30 Most Asked DBMS Interview Questions

Modern software systems heavily depend on Database Management Systems (DBMS) to make storage, retrieval, and organization of data between different applications and organizations possible. DBMS  viva questions for job seekers and IT professionals. Enterprise dependence on structured data to make critical decisions has led to high demand for people that are DBMS experts. Technical interviews in all these areas, from software development to data engineering, DBA, and system architecture frequently center around your knowledge of DBMS.

 

DBMS Interview Questions

 

Technical interviewers for DBMS positions usually ask you questions about whether you can apply normalization, indexing, transactions, concurrency control, and SQL commands in real-life situations. Using such questions, interviewers can measure how effectively candidates can design strong, effective, and secure databases.

 

Based on the latest industry statistics about 45% of technical interviews also know as DBMS viva questions are based on a question concerning DBMS, and this is an integral part of backend development and data-driven positions. A thorough understanding of these topics helps you to excel in interviews and to create state-of-the-art data systems.

 

Most Asked 30 DBMS Interview Questions And Answers

 

This blog compiles top DBMS interview questions and answers separated based on difficulty level to help you prepare thoroughly and securely for any interview. Whether you are a novice or an experienced pro who wants to solidify your fundamentals, these questions give you the confidence and readiness you require.

 

Basic DBMS Questions

 

1. What is DBMS, and why is it important?

Answer:  A Database Management System (DBMS) is a software application that enables users to define, create, manage, and manipulate databases. It provides a systematic way to store, retrieve, and manage data efficiently, ensuring that it is organized and accessible when needed. DBMS is crucial because it offers several advantages, such as data integrity, security, and consistency. Without a DBMS, managing large amounts of data would be chaotic and prone to errors. For instance, it helps in enforcing data constraints, preventing data redundancy, and ensuring that multiple users can access and modify the data concurrently without any issues. It also ensures that data is securely stored, providing backup, recovery, and disaster management tools, making it indispensable for businesses that rely on accurate, up-to-date information.

 

2. What are the different types of DBMS?

Answer: There are several types of DBMS, each with unique structures and use cases. The most commonly known types are Hierarchical DBMS, Network DBMS, Relational DBMS (RDBMS), and Object-oriented DBMS. Hierarchical DBMS organizes data in a tree-like structure where each record has a single parent, and the relationships are represented as parent-child hierarchies. Network DBMS is a more flexible system where each record can have multiple parent-child relationships, organized in a graph structure. Relational DBMS is the most widely used type and organizes data into tables, where relationships between data elements are established using foreign keys. This structure allows for efficient querying and is used in popular databases like MySQL, PostgreSQL, and SQL Server. Lastly, Object-oriented DBMS stores data in the form of objects, aligning it closely with object-oriented programming languages. Each type has its own advantages, but relational databases remain the most prevalent due to their ease of use, flexibility, and scalability.

 

3. What is the difference between a primary key and a foreign key?

Answer: A primary key is a unique identifier for each record within a database table. It ensures that each row can be uniquely identified, which is crucial for data integrity. The primary key must have unique values, and it cannot contain NULL values. On the other hand, a foreign key is a column or a set of columns that creates a link between two tables. It refers to the primary key in another table, establishing a relationship between the two tables. While the primary key ensures uniqueness within its own table, the foreign key helps in maintaining referential integrity by linking data in one table to data in another. For instance, in an employee database, the employee ID could be the primary key in the employee table, while a department ID in the employee table could act as a foreign key, linking each employee to a department in another table.

 

4. Explain normalization and its types?

Answer: Normalization is the process of organizing data in a database to eliminate redundancy and dependency. The goal is to ensure that data is stored logically, with minimal duplication, to improve data integrity and efficiency in data management. Normalization is typically performed through a series of stages, known as normal forms. The first stage, 1st Normal Form (1NF), ensures that each table column contains atomic values, meaning no repeating groups or arrays in a column. The 2nd Normal Form (2NF) builds upon 1NF and ensures that all non-key attributes are fully dependent on the primary key, eliminating partial dependency. The 3rd Normal Form (3NF) goes a step further, removing transitive dependencies, meaning non-key attributes should not depend on other non-key attributes. Higher levels of normalization, such as Boyce-Codd Normal Form (BCNF), and 4th and 5th normal forms, exist, but they are usually only necessary for more complex databases. The main benefit of normalization is that it helps in reducing the chances of anomalies like update, insertion, and deletion anomalies.

