` and `REVOKE` that deal with user permissions.
• TCL (Transaction Control Language): Includes `COMMIT`, `ROLLBACK`, and `SAVEPOINT` for handling transactions and maintaining data integrity.

Comment:

Candidates who can confidently explain each SQL command type and provide examples (like using `COMMIT` after a successful data update) show practical knowledge. A best practice approach is to ask follow-up questions—like how they’ve used DML in real scenarios or what happens if a rollback is missed. Look for those who understand not just the names, but how and when each command is used in real-world database operations.

What is the difference between CHAR and VARCHAR2 data types?

CHAR and VARCHAR2 are both used to store character strings in databases, but they handle storage differently.

• CHAR stores data in a fixed-length format. If you define a CHAR(10) and store "Cat", it will still use the full 10 characters by padding the rest with spaces.
• VARCHAR2, on the other hand, uses variable-length storage. If you define VARCHAR2(10) and store "Cat", it only uses 3 characters of storage.

Best practice tip: Use VARCHAR2 when the length of data is uncertain or likely to vary, as it saves space. Use CHAR when the data length is consistent, like specific codes or fixed-format fields.

When evaluating a candidate’s answer, look for:

• A clear understanding of storage behavior
• Awareness of performance and storage implications
• Practical usage examples in real scenarios

A strong candidate should also point out that VARCHAR2 is generally preferred in most modern applications due to its flexibility and space-saving nature.

What is a primary key?

A primary key is a unique identifier for each record in a database table. It ensures that each row remains unique and easily identifiable. A primary key column cannot have duplicate or NULL values, making it a core part of data integrity.

Comment:

This is a basic but critical database concept, especially for roles involving SQL, data analysis, or backend development. When asking this, look for candidates who mention:

• Uniqueness of the data
• No NULL values allowed
• Ensures data integrity
• May also mention composite keys (good bonus)

Best practice: Follow up by asking them to give a real-world example (e.g., customer ID or employee ID). Strong candidates will be able to connect technical definitions to practical use cases clearly and confidently.

What is a foreign key?

A foreign key is a column (or a group of columns) in a database table that creates a link between two tables. It refers to the primary key in another table and helps maintain referential integrity by ensuring that the value in one table matches a value in another.

Comment:

This is a good screening question for technical roles involving databases, such as software developers or data analysts. When a candidate explains this concept clearly, it shows they understand relational database design.

Look for responses that mention these key points:

• Linking tables together
• Enforcing data consistency
• Relational integrity

Best practice tip: Ask the candidate to give a real example of how they used foreign keys in past projects. It helps validate both their technical understanding and real-world experience.

What is the purpose of the DEFAULT constraint?

The DEFAULT constraint is used in databases to automatically assign a value to a column if no value is specified during the INSERT operation. This ensures every record has a predefined value, even if the user omits that column while adding data.

Comment:

This question helps you check the candidate’s understanding of database integrity and data consistency. A strong answer should mention that the DEFAULT constraint keeps data uniform and reduces the need for manual input, especially for fields like `status = 'active'` or `createdat = CURRENTTIMESTAMP`. It's a good follow-up to ask how they’ve used DEFAULT in past projects. Look for real-life examples to assess practical knowledge.

What is normalization in databases?

Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. This typically involves breaking down large tables into smaller, more manageable related tables and establishing relationships between them. The goal is to structure data efficiently, eliminate duplicate entries, and avoid update or deletion anomalies.

Comment:

Look for candidates who explain normalization in simple terms and can mention the basic forms (like 1NF, 2NF, 3NF) if asked. Strong answers should touch on the benefits like data consistency, easy updates, and improved accuracy. A best practice is to ask follow-up questions to see if the candidate understands when to normalize and when denormalization might be better for performance.

What is denormalization, and when is it used?

Denormalization is the process of combining normalized database tables into larger, fewer tables to improve query performance. It involves adding redundant data or repeating groups to avoid complex joins during data retrieval.

Comment:

This is a great question for screening candidates applying for roles involving database design, data engineering, or ETL processes. A strong candidate should be able to explain denormalization in simple terms and know why and when to use it.

Best practice is to use denormalization when:

• Queries across multiple tables are slow due to frequent JOINs.
• The system needs faster read performance and is optimized more for reading than writing.
• The data isn't updated frequently, so the risks of redundancy are lower.

