MS SQL Interview Questions and Answers

Introduction

Microsoft SQL is a popular system that helps people store and manage data easily. It plays a vital role in enterprise applications, business intelligence, reporting, and data analytics across various industries. Many companies use MS SQL Server to run their businesses, so they need people who are good at working with databases. This list of Microsoft SQL Interview Questions and Answers includes common questions people are asked in interviews. The questions cover Microsoft SQL concepts and also some more advanced ideas. Whether you are just a fresher or an experienced professional, these Microsoft SQL questions will help you understand the concepts and feel more confident about a technical interview. Discover our MS SQL Course Syllabus to build a strong foundation in database management.

MS SQL Interview Questions for Freshers

1. What is MS SQL Server?

MS SQL Server is a relational database management system (RDBMS) developed by Microsoft. It stores, manages, and retrieves structured data efficiently. It supports SQL and Transact-SQL (T-SQL) for performing database operations and is widely used in enterprise applications.

2. What is the difference between DBMS and RDBMS?

• DBMS
  • Stores data without creating relationships between tables.
  • Suitable for small applications.
  • Offers limited data integrity.
• RDBMS
  • Stores data in related tables.
  • Supports relationships using keys.
  • Ensures data integrity and handles large databases.

3. What are the main categories of SQL commands?

There are three main types of SQL commands.

• DDL: Creates or modifies database objects such as tables and views.
• DML: Inserts, updates, and deletes records.
• DQL: Retrieves data using the SELECT statement.
• DCL: Manages user access and permissions.
• TCL: Controls database transactions.

4. What is the difference between Primary Key and Unique Key?

• A Primary Key is special because it identifies each record in a table. Duplicate values are not permitted.
• A Unique Key also prevents duplicate values but allows one NULL value in MS SQL Server.
• A database table can contain only a single Primary Key. It can have many Unique Keys.

5. What is a Foreign Key and why is it used?

• Creates a relationship between two tables.
• References the Primary Key of another table.
• Maintains referential integrity.
• Prevents invalid or orphan records.
• Helps keep related data accurate and consistent.

6. Can you explain the differences between DELETE, TRUNCATE, and DROP in SQL?

• DELETE removes rows and can use the WHERE command.
• TRUNCATE removes all rows. Keeps the table structure.
• DROP removes the complete table, including its structure, indexes, and stored data.

7. What are Joins in SQL, and what are the main types?

Joins are used to combine data from multiple related tables based on a common column.

• There are four types of joins:
  • INNER JOIN
  • LEFT JOIN
  • RIGHT JOIN
  • FULL JOIN

Each type of join returns data.

8. What is the difference between WHERE and HAVING clauses?

• WHERE 
  • Filters rows before grouping.
  • Cannot use functions that add up or count data.
• HAVING 
  • Filters grouped results after GROUP BY.
  • Mainly used with aggregate functions such as COUNT() and SUM().

9. What is an index, and why is it useful?

An Index is something that helps the database find data faster. It improves how fast the database can get data. 

There are two types of Indexes: 

• Clustered Index
  • A Clustered Index keeps the data in order. 
• Non-Clustered Index. 
  • A Non-Clustered Index keeps pointers to the data.

10. What are Constraints in SQL?

Constraints are rules that help keep the data accurate. They make sure good data gets into the tables. There are types of Constraints, including PRIMARY KEY, FOREIGN KEY, NOT NULL, UNIQUE, CHECK, and DEFAULT. They help keep the data consistent and accurate.

11. What distinguishes UNION from UNION ALL in SQL?

• UNION combines data from two or more tables. Removes duplicates.
• UNION ALL combines data from two or more tables. Does not remove duplicates.
• UNION ALL is faster because it does not have to check for duplicates.

12. What is a subquery, and what are its types?

• A Subquery is a query that’s inside another query. There are two types of Subqueries:
• Independent Subquery: Executes once before the outer query.
• Correlated Subquery: Executes repeatedly for each row processed by the outer query.

13. What are the ACID properties in a database?

ACID properties are rules that help keep the database safe. The core ACID principles include Atomicity, Consistency, Isolation, and Durability. These properties help keep the data accurate and consistent even if something goes wrong.

14. What is a View in SQL?

• A View is a virtual table created using a SELECT query.
• It does not store data physically.
• Simplifies complex queries.
• Improves security by restricting direct access to base tables.

15. How do you find duplicate rows in an MS SQL table?

To find duplicate rows, you can group records and count their occurrences. The GROUP BY command groups values, and HAVING COUNT(*) > 1 returns only records that appear more than once.

• For example:

SELECT Email, COUNT(*)

FROM Users

GROUP BY Email

HAVING COUNT(*) > 1;

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MS SQL Interview Questions for Experienced Candidates

1. What is the difference between a Clustered Index and a Non-Clustered Index?

• A Clustered Index keeps the actual table data in order based on the indexed column. A table can have one clustered index because data can be arranged in only one physical order.
• A Non-Clustered Index keeps the index separate from the table data. Has pointers to the corresponding rows. You can create non-clustered indexes on a table to improve query performance with a Clustered Index and a Non-Clustered Index.