 

5. What is indexing in DBMS, and why is it important?

Answer: Indexing in a DBMS is a technique used to improve the speed of data retrieval operations on a database table. An index is created on columns that are frequently used in search queries, allowing the database to find data more efficiently, just like how an index in a book helps you locate information quickly. Without indexing, the database would have to scan every row in a table for a query, which can be time-consuming, especially for large datasets. Indexes, however, create a data structure that allows quicker lookups, reducing query response times significantly. While indexing enhances read performance, it can impact write operations, such as insertions, updates, and deletions, because the index must be updated each time data in the indexed columns changes. Therefore, indexing is a trade-off, optimizing search operations while potentially slowing down write operations. It's essential to choose which columns to index wisely to balance performance.

 

DBMS Technical Interview Questions

 

6. What is a transaction in DBMS?

Answer: A transaction in DBMS refers to a sequence of operations performed on the database that are treated as a single unit of work. Transactions are used to ensure data consistency and integrity, especially in multi-user environments. A transaction must follow the ACID properties, which are Atomicity, Consistency, Isolation, and Durability. Atomicity ensures that all operations within the transaction are completed successfully or not at all. Consistency ensures that the database moves from one valid state to another. Isolation ensures that the operations of one transaction do not affect others. Durability guarantees that once a transaction is committed, the changes are permanent, even in the case of system failures. Transactions are essential for ensuring that databases maintain accurate, reliable, and consistent data, even in the event of failures.

 

7. What is referential integrity?

Answer: Referential integrity is a concept in relational databases that ensures that relationships between tables are maintained consistently. It means that foreign keys must always reference valid primary keys in another table, ensuring that data remains accurate and logically connected. For example, if a foreign key in the “Orders” table points to the “Customers” table, referential integrity ensures that no order can exist without a valid customer ID. If an attempt is made to insert a record with a foreign key that doesn’t match an existing primary key, the DBMS will prevent the operation. Similarly, referential integrity ensures that if a record in the primary table is deleted, the related records in the foreign table are either deleted or updated accordingly, depending on the defined constraint (such as CASCADE or RESTRICT).

 

8. What is a deadlock in DBMS, and how can it be avoided?

Answer: A deadlock in DBMS occurs when two or more transactions are blocked forever, each waiting for the other to release a lock on a resource. This creates a situation where none of the transactions can proceed. Deadlocks are a serious issue in database systems, especially in environments with high concurrency. To avoid deadlocks, DBMS systems use various techniques like timeout mechanisms, where transactions are aborted if they have been waiting for a resource for too long. Another method is to lock resources in a predefined order, ensuring that transactions do not wait for each other in a circular manner. Additionally, deadlock detection algorithms can identify and resolve deadlocks by aborting one or more transactions to break the cycle. Effective transaction management and locking mechanisms help in minimizing the risk of deadlocks in DBMS systems.

 

9. What is a schema in DBMS?

Answer: A schema in DBMS refers to the structure or blueprint of a database that defines how data is organized and how relationships between the data are established. It includes definitions of tables, views, indexes, and other database objects, but it does not contain the actual data itself. A schema provides a logical view of the database, detailing how the data is stored, organized, and how various tables relate to one another. In simple terms, it serves as a design blueprint that guides how data will be structured within the database. There can be multiple schemas in a database, especially in large systems that manage complex data structures, ensuring that data is grouped appropriately and can be accessed efficiently.

 

10. What are the ACID properties in DBMS?

Answer: The ACID properties are a set of four principles that ensure reliable processing of database transactions. These properties are essential for maintaining the integrity of the database in multi-user environments, especially when multiple transactions are running concurrently. The ACID acronym stands for:

Atomicity: This property ensures that a transaction is treated as a single unit. It either completes entirely or not at all, meaning if one part of the transaction fails, the entire transaction is rolled back.

Consistency: Ensures that the database moves from one valid state to another after a transaction. All data integrity constraints must be satisfied during the transaction.

Isolation: Ensures that transactions are executed independently without interference from other transactions. The intermediate states of transactions are invisible to others.

Durability: Once a transaction is committed, it is permanent, and the changes made by it will survive system failures, ensuring data is not lost.

