oracle interview questions

Oracle is one of the most popular Relational Database Management Systems in the world. Countless companies and organizations have adopted it as their primary database system, making it an essential skill for database professionals. As such, it's not surprising that Oracle experience is highly sought after by hiring managers. However, getting an Oracle job requires more than just experience; preparation is key, which means understanding the types of questions you should expect.

In this article, we take a comprehensive look at some of the most common Oracle interview questions and how to answer them effectively.

  1. What is Oracle, and how does it differ from other databases?

This is a fundamental question that you'll undoubtedly encounter during an Oracle interview. As such, it is critical that you know how to answer it.

Oracle is a highly sophisticated and robust Relational Database Management System that enables users to manage and store large amounts of data. It is based on SQL (Structured Query Language), and allows applications to access and manipulate data stored in it. Oracle is particularly popular for its scalability, security, and flexibility, making it a preferred choice for large enterprises.

In comparison to other relational database management systems, Oracle tends to be more scalable, stable, and secure. It also boasts better performance, robustness, and the ability to manage large amounts of data. These characteristics make it ideal for enterprise-level applications that handle high volumes of data.

  1. Explain ACID characteristics of a transaction.

ACID stands for Atomicity, Consistency, Isolation, and Durability, and is a set of properties that ensure database transactions are processed reliably. These properties guarantee that data reflects correctly in the database.

  • Atomicity: This property ensures that a transaction occurs as a single unit of work. Either the entire transaction occurs, or it rolls back if any part of the transaction fails. It aims to maintain data integrity by ensuring that a sequence of operations occur as a single, indivisible action.

  • Consistency: Consistency means that any transaction must leave the database in a valid state. It ensures that data is valid and prevents any errors that would undermine the correctness of the database.

  • Isolation: Isolation ensures that transactions that are running concurrently with one another do not conflict. It prevents them from accessing or modifying each other's data. This property helps to ensure database consistency and reduce concurrency issues.

  • Durability: Durability means that once a transaction has been committed, it is permanent. This property guarantees that any changes made to the database are recorded permanently and can survive failures, corruption, or crashes.

  1. How do you optimize performance?

Optimizing performance is crucial to Oracle's database functionality. A slow database can be a major issue that causes various business complications. Therefore, it's essential to know how to optimize it. There are different techniques to do this, and here are some of the most effective ones:

  • Indexes: Indexes help speed up queries. They allow the database to access data quickly, so users don't have to wait for what feels like an eternity for a result.

  • Database Tuning: Database tuning involves analyzing the database and fine-tuning settings, such as system settings, network settings, memory allocation, and more, to maximally optimize performance.

  • Partitioning: Partitioning involves dividing large tables into smaller ones. Smaller tables occupy less space, making it easier to access and query them, resulting in better performance.

  • Clustering: Clustering involves grouping related data into the same disk so that they can be quickly accessed by the database.

  1. What are the different types of joins in Oracle?

Joins are an essential way of combining data from different tables and retrieving relevant information. Oracle database supports several types of joins; however, the most commonly used are four types: inner join, left join, right join, and full outer join.

  • Inner Join: Inner join is the most commonly used join type. It selects only those records that have a match in both tables.

  • Left Join: Left join returns all the records from the left table and matching records from the right table. Null values appear in the result when there are no matches.

  • Right Join: Right join does the opposite of the left join. It returns all the records from the right table and any matching records from the left table.

-Full Outer Join. This type of join returns all the rows from both the tables, along with the matching records. It also returns null for non-matching rows.

  1. What are the different types of constraints in Oracle?

Constraints are used to ensure data accuracy, integrity, and consistency. There are several types of constraints in Oracle, and it's vital to understand each one of them. Here are some of the most commonly used constraints:

  • Primary key constraint: This constraint enforces the uniqueness of a column. This constraint is the primary reference key in most tables and is usually the value of an ID field.

  • Unique key constraint: Unique key constraint is similar to the primary key constraint, but it allows you to have one or more columns with unique values.

  • Foreign key constraint: This constraint enforces the relationship between tables and ensures the referential integrity of database rows.

  • NOT NULL constraint: This constraint ensures that a column does not contain null values.

  • Check constraint: Check constraint is used to limit the data that can be inserted into a column.

