You Need to Know these SQL Server Interview Questions

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?

Benefits:

• 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.

Drawbacks:

• 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.

• Prevention:

  • 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.

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