Top 40 Oracle Interview Questions & Answers

When preparing for an interview for the position of a Database Administrator (DBA) or even a Data Analyst, a solid grasp of Oracle Database concepts is crucial. To help you succeed, we've compiled a comprehensive list of the top 40 Oracle Interview Questions and Answers.  

According to Glassdoor, a Database Administrator earns an average of around £50,000 per year. With such lucrative pay scales, the interviews are bound to be a test of your technical expertise. Check out these top 40 Oracle Interview Questions and Answers and start preparing for these interview questions to crack your DBA interview. 

Table of Contents 

1) Oracle Interview Questions and Answers 

     a) Basic Oracle Interview Questions 

     b) Intermediate Oracle Interview Questions 

     c) Advanced Oracle Interview Questions 

2) Conclusion 

Oracle Interview Questions and Answers 

Whether you're preparing for basic, intermediate, or advanced queries, this resource equips you with insights to confidently address a range of topics. From ACID properties to Materialised Views, this blog has all the Oracle Interview Questions and Answers you need. Let's delve into this repository of knowledge and elevate your readiness for Oracle interviews through these Oracle Interview Questions.  

Basic Oracle Interview Questions 

1) What is Oracle Database? 

Oracle Database is a Relational Database Management System (RDBMS) that enables efficient storage, retrieval, and management of structured data. 

2) What is SQL? 

Structured Query Language (SQL) is a language the enables communication with and manipulation of databases. 

3) Explain the difference between SQL and PL/SQL. 

SQL (Structured Query Language) is used for querying and manipulating databases, while PL/SQL (Procedural Language/SQL) is Oracle's extension for procedural programming, allowing for more complex data processing within the database. 

4) What is a Primary Key? 

A Primary Key in a database is a unique identifier for each record in a table. It ensures data integrity, facilitates searches, and forms the basis for relational connections. 

5) What are Indexes in Oracle? 

Indexes in Oracle are data structures that enhance database performance by enabling quicker data retrieval. They work like a book's index, allowing faster access to specific data within a table. 

6) What is the difference between a View and a Table? 

In Oracle, a View is a virtual table that displays data from one or more tables based on predefined queries. A Table, on the other hand, is a physical storage structure containing actual data. 

7) How can you prevent SQL injection in Oracle? 

To prevent SQL injection in Oracle, employ parameterised queries and bind variables. This technique ensures input values are treated as data, thwarting malicious attempts to manipulate the SQL query. 

8) What is the purpose of the COMMIT statement? 

The COMMIT statement in Oracle finalises and permanently applies all the changes made within a transaction to the database, ensuring data consistency and durability. 

9) Explain the difference between CHAR and VARCHAR2 data types. 

In Oracle, the CHAR data type stores fixed-length strings, and padding with spaces if needed. VARCHAR2 stores variable-length strings, using only the required space. 

10) What is the purpose of the NVL function? 

The NVL function in Oracle serves to replace null values with a specified alternate value. This ensures smoother data handling and accurate results in queries and calculations. 

11) What is the purpose of the ROLLBACK statement? 

The ROLLBACK statement in Oracle serves to undo any uncommitted changes made during a transaction, restoring the database to its previous consistent state, and ensuring data integrity. 

12) How can you retrieve the current date and time in Oracle? 

To retrieve the current date and time in Oracle, you can use the SQL function ‘SYSDATE’, which provides the system's current date and time information accurately. 

13) Explain the difference between a Left Outer Join and a Right Outer Join. 

A Left Outer Join returns all rows from the left table and matching rows from the right table, while a Right Outer Join returns all rows from the right table and matching rows from the left table. 

14) What is the purpose of the UNIQUE constraint? 

The purpose of the UNIQUE constraint in Oracle is to ensure that the values in a specific column or set of columns are distinctive and different across the table, preventing duplicate entries. 

15) What is the difference between a Transaction and a Session? 

In Oracle, a Transaction is a sequence of database operations treated as a single unit, ensuring data consistency. A Session, on the other hand, is a connection established by a user to interact with the database. Transactions affect data, while Sessions facilitate user interactions
 

 

Intermediate Oracle Interview Questions 

16) What is Normalisation? 