2. What are Common Table Expressions (CTEs), and when do you use Recursive CTEs?

• A CTE is a temporary, named result set that simplifies complex SQL queries.
• It improves query readability and organization.
• Can be used with SELECT, INSERT, UPDATE, and DELETE statements.
• Recursive CTEs are useful for data like employee-manager relationships.

3. How do you find and eliminate duplicate records in a table?

• Find duplicates with GROUP BY and HAVING COUNT(*) > 1.
• Use ROW_NUMBER() with a CTE to spot duplicates.
• Delete records where the row number is greater than one.
• Helps keep unique records in a table.

4. What are Window Functions? Explain ROW_NUMBER, RANK, and DENSE_RANK.

Window functions do calculations across a group of rows without combining them into a single result.

Common functions include 

• ROW_NUMBER(), which assigns numbers.
• RANK(), which skips ranking numbers after duplicates.
• DENSE_RANK(), which assigns ranks without gaps.

These functions are widely used for ranking, reporting, and data analysis with Window Functions.

5. What is a deadlock, and how can you minimize its occurrence?

• A deadlock occurs when transactions wait for each other indefinitely.
• Keep transactions short.
• Access database objects in a consistent order.
• Create proper indexes to reduce locking.
• Choose suitable transaction isolation levels.

6. What is the difference between a Correlated Subquery and a Non-Correlated Subquery?

• Correlated Subquery
  • Depends on the outer query.
  • Executes once for each row processed by the outer query.
  • Useful for row-by-row comparisons.
  • May be slower for large datasets.
• Non-Correlated Subquery
  • Runs independently of the outer query.
  • Executes only once.
  • Passes the result to the main query.
  • Generally provides better performance.

7. Explain Transaction Isolation Levels in SQL Server.

Transaction isolation levels decide how SQL Server handles transactions.

• The available levels include Read Uncommitted, Read Committed, Repeatable Read, Serializable, and Snapshot.
• Each level provides a balance between data consistency and system performance.

8. How do you optimize a slow-running SQL query?

• Review the execution plan.
• Create or update indexes.
• Update database statistics.
• Avoid unnecessary functions in WHERE clauses.
• Simplify complex queries using CTEs or temporary tables.

9. What is the difference between an Index Scan and an Index Seek?

• Index Scan
  • Reads the entire index to find matching records.
  • Takes more time for large tables.
  • Occurs when SQL Server cannot directly locate the required data.
• Index Seek
  • Directly locates the required records using the index.
  • Provides faster query performance.
  • Occurs when suitable indexes are available.

10. Explain the difference between COALESCE() and ISNULL() in SQL Server.

• COALESCE()
  • Returns the first non-NULL value from multiple expressions.
  • Accepts two or more arguments.
  • Follows the ANSI SQL standard.
• ISNULL()
  • Replaces a NULL value with another value.
  • Accepts only two arguments.
  • Is specific to Microsoft SQL Server.

11. What are Database Triggers? When should they be avoided?

• Triggers happen automatically after INSERT, UPDATE, or DELETE.
• Used to enforce business rules.
• Can maintain audit logs.
• Use them carefully because they may affect performance.

12. What is Partition Pruning and why is it beneficial?

• It helps with partitioned tables.
• Reads only needed partitions.
• Reduces disk work.
• Speeds up queries on large datasets.

13. How would you retrieve the Nth-highest salary from an Employee table?

• Use ROW_NUMBER() DENSE_RANK() or OFFSET-FETCH.
• Sort salaries in descending order.
• Return the required ranked salary.
• Commonly asked in SQL interviews.

14. What is Parameter Sniffing, and how do you mitigate its negative effects?

Parameter Sniffing occurs when SQL Server creates an execution plan based on the parameter values used during the execution of a query or stored procedure. While this often improves performance, it can also slow down future executions if different parameter values require a different execution plan.

15. What is the difference between Database Page Splits and Index Fragmentation? How do you fix them?

• Page Split
  • Occurs when new data is inserted into a full data page.
  • SQL Server creates a new page and moves some records.
  • Can increase storage overhead and reduce performance.
• Index Fragmentation
  • Occurs when index pages become disorganized.
  • Slows down data retrieval and query performance.
  • Can be reduced using ALTER INDEX REORGANIZE or ALTER INDEX REBUILD.

Conclusion

In conclusion, knowing the basics of MS SQL is really important for doing well in technical interviews and having a good career in database management. This set of MS SQL Interview Questions and Answers covers both hard topics commonly asked by employers. Practicing these questions and understanding the concepts can improve technical knowledge, problem-solving abilities, and help you feel more confident in interviews. MS SQL is key to getting ready for interviews, whether you are applying for a job or a senior role. These questions help prepare for interviews and grow in your career on the run with MS SQL. Get expert career guidance from our Training and Placement Institute in Chennai to prepare for successful job opportunities.