Watch out for answers that ignore the trade-offs, like the increased chance of data inconsistency. A well-rounded answer should touch on both the benefits (performance boost) and drawbacks (redundant data, maintenance overhead).

What is a query in SQL?

A query in SQL is a command used to communicate with a database. It allows you to retrieve, modify, insert, or delete data. The most common type is a SELECT query, which pulls data based on set conditions. For example:

SELECT name, email FROM users WHERE status = 'active';

This SQL query fetches the name and email of users whose status is "active."

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Comment:

This question helps you assess whether the candidate understands the basics of working with relational databases. Listen for responses that mention SELECT, WHERE clauses, and the purpose of querying a database. A good answer explains that queries are used to retrieve or manipulate data, with real-world examples. Strong candidates may also refer to other types of queries like INSERT, UPDATE, and DELETE. Best practice: ask for a follow-up example or scenario where they’ve written or modified SQL queries in a previous role.

What are the different operators available in SQL?

When interviewing candidates for SQL-related roles, a great technical screening question is: "What are the different operators available in SQL?"

This question helps you assess their foundational understanding of SQL, which is crucial for data manipulation and querying. A strong candidate should be familiar with the different types of operators and know when and how to use each one.

Here's what you should look for in a solid answer:

• Arithmetic Operators: Used for basic mathematical operations
• `+` (addition), `-` (subtraction), `*` (multiplication), `/` (division), `%` (modulus)

• Comparison Operators: Used to compare values
• `=`, `!=`, `<>`, `>`, `<`, `>=`, `<=`

• Logical Operators: Used for combining multiple conditions
• `AND`, `OR`, `NOT`

• Set Operators: Combine results from multiple queries
• `UNION`, `INTERSECT`, `EXCEPT` (also known as MINUS in some databases)

• Special Operators: Used for filtering data in more complex ways
• `BETWEEN`, `IN`, `LIKE`, `IS NULL`

• Concatenation Operators: Combine strings
• `||` (used in Oracle and PostgreSQL), `+` (used in SQL Server)

Comment:

The ideal response isn’t just listing operators—they should also show basic examples or describe use cases. This demonstrates real-world knowledge beyond memorization. Watch for answers limited to only a few operator types—that’s a red flag the candidate might only have surface-level understanding. As a best practice, follow up by asking them to write a query using several types of operators in one example to evaluate fluency.

What is a view in SQL?

A view in SQL is like a virtual table that is created using a `SELECT` query. It doesn’t store data directly; instead, it displays data pulled from one or more actual tables. Think of it as a saved query that you can use like a table.

This helps in situations where you want to:

• Simplify complex queries by turning them into reusable views
• Restrict access to sensitive data by only showing specific columns or rows
• Improve code clarity by keeping database logic clean

For example, instead of writing a long `JOIN` every time, create a view and just select from it.

Some views can be updated—if they meet specific conditions like no use of `GROUP BY`, `DISTINCT`, or aggregated functions.

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Best Practice: Use views when you want to protect sensitive parts of your tables or keep recurring queries tidy. But don’t overuse them if performance is a concern. Make sure to test if a view affects query speed.

What is the purpose of the UNIQUE constraint?

The UNIQUE constraint ensures that all values in a column—or a group of columns—are distinct. It means no two rows can have the same value in that specific column or combination of columns.

Comment:

This is a great question to ask candidates applying for any role that involves working with databases or SQL. The ideal answer should explain that the UNIQUE constraint is used to prevent duplicate entries in a table, helping maintain data integrity.

Look for candidates who can give a clear example, like applying a UNIQUE constraint on an email column to ensure no two users can register with the same email. That shows practical understanding.

Best practice tip: Encourage candidates to explain why and when they’d use UNIQUE over other constraints like PRIMARY KEY, showing they understand the differences.

What are the different types of joins in SQL?