These properties help maintain data reliability and consistency even when the database system faces failures or heavy loads.

 

DBMS and SQL Interview Questions

 

11. What are joins in SQL? Can you explain the different types of joins?

Answer: Joins in SQL are used to combine rows from two or more tables based on a related column, often involving a primary key and a foreign key. The most common types of joins are INNER JOIN, LEFT JOIN (or LEFT OUTER JOIN), RIGHT JOIN (or RIGHT OUTER JOIN), and FULL JOIN. An INNER JOIN returns only the rows where there is a match in both tables. If there is no match, those rows are excluded. The LEFT JOIN returns all rows from the left table, along with the matching rows from the right table; if no match is found, NULL values are returned for the right table’s columns. Similarly, the RIGHT JOIN returns all rows from the right table, with NULLs for non-matching rows from the left table. A FULL JOIN combines the results of both LEFT and RIGHT joins, returning all rows when there is a match in either table. These joins help in retrieving related data from multiple tables, allowing you to analyze complex data relationships.

 

12. What is the difference between DELETE and TRUNCATE in SQL?

Answer: Both DELETE and TRUNCATE are used to remove data from a table, but there are key differences between the two. DELETE is a DML (Data Manipulation Language) command that removes records from a table one row at a time and allows the deletion of specific rows based on a condition (using a WHERE clause). It also logs each row deletion, which means it can be rolled back if required. On the other hand, TRUNCATE is a DDL (Data Definition Language) command that removes all rows from a table without logging individual row deletions. It is faster than DELETE because it doesn't generate row-level logs, and it cannot be used with a WHERE clause. TRUNCATE is irreversible and cannot be rolled back unless a transaction is used. Unlike DELETE, TRUNCATE also resets any identity column values to the seed value.

 

Also Check -  Know Advance Concept of SQL

 

13. What is the difference between CHAR and VARCHAR in SQL?

Answer: Both CHAR and VARCHAR are data types used to store string values in SQL, but they differ in how they store data and their efficiency. CHAR (short for character) is used for storing fixed-length strings. If a string is shorter than the defined length, it is padded with spaces to meet the specified length. For example, a CHAR(10) field will always store exactly 10 characters, padding shorter strings with spaces. On the other hand, VARCHAR (short for variable character) stores variable-length strings, meaning it only uses as much space as needed for the actual string content. For instance, a VARCHAR(10) field can store strings of length 1 to 10, without any padding. VARCHAR is generally more space-efficient than CHAR because it only allocates the exact amount of space needed for the data, making it more suitable for fields where string length varies.

 

14. What is the purpose of the GROUP BY clause in SQL?

Answer: The GROUP BY clause in SQL is used to group rows that have the same values in specified columns into summary rows, often for the purpose of performing aggregate functions like COUNT(), SUM(), AVG(), MIN(), or MAX(). For example, if you want to calculate the total sales per region, you would use the GROUP BY clause to group the data by region and then apply an aggregate function like SUM() to calculate the total sales for each region. The GROUP BY clause is often used in combination with aggregate functions to produce meaningful insights from the data, such as calculating averages or totals for specific categories.

 

DBMS IMP Questions

 

15. What is a view in DBMS?

Answer: A view in DBMS is a virtual table that provides a way to represent data from one or more tables in a specific manner without actually storing the data itself. Views are defined by SQL queries, and they can be used to simplify complex queries, present data in a more user-friendly format, or provide an extra layer of security by limiting access to certain parts of the database. When a user queries a view, the DBMS executes the underlying SQL query and presents the result as though it were a regular table. Views do not store data physically but instead provide a dynamic representation of data from the base tables. They are particularly useful in situations where complex joins or aggregation are required frequently, as views can encapsulate these complex operations.

 

16. What is a composite key?

Answer: A composite key is a primary key that consists of two or more columns in a table, used together to uniquely identify a record. In cases where a single column cannot uniquely identify a record, combining multiple columns creates a composite key. For example, in an order details table, a combination of OrderID and ProductID may be used as a composite key to uniquely identify each record, as a single column (such as OrderID) might not be unique enough on its own. Composite keys are essential when data is normalized, and individual columns do not have unique values to serve as a primary key.

