Unknown top 25 MS SQL interview questions

5 min readApr 30, 2024

The top 25 MS SQL interview questions along with detailed explanations and categorized into different topics:

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Basic SQL Concepts:

What is SQL and its purpose?

SQL (Structured Query Language) is a standard language for managing and manipulating relational databases. Its purpose is to perform tasks such as querying, updating, inserting, and deleting data in databases.

What are the different types of SQL commands?

SQL commands can be categorized into four main types:
Data Definition Language (DDL) for defining database structures.
Data Manipulation Language (DML) for manipulating data.
Data Control Language (DCL) for controlling access to data.
Transaction Control Language (TCL) for managing transactions.

Querying Data:

What is the difference between SELECT and SELECT DISTINCT?

SELECT retrieves all records from a table, while SELECT DISTINCT retrieves only unique records, eliminating duplicates.

Explain the difference between WHERE and HAVING clauses.

WHERE is used to filter rows before grouping in a query, while HAVING is used to filter groups after grouping has occurred.

Joins and Relationships:

What are the different types of SQL joins?

SQL joins include INNER JOIN, LEFT JOIN (or LEFT OUTER JOIN), RIGHT JOIN (or RIGHT OUTER JOIN), and FULL JOIN (or FULL OUTER JOIN).

Explain the difference between INNER JOIN and OUTER JOIN.

INNER JOIN returns only matching rows from both tables, while OUTER JOIN returns all rows from at least one of the tables, including unmatched rows.

Indexing and Performance Optimization:

What is an index in SQL and why is it used?

An index is a database object that improves the speed of data retrieval operations on a table. It allows SQL queries to quickly locate rows based on the indexed column values.

What are clustered and non-clustered indexes?

A clustered index determines the physical order of rows in a table, while a non-clustered index does not affect the physical order of rows but improves query performance by creating a separate data structure.

Stored Procedures and Functions:

What is a stored procedure and how is it different from a function?

A stored procedure is a precompiled collection of SQL statements stored in the database and can be executed by calling its name. A function is a precompiled SQL statement that returns a single value and can be used in SQL queries.

What are the advantages of using stored procedures?

Some advantages of stored procedures include improved performance, enhanced security, reduced network traffic, and better code modularity and reusability.

Transactions and Concurrency:

Explain ACID properties in the context of database transactions.

ACID stands for Atomicity, Consistency, Isolation, and Durability, which are key properties that ensure the reliability and integrity of database transactions.

What is a deadlock in SQL Server? How can it be avoided?

A deadlock occurs when two or more transactions are waiting for each other’s locks and cannot proceed. Deadlocks can be avoided by ensuring consistent locking order, minimizing transaction duration, and using appropriate isolation levels.

Backup and Recovery:

What are the different types of database backups in SQL Server?

SQL Server supports several types of backups, including full backups, differential backups, transaction log backups, and file or filegroup backups.

Explain the process of restoring a database backup in SQL Server.

To restore a database backup in SQL Server, you need to use the RESTORE DATABASE command, specifying the backup file and desired restore options such as WITH REPLACE or WITH RECOVERY.

Security and Permissions:

How do you grant and revoke permissions in SQL Server?

Permissions can be granted and revoked using the GRANT and REVOKE statements, specifying the type of permission (e.g., SELECT, INSERT, UPDATE) and the target object (e.g., table, stored procedure).

What is SQL injection and how can it be prevented?

SQL injection is a security vulnerability that occurs when malicious SQL code is inserted into input fields of an application, allowing attackers to execute unauthorized SQL queries. It can be prevented by using parameterized queries, input validation, and proper authentication mechanisms.

Advanced SQL Concepts:

What are Common Table Expressions (CTEs) and when are they used?

CTEs are temporary result sets that are defined within the scope of a single SQL statement. They are often used to simplify complex queries, improve readability, and enable recursive queries.

Explain window functions and provide examples of their usage.

Window functions are SQL functions that perform calculations across a set of rows related to the current row. Examples include ROW_NUMBER(), RANK(), and NTILE(), which are commonly used for analytical tasks such as ranking and partitioning data.

Performance Tuning and Optimization:

What are the different tools and techniques available for performance tuning in SQL Server?

Performance tuning in SQL Server involves various tools and techniques such as query optimization, index tuning, database design optimization, and server configuration tuning.

Explain query optimization and provide examples of optimization techniques.

Query optimization involves analysing and restructuring SQL queries to improve performance. Techniques include using proper indexing, rewriting queries, avoiding unnecessary joins, and optimizing data access patterns.

Database Design and Normalization:

What is database normalization and why is it important?

Database normalization is the process of organizing data in a relational database to reduce redundancy and dependency. It improves data integrity, reduces storage space, and enhances query performance.

Explain the different normal forms in database normalization.

The different normal forms (NF) in database normalization include First Normal Form (1NF), Second Normal Form (2NF), Third Normal Form (3NF), and Boyce-Codd Normal Form (BCNF), each with specific rules for eliminating data redundancy and dependency.

Data Migration and Integration:

What are the common methods for data migration in SQL Server?

Data migration in SQL Server can be performed using various methods such as backup and restore, detach and attach, Import/Export Wizard, Bulk Copy Program (BCP), and SQL Server Integration Services (SSIS).

Explain the role of SQL Server Integration Services (SSIS) in data integration.

SQL Server Integration Services (SSIS) is a powerful ETL (Extract, Transform, Load) tool provided by Microsoft for building data integration and workflow solutions. It enables developers to extract data from various sources, transform it according to business rules, and load it into destination systems.

Data Warehousing and Business Intelligence:

What is a data warehouse and how does it differ from a traditional database?

A data warehouse is a central repository of integrated data from multiple sources, designed for querying and analysis to support decision-making processes. Unlike traditional databases optimized for transactional processing, data warehouses are optimized for analytical queries and reporting.

Conclusion:

These top 25 MS SQL interview questions cover a broad range of topics essential for mastering SQL Server administration, database development, and data management. By understanding these concepts and practicing with real-world scenarios, candidates can confidently tackle MS SQL interviews and excel in their careers as database professionals.