This is a great technical question to evaluate a candidate’s SQL knowledge and data handling skills. You’re not just looking for someone to list joins—you want to hear clear understanding with simple examples if possible. Here’s what you should expect in a solid answer:

• INNER JOIN: Returns only the rows that have matching values in both tables. This is the most commonly used join.
• LEFT JOIN (LEFT OUTER JOIN): Returns all rows from the left table, and the matched rows from the right table. If there's no match, NULLs are returned for columns from the right table.
• RIGHT JOIN (RIGHT OUTER JOIN): The opposite of LEFT JOIN. Returns all rows from the right table, and the matched rows from the left table.
• FULL JOIN (FULL OUTER JOIN): Combines LEFT and RIGHT JOIN. Returns all rows when there’s a match in either table, filling in NULLs where there’s no match.
• CROSS JOIN: Returns the Cartesian product of the two tables. That means every row from the first table is paired with every row from the second. It’s rarely used but has its place in specific scenarios.
• SELF JOIN: This is when a table is joined to itself. It’s often used to deal with hierarchical or tree-structured data like employee-manager relationships.

Best practice: A strong candidate shouldn’t just list these types—they should explain their purpose with a small use case or example. Be sure to ask follow-up questions like, “When would you use a LEFT JOIN over an INNER JOIN?” to explore depth of knowledge.

What is the difference between INNER JOIN and OUTER JOIN?

This is a great question to ask when screening candidates for SQL knowledge, especially for roles that require working with relational databases. It helps you understand how deep their understanding goes with data relationships and query logic.

INNER JOIN returns only the rows from both tables where there is a match. If there's no match, the row is excluded from the result.

OUTER JOIN, on the other hand, brings back all the rows from one table and the matching rows from the other. If a match doesn't exist, it still includes the unmatched rows but fills in `NULL` for the missing data. There are three types:

• LEFT OUTER JOIN: All rows from the left table, matched or not.
• RIGHT OUTER JOIN: All rows from the right table, matched or not.
• FULL OUTER JOIN: All rows from both tables, matched or not.

Best Practice Tip: Pay attention to how clearly a candidate can distinguish these joins. Can they explain scenarios or use cases? Bonus points if they can talk about performance impact or indexing when using different JOIN types.

What is the purpose of the GROUP BY clause?

The `GROUP BY` clause in SQL is used to collect data across multiple records and group the results by one or more columns. It works hand-in-hand with aggregate functions like:

• `COUNT()` — to count the number of records in each group
• `SUM()` — to get the total of a column per group
• `AVG()` — to calculate the average
• `MAX()` and `MIN()` — to find the highest or lowest value in each group

This clause is very useful when you want summary results from your data.

Comment:

Candidates should explain that the `GROUP BY` clause organizes rows that have the same values in specified columns into summary rows. A solid answer includes mention of its use with aggregate functions like `COUNT`, `SUM`, or `AVG`.

Best practice: Look for candidates who point out that `GROUP BY` comes after the `WHERE` clause and before `ORDER BY` in a SQL query. They should also be aware that all selected columns either need to be aggregated or included in the `GROUP BY` clause.

A well-rounded answer shows both a technical understanding and an awareness of real-world usage.

What are aggregate functions in SQL?

Aggregate functions in SQL are used to perform calculations on multiple rows of data and return just one value as a result. These functions are often used with the `GROUP BY` clause to summarize information from your dataset in a meaningful way.

Common aggregate functions include:

• COUNT(): Returns the number of rows that match a given condition.
• SUM(): Adds up all the values in a numeric column.
• AVG(): Calculates the average of the values in a column.
• MIN(): Retrieves the smallest value from a column.
• MAX(): Retrieves the largest value from a column.

Comment:

This question is great for testing a candidate’s basic understanding of SQL data analysis. You're checking if they know how to summarize data and extract meaningful insights. A strong candidate should not only list the functions but also explain when and why to use each one. The best practice approach is to ask for a real-life example. For instance, how they’ve used `GROUP BY` with `SUM()` in a sales report. Look for clear, confident explanations with practical use cases.

What is a subquery?

A subquery is a query inside another SQL query. It helps break down complex questions by nesting one query within another. Subqueries are often found in the `WHERE`, `FROM`, or `SELECT` clauses.

There are two main types:

• Non-Correlated Subquery: Runs once and returns a result used by the outer query. It doesn't rely on the outer query to execute.
• Correlated Subquery: Depends on the outer query and runs for each row processed by it.

Comment:

Asking this question helps you see how well a candidate understands SQL and handles complex data retrieval. A strong answer should clearly explain both types, with examples if possible. The best practice is to look for candidates who know when and why to use subqueries—and can explain the performance impact of correlated subqueries.

