You Need to Know these SQL Server Interview Questions
Questions & Answers to get you through the initial phone interview
My technical recruiter friends have emailed me the top questions they ask when SQL Server is a core skill needed in a job.
Below is a compilation of those questions. I’ve made the answers in simple bullet points.
Print this page and have it handy for your phone interview. Think of it as a quick reference guide, so you don’t get stuck.
Note, they normally ask only 3 or 4 of these questions on a 1st or 2nd phone/video interview. Generally, easier questions are asked first.
SQL Server Interview Questions & Answers
Question 1: What is the difference between clustered and non-clustered indexes in SQL Server?
Clustered indexes: sort and store data rows based on their key values, only one per table, and directly affect the physical order of data storage.
Non-clustered indexes: store a separate copy of the indexed data with a reference (pointer) to the actual data, multiple per table, and do not affect the physical order of data storage.
Clustered indexes improve query performance for range-based queries and sorting.
Non-clustered indexes improve performance for specific queries (e.g., using WHERE, JOIN, or GROUP BY clauses) and don't impact data modification operations as much.
Question 2: Explain the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN in SQL Server.
INNER JOIN: returns rows where there's a match between the joined tables based on the specified condition.
LEFT JOIN: returns all rows from the left table and the matched rows from the right table; unmatched rows are filled with NULL values.
RIGHT JOIN: returns all rows from the right table and the matched rows from the left table; unmatched rows are filled with NULL values.
FULL OUTER JOIN: returns all rows from both tables, filling unmatched rows with NULL values.
Question 3: What are the primary differences between DELETE, TRUNCATE, and DROP statements in SQL Server?
DELETE: removes one or more rows from a table based on a specified condition, can be rolled back, and triggers are fired.
TRUNCATE: removes all rows from a table, faster than DELETE, can be rolled back in certain cases, and triggers are not fired.
DROP: completely removes the table with its data and schema, cannot be rolled back, and triggers, constraints, and other dependencies are also removed.
DELETE and TRUNCATE are data manipulation operations, while DROP is a data definition operation.
Question 4: What is normalization, and what are its different forms (1NF, 2NF, 3NF)?
Normalization: the process of organizing a database to reduce redundancy, increase consistency, and improve query performance.
1NF (First Normal Form): ensures that each column contains atomic values and each row has a unique identifier (primary key).
2NF (Second Normal Form): in addition to 1NF, removes partial dependencies by ensuring that non-key attributes are fully dependent on the primary key.
3NF (Third Normal Form): in addition to 2NF, removes transitive dependencies by ensuring that non-key attributes are directly dependent on the primary key and not on other non-key attributes.
Question 5: What is the difference between a primary key and a unique key constraint in SQL Server?
Primary key: a column or a set of columns that uniquely identifies each row in a table, cannot have NULL values, and each table can have only one primary key.
Unique key: a column or a set of columns that uniquely identifies each row in a table, can have NULL values, and multiple unique keys can be defined per table.
Both primary and unique keys enforce uniqueness, ensuring data integrity and consistency.
Primary keys are often used as a reference for foreign key constraints, while unique keys can be used when multiple unique identifiers exist.
Question 6: Explain the difference between a stored procedure and a user-defined function in SQL Server.
Stored procedures: precompiled and reusable SQL code, can perform data manipulation and return multiple values, and support output parameters and control-of-flow statements.
User-defined functions: reusable SQL code that returns a single value, table, or a table-valued result, can be used in SELECT statements, and do not support output parameters or control-of-flow statements.
Both can help modularize and reuse code, improving code organization, maintainability, and performance.
Stored procedures are generally used for complex business logic, while user-defined functions are used for calculations or data manipulation that must be part of a query.
Question 7: What are the benefits and drawbacks of using stored procedures in SQL Server?
Code reusability: stored procedures can be called multiple times, reducing code duplication and improving maintainability.
Performance: precompiled execution plans speed up query execution and reduce server overhead.
Centralized logic: simplifies maintenance, debugging, and updating by keeping business logic in one place.
Security: allows granting execution permissions without providing direct access to the underlying tables, enhancing data protection.
Limited portability: stored procedures are specific to the database platform, making migration between platforms more challenging.
Debugging complexity: debugging stored procedures can be more difficult than debugging inline SQL code or application code.
Development challenges: working with stored procedures may require specialized knowledge, which can be a learning curve for some developers.
Version control: integrating stored procedures with standard version control systems can be more difficult than managing application code.
Reduced flexibility: changes to the stored procedures may require server access, making it harder to deploy updates in certain environments.
Question 8: What is the difference between a scalar function and a table-valued function in SQL Server?
Scalar function: returns a single value, can be used in a SELECT clause or other expressions, and accepts one or multiple input parameters.
Table-valued function: returns a table as output, can be used in a SELECT statement like a table, and accepts one or multiple input parameters.
Scalar functions are useful for calculations or transformations on single values, while table-valued functions are used for returning multiple rows or columns of data.
Both types of functions help modularize and reuse code, but address different use cases and output requirements.
Question 9: What is the purpose of using SQL Server Profiler, and what can it monitor?
SQL Server Profiler: a graphical tool that allows monitoring and analyzing SQL Server events, helping diagnose and troubleshoot performance issues.
Monitors: T-SQL statements, stored procedures, and triggers execution; login and logout events; deadlocks; data and schema changes; and query performance statistics.
Profiler helps identify slow-running queries, optimize indexes, find bottlenecks, and improve overall database performance.