 

17. What is the difference between UNION and UNION ALL?

Answer: Both UNION and UNION ALL are SQL operators used to combine the results of two or more SELECT queries, but there is a significant difference in how they treat duplicate rows. UNION combines the result sets of multiple SELECT queries, but it removes any duplicate rows, ensuring that only unique records are returned in the final result. This process involves sorting and removing duplicates, which may increase processing time. On the other hand, UNION ALL combines the result sets without removing duplicates, meaning all records, even duplicates, are included in the final output. As a result, UNION ALL is faster than UNION because it doesn't need to perform the extra operation of removing duplicates.

 

18. What is a stored procedure in DBMS?

Answer: A stored procedure is a precompiled collection of SQL statements that can be executed as a single unit. It is stored and executed directly within the database, making it faster and more efficient than issuing multiple SQL queries from an application. Stored procedures are used to encapsulate business logic and database operations, providing a way to manage and execute complex operations in the database. They can accept parameters, perform operations, and return results to the calling program or user. By using stored procedures, developers can promote code reusability, improve performance, and ensure better security, as they can help prevent SQL injection attacks by using parameterized queries.

 

19. What is a trigger in DBMS?

Answer: A trigger is a special type of stored procedure in DBMS that is automatically executed (or "triggered") when certain events occur in the database. Triggers are typically used to enforce business rules, maintain data integrity, or automatically update data in response to certain actions like inserts, updates, or deletes. For example, a trigger could be set to automatically update the total price of an order whenever a product's price is modified in the products table. Triggers can be set to fire before or after specific operations (such as before an insert or after an update). While triggers are powerful, they should be used carefully, as they can introduce performance overhead or unintended side effects if not properly managed.

 

20. What is the difference between OLTP and OLAP?

Answer: OLTP (Online Transaction Processing) and OLAP (Online Analytical Processing) are two types of systems designed for different purposes. OLTP systems are used for managing day-to-day transactional data and are optimized for speed and efficiency in processing a high volume of short online transactions, such as inserting, updating, and deleting records. Examples of OLTP systems include banking applications, e-commerce platforms, and customer relationship management systems. OLAP systems, on the other hand, are designed for complex querying and data analysis, often involving large volumes of historical data. OLAP systems are optimized for read-heavy operations, enabling users to analyze trends, make forecasts, and generate reports. OLTP focuses on current transactional data, while OLAP focuses on aggregating and analyzing historical data.

 

DBMS Interview Questions for Freshers

 

21. What is a data dictionary in DBMS?

Answer: A data dictionary in DBMS is a centralized repository that stores metadata, which is data about data. It contains information about the structure of the database, such as tables, columns, data types, constraints, and relationships. Essentially, it acts as a catalog for the database, providing detailed information about how the data is organized and how different elements within the database relate to each other. The data dictionary is essential for database administrators and developers because it helps in understanding the database design, ensuring consistency, and supporting various operations such as queries, backups, and restores. It is automatically maintained by the DBMS and is essential for the efficient functioning of the database system.

 

22. What is a B-tree in DBMS?

Answer: A B-tree (Balanced Tree) is a self-balancing data structure used to organize data in a way that allows for efficient searching, insertion, and deletion operations. In DBMS, B-trees are commonly used to implement indexes because they allow for fast access to data. A B-tree is organized in such a way that the data is stored in nodes with multiple children, making it a balanced tree where each node can contain more than one key. This structure allows for logarithmic time complexity for search operations, making it highly efficient for large datasets. The B-tree automatically keeps itself balanced by ensuring that data remains evenly distributed across the tree, which prevents performance degradation in large-scale systems.

 

23. What is a clustered index in DBMS?

Answer: A clustered index in DBMS is a type of index in which the data rows are stored in the same order as the index itself. In other words, the table's data is physically organized based on the clustered index. This means that there can only be one clustered index per table because the data can only be sorted in one way. A clustered index is typically created on the primary key of a table, as it ensures fast retrieval of data when searching for rows based on the indexed column. However, when rows are inserted, updated, or deleted, the DBMS needs to reorganize the data to maintain the order, which can have a performance impact in certain cases.

