10 Microsoft SQL Server Interview Questions and Answers

Microsoft SQL Server is a robust relational database management system widely used for enterprise-level database solutions. Known for its scalability, security features, and integration capabilities, SQL Server supports a variety of data analytics, business intelligence, and transaction processing applications. Its comprehensive suite of tools and services makes it a preferred choice for managing and analyzing large volumes of data efficiently.

This article offers a curated selection of interview questions designed to test your knowledge and proficiency with Microsoft SQL Server. By reviewing these questions and their detailed answers, you will be better prepared to demonstrate your expertise and problem-solving abilities in SQL Server during your upcoming interviews.

Microsoft SQL Server Interview Questions and Answers

1. Write a SQL query to retrieve all records from a table named ‘Employees’ where the ‘Department’ is ‘Sales’.

To retrieve all records from a table named ‘Employees’ where the ‘Department’ is ‘Sales’, use the following SQL query:

SELECT * FROM Employees WHERE Department = 'Sales';

2. Write a SQL query to join two tables, ‘Orders’ and ‘Customers’, to find all orders placed by customers from ‘New York’.

To join two tables, ‘Orders’ and ‘Customers’, and find all orders placed by customers from ‘New York’, use an SQL INNER JOIN. This selects records with matching values in both tables, joining on the ‘CustomerID’ column and filtering for customers from ‘New York’.

Example:

SELECT Orders.OrderID, Orders.OrderDate, Customers.CustomerName, Customers.City
FROM Orders
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID
WHERE Customers.City = 'New York';

3. Write a SQL query to create a view that shows the total sales per customer from the ‘Sales’ table.

A SQL view is a virtual table based on an SQL query’s result-set. Views simplify complex queries by encapsulating them into a single, reusable query. To create a view showing total sales per customer from the ‘Sales’ table:

CREATE VIEW TotalSalesPerCustomer AS
SELECT CustomerID, SUM(SalesAmount) AS TotalSales
FROM Sales
GROUP BY CustomerID;

This creates a view named ‘TotalSalesPerCustomer’ that aggregates total sales for each customer by summing the ‘SalesAmount’ column and grouping by ‘CustomerID’.

4. Describe three techniques you would use to improve the performance of a slow-running query.

To improve the performance of a slow-running query, consider these techniques:

• Indexing: Create indexes on columns frequently used in WHERE clauses, JOIN conditions, and ORDER BY clauses to enhance query performance. Balance the number of indexes to avoid increased maintenance overhead and slower write operations.
• Query Optimization: Optimize the query by using efficient SQL constructs, avoiding unnecessary columns in SELECT statements, and ensuring appropriate use of subqueries and joins. Use query execution plans to identify bottlenecks.
• Database Design: Ensure a well-designed database schema. Normalize the database to reduce redundancy and improve data integrity. In some cases, denormalization might be beneficial for read-heavy workloads. Partitioning large tables and using appropriate data types can also improve performance.

5. What are the differences between CHAR, VARCHAR, and NVARCHAR data types, and when would you use each?

CHAR, VARCHAR, and NVARCHAR are data types for storing character data, each with distinct differences and use cases.

1. CHAR (Fixed-Length Character Data)

• Stores fixed-length character data. The length is defined when the table is created, using the same storage regardless of actual data length.
• Use CHAR for data entries expected to be of the same length, like fixed-length codes.

2. VARCHAR (Variable-Length Character Data)

• Stores variable-length character data, using storage as needed for actual data plus an additional byte for length.
• Use VARCHAR for data entries that vary significantly in length, like names or descriptions.

3. NVARCHAR (Variable-Length Unicode Character Data)

• Similar to VARCHAR but stores variable-length Unicode character data, using two bytes per character.
• Use NVARCHAR for data entries needing support for multiple languages or special characters.