Assists in understanding the flow of application code, identifying potential security risks, and auditing database activity.
Question 10: What are the main differences between SQL Server Authentication and Windows Authentication?
SQL Server Authentication: users are authenticated using SQL Server credentials (username and password), managed within SQL Server, and can be used across different Windows domains or non-Windows platforms.
Windows Authentication: users are authenticated using their Windows credentials, managed by the Windows operating system, and is considered more secure than SQL Server Authentication.
SQL Server Authentication is more flexible, but less secure due to potential password vulnerabilities and the need for manual password management.
Windows Authentication provides tighter security, simplified user management, and supports advanced features like Kerberos authentication and password policies.
Question 11: What is a deadlock, and how can it be resolved or prevented in SQL Server?
Deadlock: a situation where two or more transactions are waiting for each other to release a locked resource, causing a standstill and preventing the transactions from completing.
Resolution: SQL Server automatically detects and resolves deadlocks by choosing a victim transaction and rolling it back, allowing other transactions to proceed.
Access objects in a consistent order to avoid circular waiting.
Keep transactions short and focused to minimize lock duration.
Use appropriate isolation levels to reduce lock contention.
Implement retries for the victim transaction, allowing it to reattempt after a deadlock situation.
Use query hints like NOLOCK or ROWLOCK to control the locking behavior.
Question 12: What is the purpose of the SQL Server Agent, and what types of tasks can it automate?
SQL Server Agent: a background service that automates and schedules administrative tasks, such as running T-SQL scripts, executing SSIS packages, or managing backups.
Automatable tasks: backups and maintenance tasks, database health monitoring, report generation, data imports and exports, and executing stored procedures or scripts at specified intervals.
Enhances reliability by automating repetitive tasks, reducing human error, and ensuring tasks run even during off-hours.
Facilitates central management of tasks and monitoring job execution history, simplifying administration.
Question 13: What is the difference between the HAVING and WHERE clauses in SQL Server?
WHERE clause: filters rows based on specified conditions before any aggregation, grouping, or calculation occurs, and operates on individual rows.
HAVING clause: filters rows after aggregation, grouping, or calculation has taken place, and operates on the result of the GROUP BY clause.
The WHERE clause is used to filter data based on column values, while the HAVING clause is used to filter data based on aggregate function results (e.g., COUNT, SUM, AVG).
Both clauses help refine query results, but they are used at different stages of the query execution process.
Question 14: What is the purpose of using the PIVOT and UNPIVOT operators in SQL Server, and when should they be used?
PIVOT: transforms rows into columns, consolidating data and creating a summary table, often used for creating cross-tabular reports or aggregating data for analysis.
UNPIVOT: transforms columns into rows, normalizing data and making it easier to query, often used for transforming denormalized or pivoted data into a more query-friendly format.
Both operators help reshape data and facilitate reporting, analysis, and querying.
PIVOT is used when aggregating data and creating a summary view, while UNPIVOT is used when normalizing data and preparing it for further querying or processing.
Question 15: What is the purpose of using the SQL Server Integration Services (SSIS), and what are its main components?
SSIS: an ETL (Extract, Transform, Load) tool that facilitates data integration, data transformation, and data migration between various data sources and SQL Server.
Main components: Control Flow (orchestrates tasks and containers), Data Flow (manages data movement and transformations), and Event Handlers (handles events raised during package execution).
SSIS is used for data integration, data cleansing, data warehousing, and data migration tasks, as well as for automating administrative tasks (e.g., backups, data synchronization).
Provides a visual development environment, simplifying the design and management of complex data workflows.
Question 16: What are the main differences between the CHAR, VARCHAR, and NVARCHAR data types in SQL Server?
CHAR: a fixed-length character data type, with the storage size equal to the maximum size defined, and any unused space is padded with spaces.
VARCHAR: a variable-length character data type, with the storage size equal to the actual length of the data plus 2 bytes for overhead, and supports a maximum size of 8,000 characters.
NVARCHAR: a variable-length Unicode character data type, with the storage size equal to twice the actual length of the data plus 2 bytes for overhead, and supports a maximum size of 4,000 characters.
CHAR is suitable for storing fixed-length data, while VARCHAR and NVARCHAR are suitable for storing variable-length data with varying space requirements.
Question 17: What is the purpose of using the SQL Server Reporting Services (SSRS), and what are its main features?
SSRS: a server-based reporting platform that enables the creation, deployment, and management of various types of reports and data visualizations, providing insights and business intelligence from the data stored in SQL Server.
Main features: Report Designer (a visual report creation tool), Report Server (a centralized report repository and execution engine), and Report Manager (a web-based interface for managing and delivering reports).
SSRS supports various report types, including tabular, matrix, chart, and map-based reports, as well as custom visualizations using embedded code or custom assemblies.
Provides extensive formatting and data presentation options, including conditional formatting, grouping, sorting, and drill-down capabilities.
Question 18: What is the purpose of using SQL Server Analysis Services (SSAS), and what are its main components?
SSAS: a multidimensional data analysis and data mining platform that helps organizations gain insights from their data, supporting both OLAP (Online Analytical Processing) and data mining capabilities.
Main components: Multidimensional (OLAP) databases (cubes), Tabular databases (using the in-memory VertiPaq engine), and Data Mining Models.
SSAS enables the creation and management of complex data models, the development of KPIs (Key Performance Indicators), and the analysis of large datasets.
Provides faster query performance and data analysis capabilities through pre-aggregated data structures and caching mechanisms.
All the best. Cheers Shano.