What is the difference between the WHERE and HAVING clauses?

WHERE and HAVING may seem similar, but they serve different roles in SQL queries.

• WHERE is used to filter rows before any grouping is done.
• HAVING is used to filter groups after the `GROUP BY` clause has been applied.

A best practice is to use WHERE for filtering raw data and HAVING when you need to filter based on aggregate values like `SUM()`, `COUNT()`, or `AVG()` after grouping.

For example:

SELECT department, COUNT(*) FROM employees WHERE active = 1 GROUP BY department HAVING COUNT(*) > 5;

Here, `WHERE active = 1` filters employees before grouping, while `HAVING COUNT(*) > 5` filters departments after counting them.

Comment:

This question helps you check if the candidate understands how SQL filtering works in both row-level and group-level operations. A strong candidate should clearly differentiate WHERE and HAVING. Also, listen for examples in their explanation—real examples often show a deeper level of understanding.

What are indexes, and why are they used?

Indexes are special data structures in databases that make it faster to find and retrieve specific data from a table. Think of them like the index at the back of a book—you don’t have to read every page to find what you need. Instead of scanning an entire table row by row, a database uses the index to jump directly to the relevant data.

Indexes are used to improve the speed of data retrieval operations, especially for large tables. They can significantly reduce the time it takes to run queries that filter or sort by specific columns. However, it's important to know that while indexes improve read performance, they can slow down write operations like insert, update, or delete because the index also needs to be updated.

Types of indexes to know:

• Clustered Index: Organizes the data rows in the table based on the index key. A table can have only one clustered index.
• Non-Clustered Index: Holds a separate structure from the data rows and uses pointers to find specific rows. Tables can have many non-clustered indexes.
• Unique Index: Makes sure that no duplicate values exist in the indexed column.
• Composite Index: An index that covers more than one column, useful for queries that filter on multiple fields.

Best practice tip: Don’t overuse indexes. Use them on columns that are often used in WHERE, JOIN, or ORDER BY clauses, and monitor their impact on your database's performance over time.

What is the difference between DELETE and TRUNCATE commands?

This question is a great way to assess a candidate’s understanding of database operations—especially if you're hiring for data-driven or backend-focused roles. It helps you see if they know how data removal works on a fundamental level.

Best practice: Ask this if your role involves handling large datasets, SQL databases, or backend maintenance.

Here’s what to look for in their answer:

• DELETE is a Data Manipulation Language (DML) command. It removes records row by row and logs each deletion in the transaction log. Because of this, DELETE commands can be rolled back and can use a `WHERE` clause to specify which rows to delete.

• TRUNCATE is a Data Definition Language (DDL) command. It removes all rows in a table at once, does not log individual deletions, and is much faster for large datasets. It cannot use a `WHERE` clause and usually cannot be rolled back.

What makes a good response: An ideal candidate will mention that:

• DELETE is more flexible because it allows row filtering with WHERE.
• TRUNCATE is faster, but less flexible.
• DELETE is logged per row, so it's slower but safer.
• TRUNCATE is minimally logged and resets identity columns (depending on the database system).

Red flags: If a candidate says DELETE and TRUNCATE are the same, or doesn’t mention transaction behavior or logging differences, it shows a gap in SQL fundamentals.

What is the purpose of the SQL ORDER BY clause?

The `ORDER BY` clause in SQL is used to sort the result set of a query. It arranges the data based on one or more columns in either ascending (ASC) or descending (DESC) order. By default, it sorts in ascending order.

Example:

SELECT first_name, last_name FROM employees ORDER BY last_name ASC;

This will return the list of employees sorted by their last names in alphabetical order.

Comment:

The `ORDER BY` clause sorts the result set of a query in either ascending (default) or descending order, based on one or more columns. This helps present the data in a more meaningful or readable sequence.

What to look for in candidates:

• Clear understanding of how sorting works in SQL.
• Ability to apply `ORDER BY` in practical queries.
• Awareness that it can improve data reporting and usability.
• Mentioning that sorting happens after filtering (i.e., after `WHERE` clause is applied) shows depth of SQL knowledge.

Best practice: Watch for whether the candidate knows how `ORDER BY` affects performance with large datasets, especially when combined with `LIMIT`.

What are the differences between SQL and NoSQL databases?