6. Write a SQL query using dynamic SQL to select records from a table whose name is passed as a parameter.

Dynamic SQL allows constructing and executing SQL statements at runtime, useful when the table name or other query parts are unknown until execution.

Example of using dynamic SQL to select records from a table whose name is passed as a parameter:

CREATE PROCEDURE SelectFromTable
    @TableName NVARCHAR(128)
AS
BEGIN
    DECLARE @SQL NVARCHAR(MAX)
    SET @SQL = N'SELECT * FROM ' + QUOTENAME(@TableName)
    EXEC sp_executesql @SQL
END

In this example, the SelectFromTable stored procedure takes a table name as a parameter. The QUOTENAME function safely includes the table name in the SQL statement, preventing SQL injection attacks. The sp_executesql system stored procedure executes the dynamically constructed SQL statement.

7. Explain the steps involved in performing a full backup and restore of a database.

Performing a full backup and restore of a database involves:

1. Full Backup:

• Use SQL Server Management Studio (SSMS) or T-SQL commands to start the backup process.
• Choose the database, backup type (full), and destination for the backup file.
• Run the backup operation, ensuring the backup file is created successfully.

2. Restore:

• Ensure the database to be restored is not in use and you have necessary permissions.
• Use SSMS or T-SQL commands to start the restore process.
• Choose the backup file, target database, and any additional options like overwriting the existing database.
• Run the restore operation, ensuring the database is restored successfully.

8. Explain the concept of database normalization and its importance.

Database normalization structures a relational database to reduce data redundancy and improve data integrity. The primary goals are to eliminate redundant data and ensure data dependencies make sense.

Normalization involves several stages, known as normal forms:

• First Normal Form (1NF): Ensures the table has a primary key and all columns contain atomic values.
• Second Normal Form (2NF): Achieved when the table is in 1NF and all non-key attributes are fully functional dependent on the primary key.
• Third Normal Form (3NF): Achieved when the table is in 2NF and all attributes are functionally dependent only on the primary key.
• Boyce-Codd Normal Form (BCNF): A stronger version of 3NF where every determinant is a candidate key.

Normalization is important because it:

• Reduces data redundancy, saving storage space and improving performance.
• Improves data integrity by ensuring data is logically stored and related.
• Makes it easier to maintain and update the database, as changes in one place do not require changes in multiple places.

9. Discuss best practices for securing a SQL Server environment.

Securing a SQL Server environment involves several practices to ensure data integrity, confidentiality, and availability:

• Authentication: Use Windows Authentication, enforce strong passwords, and consider multi-factor authentication (MFA).
• Authorization: Grant users the minimum permissions necessary, use roles to manage permissions, and avoid granting permissions directly to users.
• Encryption: Encrypt sensitive data at rest and in transit using Transparent Data Encryption (TDE) and Secure Sockets Layer (SSL).
• Auditing and Monitoring: Enable auditing to track access and changes, use SQL Server Audit and Extended Events, and regularly review logs.
• Patch Management: Keep SQL Server and the operating system up to date with security patches and updates.
• Backup and Recovery: Implement a robust backup strategy, regularly test backups, and store them securely.
• Network Security: Use firewalls to restrict access, implement Virtual Private Networks (VPNs) for remote access, and isolate the SQL Server in a secure network segment.

10. Explain the role of SQL Server Integration Services (SSIS) and provide an example use case.

SQL Server Integration Services (SSIS) is a tool for data integration and workflow applications. It extracts data from various sources, transforms it according to business rules, and loads it into a destination database.

Example Use Case:
A retail company needs to consolidate sales data from multiple stores into a central data warehouse for reporting. Each store maintains its sales data locally, and the data needs to be extracted, transformed, and loaded into the central warehouse daily. SSIS can automate this process.

• Extract: SSIS connects to each store’s database and extracts sales data.
• Transform: The data is cleaned and transformed for consistency, such as standardizing date formats and removing duplicates.
• Load: The transformed data is loaded into the central data warehouse.