  1. Describe Oracle distributed Database system.

An Oracle Distributed Database System is a system where a database can be spread over multiple machines connected by a network. This system is designed to improve database performance, provide data availability, and reduce the cost associated with maintaining a large database.

In a distributed database system, each machine has a separate database instance, which means that users can access different parts of the database simultaneously, with each machine working on its part of the database. The system uses Oracle Shared Server Architecture to ensure that users can access different parts of the database and maintain connection.

The key benefit of a distributed database system is that it provides high availability and data integrity. It ensures that data remains accessible, even in the event of hardware failure, power outages, or any other event that can cause system downtime or data loss.


The Oracle interview questions above are some of the most commonly asked in Oracle interviews. Answering them requires a thorough understanding of Oracle architecture, performance, administration, and best practices. Using the tips and guidelines provided in this article will help you to confidently answer any Oracle-related questions that may come your way during a job interview. Make sure to practice, study, and stay updated, as Oracle is a constantly evolving technology that demands persistent learning.

I'd be happy to provide more details on the previous topics.

  1. What is Oracle and how does it differ from other databases?

Oracle is an enterprise-level Relational Database Management System (RDBMS) that is widely used in various sectors, including finance, healthcare, retail, and more. It is a full-featured database system that allows users to store and manage large amounts of data.

What sets Oracle apart from other databases is its scalability, security, robustness, and flexibility. It offers high availability and disaster recovery options, including real-time data replication and automatic failover. Its built-in security features, such as user authentication, access control, and encryption, make it a trusted choice for organizations dealing with sensitive data.

  1. Explain ACID characteristics of a transaction.

The ACID (Atomicity, Consistency, Isolation, and Durability) properties form a set of guidelines that database management systems use to guarantee data consistency. Here is a brief explanation of each of these properties:

  • Atomicity: This characteristic ensures that a transaction is either fully executed or not executed at all. If any part of a transaction fails, all changes made to the database are rolled back to their previous state.

  • Consistency: The consistency property ensures that the database is in a valid state both before and after a transaction is executed. In other words, the transaction must adhere to all the relevant business rules and constraints.

  • Isolation: This feature ensures that multiple transactions are executed concurrently without interfering with each other, thus preventing data corruption or loss.

  • Durability: Durability ensures that once a transaction is committed, it remains permanent even in the face of hardware or software failure. The data that was committed will remain available even after a system failure.

  1. How do you optimize Oracle performance?

Performance tuning is a crucial aspect of managing an Oracle database system. Here are some performance optimization strategies you can use:

  • Use indexes: An index improves the database query’s performance by making it easier to retrieve data. Creating the appropriate indexes on tables can result in considerable improvements in query performance.

  • Optimize SQL statements: Use efficient SQL statements to reduce the time taken to retrieve data. Eliminate unnecessary queries, optimize the existing one, and reduce table joins.

  • Use partitioning: Partitioning can divide up large tables into smaller, more manageable pieces. This technique helps improve query performance and efficiency.

  • Configure memory: Allocate the right amount of memory so that the system can operate optimally. Configuring a good memory strategy helps avoid unnecessary I/O operations.

  • Monitor Performance: Good performance monitoring involves looking out for problem areas and rectifying them before they cause system-wide issues. Tracking performance metrics can help you identify system bottlenecks and refine your optimization strategies.

  1. What are the different types of joins in Oracle?

A join is one of the most frequently used operations in SQL and allows the consolidation of data from multiple tables. Oracle supports four types of joins:

  • Inner Join: Inner join returns only the matching rows from both tables. It is the most commonly used join type.

  • Left Join: Left join returns all records from the left table and only the matching records from the right table if any. In case there are no matching records, then nulls will be returned.

  • Right Join: Right join returns all records from the right table and only the matching records from the left table if any. In case there are no matching records, then nulls will be returned.

  • Full Outer Join: Full outer join returns all records from both tables. If any records do not have matches, nulls will be returned.

  1. What are the different types of constraints in Oracle?

A constraint is a rule that is applied to data in a table. It ensures that the data stored in the table is accurate, consistent, and fits a certain set of criteria. Oracle supports several types of constraints, including:

  • Primary key constraint: A primary key constraint uniquely identifies a record in a table.
  • Unique constraint: A unique constraint requires that the values in a column (or a group of columns) are unique.
  • Foreign key constraint: A foreign key constraint establishes a relationship between two tables, with the child table referencing the parent table, using a common set of columns.
  • NOT NULL constraint: This constraint ensures that a column cannot contain a null value.
  • Check constraint: A check constraint enforces a condition or a set of conditions on the data in a column.
  1. Describe Oracle distributed Database system.