This question helps you understand a candidate’s technical depth in working with databases — a crucial part of most backend development and data engineering roles. You're not just testing knowledge; you're looking for clarity and practical understanding.

Comment:

SQL Databases:

• Store data in structured tables (rows and columns)
• Have a fixed schema
• Follow ACID (Atomicity, Consistency, Isolation, Durability) properties
• Are ideal for structured data and reliable transactions (e.g., banking systems)

NoSQL Databases:

• Use flexible, schema-less structures like key-value pairs, document stores, wide-column, and graph databases
• Designed for horizontal scaling and handling huge volumes of data
• Focus on performance and scalability over strict consistency
• Best suited for unstructured or semi-structured data and real-time applications (e.g., large-scale web apps, IoT, social media)

Best practice: A well-rounded candidate should know when and why to choose SQL over NoSQL (or vice versa). Look for answers that go beyond definitions and give a real-world example — that’s a sign of hands-on experience.

What is a table in SQL?

A table in SQL is a structured collection of data that’s organized into rows and columns. Each column holds a specific type of data (like text, numbers, or dates), and each row represents a single record related to that data.

Comment:

This is a foundational SQL question that's great for gauging a candidate's basic understanding of databases. A good answer should mention:

• Tables are structured in rows and columns
• Columns define the data types
• Rows represent individual records

Best Practice: Look for candidates who also mention how tables relate to each other using keys—this shows a deeper understanding beyond just the definition. Use this question early in the screening to assess fundamental knowledge before diving into more complex queries or joins.

What are the types of constraints in SQL?

SQL constraints are rules applied to table columns to maintain data accuracy and integrity. Asking this question during a technical interview helps you understand if the candidate grips essential database fundamentals.

Comment:

A strong candidate should mention common SQL constraints like:

• NOT NULL – Ensures the column can’t have a NULL (empty) value.
• UNIQUE – Guarantees each value in a column is distinct.
• PRIMARY KEY – Uniquely identifies each row. Combines NOT NULL and UNIQUE.
• FOREIGN KEY – Maintains referential integrity. Links a record to a key column in another table.
• CHECK – Makes sure values meet a specific condition (like age > 18).
• DEFAULT – Sets a default value for a column when none is given.

Best practice: Look for candidates who can not only list these but also give a real-world example of when they’ve used each. That's where real understanding shows.

What is a cursor in SQL?

A cursor in SQL is a database object that lets you fetch and manipulate rows in a result set, one row at a time. While SQL is typically set-based—meaning operations work on whole tables or result sets—cursors are used when you need to process rows individually in a loop-like fashion.

They’re especially useful for row-by-row processing, such as complex logic that can’t be easily accomplished with standard SQL queries.

There are four main types of cursors, especially in SQL Server:

• STATIC: Takes a snapshot of the result set. Doesn’t reflect changes in the database after the cursor is opened.
• DYNAMIC: Reflects all changes (additions, updates, deletions) made to the rows while the cursor is open.
• FORWARD_ONLY: Can only move forward through the result set. Lightweight and faster than other cursor types.
• KEYSET: Uses a set of keys to identify rows. You see changes to non-key columns, but the set of rows remains fixed.

Comment:

Ask this question to test a candidate’s understanding of row-by-row operations in SQL. Many beginners struggle with cursors, so a solid explanation shows deeper database knowledge. A good answer should touch on when and why to use a cursor, and mention at least a couple of its types. Best practice: prefer set-based operations unless row-level logic is truly needed.

What is a trigger in SQL?

A trigger in SQL is a special kind of stored procedure that runs automatically when a specific event happens in a database table. These events are usually INSERT, UPDATE, or DELETE actions.

Triggers are commonly used to:

• Enforce business rules automatically
• Maintain data consistency
• Keep audit logs without needing extra code in applications
• Prevent invalid transactions or notify systems of changes

There are two main types of triggers:

• BEFORE Triggers: Execute before the data change
• AFTER Triggers: Execute after the data change

Comment:

This question helps you find out if the candidate understands automated processes in databases and how they relate to maintaining data quality. Good answers will mention triggers firing on events like INSERT, UPDATE, DELETE, and may include real-life examples like auditing or validation. Best practice: Look for someone who not only defines triggers but also explains why and when to use them effectively.

What is the purpose of the SQL SELECT statement?