Normalisation is a vital database design process that minimises data redundancy and dependency. By structuring data into separate tables, each with a specific purpose, Normalisation enhances data integrity and simplifies maintenance. This approach optimises database efficiency, ensuring consistent and reliable information storage and retrieval.  

17) Explain the difference between UNION and UNION ALL. 

In the context of SQL queries, the ‘UNION’ operator combines and returns distinct rows from multiple SELECT statements, eliminating duplicates. Conversely, ‘UNION ALL’ also merges results from multiple queries but includes all rows, including duplicates. The choice depends on whether you want distinct results or require all entries, respectively. 

18) What is the purpose of the GROUP BY clause? 

The purpose of the GROUP BY clause in SQL is to categorise and group rows from a table based on specific column values. It enables aggregation functions like COUNT, SUM, AVG, etc., to be applied to these groups, providing summarised information and aiding in data analysis. 

19) What is a Foreign Key? 

A Foreign Key in Relational Databases establishes a link between two tables by referencing the primary key of one table in another. This relationship enforces data integrity and maintains referential integrity, ensuring consistency and accuracy when retrieving and manipulating data across related tables. 

20) What is a Subquery? 

A Subquery, often referred to as a Nested Query, is a SQL query within another query. It's used to retrieve data that will be used in the main query's conditions or calculations. Subqueries enhance query flexibility, enabling data retrieval from multiple tables or applying aggregates for more precise results. 

21) What is the difference between a Candidate Key and a Primary Key? 

In Oracle SQL, a Candidate Key and a Primary Key both hold significant roles, yet with distinctions. While a Candidate Key is a unique identifier for each record within a table, a Primary Key is a chosen Candidate Key, serving as the main identification method for the records, facilitating efficient data organisation and retrieval. 

22) Explain the difference between a Stored Procedure and a Function. 

In database management, a Stored Procedure and a Function serve distinct purposes. A set of SQL statements designed to perform specific tasks, often altering data is known as a stored procedure. On the other hand, a Function returns a single value, derived from computations or transformations, and is commonly used within SQL queries. 

23) What is the difference between a Correlated Subquery and a Non-correlated Subquery? 

In SQL, the distinction between a Correlated Subquery and a Non-correlated Subquery lies in their interaction with the Outer Query. A Correlated Subquery references values from the Outer Query, influencing its execution for each row processed. On the other hand, a Non-correlated Subquery operates independently, obtaining data without relying on the Outer Query's context. 

24) What is a Sequence in Oracle? 

A Sequence in Oracle is a database object used to generate unique numeric values sequentially. It's commonly used to provide primary key values for tables. Sequences ensure that each value is unique and can be incremented or decremented based on defined steps. This enhances data integrity and simplifies data management. 

25) What is a Bitmap Index? 

A Bitmap Index is a data structure in Oracle that efficiently represents binary attributes, like Yes/No or True/False values. It uses bitmaps to index data, where each bit corresponds to a unique attribute value. This indexing method accelerates queries involving multiple attributes and enhances query performance by minimising disk I/O. 

26) Explain the difference between a Database Trigger and a Stored Procedure. 

A Database Trigger and a Stored Procedure are both database objects, but they serve distinct purposes. A Trigger is automatically executed in response to specific events, like data changes, while a stored procedure is a reusable set of SQL statements that can be invoked manually. Triggers are event-driven, whereas Stored Procedures are invoked by application code. 

27) What is the difference between a Synonym and an Alias? 

In the context of databases, a Synonym and an Alias are terms used interchangeably, but they serve distinct purposes. A Synonym is an alternate name for an object, aiding in simplifying complex queries. On the other hand, an Alias is a temporary renaming of a table or column in a query, enhancing readability. 

28) Explain the difference between a Hot Backup and a Cold Backup. 

In Oracle databases, a Hot Backup and a Cold Backup differ in timing and accessibility. A Hot Backup is taken while the database is operational, allowing users to access data. In contrast, a Cold Backup is conducted while the database is offline, ensuring data consistency but temporarily restricting user access. 