A distributed database system is a database that is spread across multiple physical locations interconnected by a network. In an Oracle distributed database system, each location has a separate database instance, and each instance works together to provide a single database environment. A distributed database system has several advantages, such as data availability, better performance, and reduced maintenance costs.

Oracle Shared Server Architecture, also known as multi-threaded server (MTS), is used to manage simultaneous connections to a distributed database. The shared server architecture helps reduce resource consumption and minimizes the overhead of starting new processes. It provides a more efficient way of managing client-server connections to the database, allowing more users to connect simultaneously.

In conclusion, Oracle is a complex database system with a wide range of features and capabilities. Having a deep understanding of its architecture, performance optimization strategies, constraints, and distributed database systems is essential to being a successful Oracle database administrator.

Popular questions

Sure, here are five common Oracle interview questions and answers to prepare for:

  1. What is the difference between a clustered index and a non-clustered index in Oracle?

A clustered index determines the physical order of data in a table based on its key columns. In a clustered index, the data of the table is stored in the index structure itself. Non-clustered indexes, on the other hand, do not store the table's data, but rather only a copy of the table's key columns linked to the actual table data. Non-clustered indexes allow for faster access to data when searching for values in the indexed columns.

  1. What is a sequence in Oracle, and how is it used?

A sequence is a database object in Oracle that generates unique numbers in a specific order. Sequences are commonly used to provide unique values for primary keys and other purposes in tables. Sequences supply unique values irrespective of concurrent transaction activity in the database.

To create a sequence, you use the CREATE SEQUENCE statement. Once you have created a sequence, you can reference it in an INSERT statement to automatically generate the next sequence number for a new row.

  1. How do you backup and restore an Oracle database?

To backup an Oracle database, you can use Oracle Recovery Manager (RMAN) or perform a database export. To use RMAN, first make sure that backup configuration is set up properly. Then, you can use the RMAN backup command to back up either the whole database or individual data files.

To restore an Oracle database, copy the backup files to the target database server. RMAN can then be used to restore the data. Use the RMAN set newname command to rename the data files to their proper location. You can then use the RMAN restore command to recover the database files.

  1. What is the difference between UNION and UNION ALL in Oracle?

The UNION and UNION ALL operators combine rows from multiple queries into an output result set. However, UNION removes any duplicate rows from the output set, while UNION ALL does not.

For example, consider two queries that will return four rows:

SELECT name FROM employees WHERE department = 'HR'
SELECT name FROM employees WHERE department = 'Finance'

If there are two employees with the same name in both departments, only one of them will appear in the result set. By contrast, using UNION ALL instead of UNION will return all four rows, including the duplicates.

  1. What is the difference between OLTP and OLAP databases?

An Online Transaction Processing (OLTP) database is used for transactional processing and operational tasks, such as insert, update, and delete. OLTP databases are optimized for handling high volumes of data transactions with high concurrency and low latency.

In contrast, an Online Analytic Processing (OLAP) database is used for analytical processing and data mining purposes. An OLAP database is optimized for complex queries that involve aggregations and calculations across large data sets.

OLTP databases tend to have normalized data structures with many tables, whereas OLAP databases usually have star or snowflake schemas, with fewer tables and more columns in each table. An OLTP database must have high write performance, while an OLAP database must have high read performance.


"Interview Oracle"

As an experienced software engineer, I have a strong background in the financial services industry. Throughout my career, I have honed my skills in a variety of areas, including public speaking, HTML, JavaScript, leadership, and React.js. My passion for software engineering stems from a desire to create innovative solutions that make a positive impact on the world. I hold a Bachelor of Technology in IT from Sri Ramakrishna Engineering College, which has provided me with a solid foundation in software engineering principles and practices. I am constantly seeking to expand my knowledge and stay up-to-date with the latest technologies in the field. In addition to my technical skills, I am a skilled public speaker and have a talent for presenting complex ideas in a clear and engaging manner. I believe that effective communication is essential to successful software engineering, and I strive to maintain open lines of communication with my team and clients.
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