The SQL `SELECT` statement is used to retrieve data from a database. It's the most commonly used command in SQL because it allows users to extract specific information from one or more tables.

Comment:

The `SELECT` statement retrieves data from one or more tables. It is the most commonly used command in SQL, allowing users to filter, sort, and display data based on specific criteria.

Best practice tip: When screening for SQL skills, look for candidates who understand how to use `SELECT` with clauses like `WHERE`, `ORDER BY`, `JOIN`, and `GROUP BY`. This shows they're capable of working with real-world data queries, not just basic commands.

Good answers should reflect:

• Clear understanding of the purpose: data retrieval
• Knowledge of optional clauses to refine results
• Examples of queries they've worked on in previous roles

Avoid candidates who only give textbook definitions without real usage examples.

What are NULL values in SQL?

NULL in SQL means a value is missing or unknown. It doesn't mean zero, and it’s not the same as an empty string. It simply means “nothing here.”

During candidate screening, asking about NULL values is a great way to test for foundational knowledge in SQL. This question helps reveal:

• If the candidate understands the difference between NULL and other "empty" values.
• How they handle data that’s incomplete or missing in queries.
• If they know how NULLs affect things like `JOINs`, `WHERE` clauses, or `aggregates`.

Best practice answer: Look for candidates who explain that NULL is used to represent unknown or missing information, and who can give examples—such as what happens when you try to compare two NULLs, or how to check for NULL using `IS NULL`.

Watch out for: Answers that confuse NULL with zero or blank strings. It’s a common mistake and may signal a surface-level understanding.

When assessing responses, look for clarity, real-world examples, and confidence in handling SQL behavior around NULLs.

What is a stored procedure?

A stored procedure is a precompiled group of SQL statements stored in a database. It can accept input parameters, run logic or calculations, query tables, and return either output values or full result sets.

Comment:

Stored procedures are useful when you want to centralize and reuse business logic. They help improve performance because the SQL code is compiled and stored on the server. It's a best practice to ask candidates how and when they’ve used stored procedures—this shows they understand not just what they are, but how they improve application performance and simplify complex operations. Good answers often include examples involving data validation, transformation, or automation of repetitive tasks.

💡 Remaining 528 questions...

The online article only includes the first 30 questions to keep it digestible, but we’ve put together an ebook for you with all the questions we gathered through our extensive research.

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Real-World SQL Interview Success Stories

Leading tech companies are continuously raising the bar for SQL interview questions, and candidates who prepare with real-world scenarios often see remarkable success rates. Google, Oracle, Amazon, and Microsoft have all implemented comprehensive SQL screening processes that test everything from basic query optimization to complex database design patterns.

One notable example comes from industry expert Prateek Narang, a 5-star instructor on Udemy with over 50,641 enrolled students in his SQL courses. His teaching methodology focuses on practical interview scenarios that mirror what candidates actually face at top-tier companies. Narang's approach emphasizes understanding PostgreSQL fundamentals - a database system originally developed in 1986 by Computer Science Professor Michael Stonebraker's team. This historical context often appears in technical interviews as companies value candidates who understand the evolution of database technologies.

The success stories from Narang's students highlight how proper preparation with realistic sql interview questions can dramatically improve hiring outcomes. Many students report landing positions at major tech companies after completing structured SQL interview preparation programs.

Building a comprehensive technical screening process? Combine these SQL questions with Python interview questions and Selenium interview questions for well-rounded evaluation.

Why Video Screening Software is Revolutionizing SQL Interviews

The recruitment landscape is experiencing a massive shift toward video screening technology, especially for technical roles requiring SQL expertise. Traditional phone screenings and lengthy in-person interviews are being replaced by efficient, standardized video assessments that save both time and resources.

Here's why more companies are adopting video screening for SQL interview questions:

• Consistent evaluation standards across all candidates
• Significant time savings for hiring teams
• Better candidate experience with flexible scheduling
• Reduced unconscious bias through structured assessments
• Scalable screening process for high-volume hiring

Video screening platforms allow recruiters to present real sql interview questions in a controlled environment while capturing authentic responses. This technology enables companies to identify top SQL talent faster while providing candidates with a more comfortable interview experience.

Ready to streamline your SQL candidate screening process? Discover how CandidateScreenings.com can transform your technical hiring and help you find the best SQL developers efficiently.