 

24. What is a non-clustered index?

Answer: A non-clustered index in DBMS is an index that does not alter the physical order of the data rows in the table. Instead of rearranging the actual data, a non-clustered index creates a separate data structure that stores a sorted list of pointers to the rows in the table. This allows for fast lookups based on the indexed column, but unlike clustered indexes, non-clustered indexes do not affect the storage order of the data itself. Non-clustered indexes can be created on columns other than the primary key and can improve query performance, especially when retrieving data based on non-primary key columns. A table can have multiple non-clustered indexes, which can optimize the performance of different types of queries.

 

25. What is the difference between UNION and INTERSECT?

Answer: UNION and INTERSECT are both set operations in SQL used to combine results from two or more SELECT queries, but they work in different ways. UNION returns all unique rows from the result sets of the combined SELECT queries, eliminating duplicates. It essentially merges all the results and ensures that only distinct rows are returned. On the other hand, INTERSECT returns only the rows that are common to both result sets, meaning it retrieves the intersection of the two queries. While UNION focuses on combining data, INTERSECT filters out anything that is not present in both result sets. This makes UNION suitable for including all distinct results, while INTERSECT is useful when you want to find common data between two queries.

 

DBMS Queries Interview Questions

 

26. What is data redundancy, and how can it be avoided in DBMS?

Answer: Data redundancy refers to the unnecessary repetition of data within a database, which can lead to inconsistencies and inefficiencies. For example, storing the same piece of data in multiple places can lead to errors when one instance is updated and the others are not. To avoid data redundancy, normalization is used. By breaking down data into smaller, related tables and establishing relationships through keys, normalization minimizes duplication and ensures that each piece of data is stored only once. Another approach is to enforce referential integrity, where foreign keys link data across tables instead of repeating it. These techniques help maintain data consistency, optimize storage, and improve performance.

 

27. What is a database instance?

Answer: A database instance refers to a specific running copy of a DBMS that manages the data for a particular database. It consists of the background processes, memory structures, and database files that work together to handle user requests. When a DBMS is started, it creates a database instance, which controls access to the database, performs queries, and manages the storage of data. The instance is what actually interacts with the physical data files of the database and ensures that data is retrieved, updated, and stored according to the SQL queries and transactions issued by users or applications. Each database instance is tied to a specific DBMS, and multiple instances can run concurrently, especially in large-scale systems or cloud-based environments.

 

28. What is an entity and an entity set in DBMS?

Answer: An entity in DBMS refers to any object or thing that can be distinctly identified within the domain of the database. For instance, a person, a product, or a vehicle could be considered entities in a database system. Each entity has attributes, which are specific properties that describe it. An entity set, on the other hand, is a collection of similar types of entities. For example, in a university database, a set of all students would be an entity set, with each student being an individual entity in that set. The entity set groups together similar objects and forms the basis for creating relationships and organizing data within the database.

 

29. What is a surrogate key?

Answer: A surrogate key is a unique identifier for an entity, typically used in place of a natural key. Surrogate keys are artificially generated values that have no inherent meaning, such as an auto-incrementing integer. They are often used in situations where natural keys (like email addresses or product codes) may not be suitable because they can be lengthy, prone to changes, or might contain duplicate values. Surrogate keys provide a consistent, simple, and non-changing identifier for each row in a table, ensuring data integrity and making indexing and querying more efficient. They are commonly used in data warehousing and scenarios where the data structure requires a unique identifier that is independent of the data content.

 

30. What are the advantages of using stored procedures in DBMS?

Answer: Stored procedures offer several advantages in DBMS. First, they encapsulate complex logic and business rules, making it easier to manage and maintain SQL code within the database itself. Since stored procedures are precompiled, they tend to execute faster than ad-hoc SQL queries because they are optimized by the DBMS. Stored procedures also promote code reuse—once written, they can be executed multiple times without needing to rewrite the logic. Furthermore, using stored procedures can enhance security, as they allow access to data through controlled, parameterized interfaces instead of allowing users direct access to underlying tables. They also help in reducing network traffic by executing multiple SQL operations within a single call, rather than sending individual queries over the network.

 

Conclusion - Questions on Database Management System

 

Preparing for DBMS-related interview questions requires a deep understanding of database management concepts and practices. Mastering topics such as indexing, normalization, schema design, and SQL commands will enhance your ability to answer technical and non-technical questions confidently. Whether you're a fresh graduate or an experienced professional, continuous learning and practice are key to excelling in these interviews.

 

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