29) What is the purpose of the WITH clause (Common Table Expression)? 

The WITH clause, also known as a Common Table Expression (CTE), serves the purpose of enhancing query readability and simplifying complex SQL queries. It allows users to define temporary result sets within a query, making it easier to reference and manipulate data. This aids in breaking down intricate queries into more manageable parts, promoting efficiency and maintainability. 

30) Explain the difference between a Schema and a User in Oracle. 

In Oracle, a Schema is a logical container that holds database objects like tables, views, and procedures. A User, on the other hand, is an account with a unique name and password, allowing access to a specific schema. A User can own multiple Schemas, but a Schema is associated with only one User. 

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Advanced Oracle Interview Questions 

31) What is the difference between a Clustered and a Non-clustered Index? 

A Clustered Index dictates the physical order of rows in a table, reorganising the data to match the Index. In contrast, a Non-clustered Index is a separate structure that references rows' locations without altering the actual data order. While a table can only have one Clustered Index, it can have multiple Non-clustered Indexes. This distinction impacts performance: a Clustered Index is efficient for range queries due to its data order, while Non-clustered Indexes excel in speeding up data retrieval for specific columns. 

32) What is the purpose of the ‘HAVING’ clause? 

The purpose of the ‘HAVING’ clause in SQL, particularly within the context of a query with GROUP BY, is to filter the results after the grouping has been applied. Unlike the ‘WHERE’ clause, which filters rows before grouping, the ‘HAVING’ clause works on grouped data. It allows you to set conditions on aggregated values, enabling you to retrieve specific subsets of grouped data that meet certain criteria. This is particularly useful when you need to perform aggregate functions like ‘SUM’, ‘COUNT’, ‘AVG’, etc., and then filter the results based on those aggregated values. The ‘HAVING’ clause helps to refine results and extract meaningful insights from grouped data. 

33) Explain the ACID properties in the context of Oracle Transactions. 

In Oracle Transactions, the ACID properties play a pivotal role in ensuring the reliability and consistency of data manipulations. ACID, an acronym for Atomicity, Consistency, Isolation, and Durability, outlines a set of fundamental principles that guide the behaviour of Transactions. Atomicity makes sure that a Transaction is treated as a single, indivisible unit of work, ensuring that either all its changes are applied or none at all.  

Consistency ensures that only valid data states are transitioned between Transactions. Isolation maintains the separation of concurrent Transactions to prevent interference. Durability guarantees that committed changes are permanently stored even in the face of system failures, solidifying the integrity of the database. 

34) What is an Execution Plan in Oracle? 

An Execution Plan in Oracle is a strategic roadmap that outlines how the database engine will execute a specific SQL query. It provides a detailed step-by-step guide for the database optimiser on how to retrieve and manipulate data efficiently from tables, indexes, and other database objects.  

The Plan's objective is to optimise query performance by choosing the most efficient access paths, join methods, and sorting techniques. This plan is generated after careful analysis of available statistics, database structure, and optimisation algorithms. By understanding and interpreting the execution plan, Database Administrators and Developers can fine-tune queries to enhance overall system performance. 

35) What are Materialised Views? 

A Materialised View is a precomputed and stored result of a complex query, designed to enhance data retrieval efficiency. Unlike standard views, which execute queries in real-time, Materialised Views store the query results physically in the database. This allows for quicker access to frequently requested information, reducing the need to recompute the same results repeatedly.  

Materialised Views are particularly useful in scenarios involving large datasets or intricate calculations, as they provide a snapshot of data that can be easily accessed without the need to re-run resource-intensive queries. This optimisation contributes to improved performance and response times within the Oracle database system. 

36) Explain the difference between a Full Backup and an Incremental Backup. 

A Full Backup and an Incremental Backup are two distinct strategies used in data backup and recovery processes. A Full Backup involves copying all data, files, and information from a source to a designated backup location, creating a complete snapshot of the entire dataset. On the other hand, an Incremental Backup captures only the changes that have occurred since the last backup, minimising storage requirements and backup time. While a Full Backup ensures a comprehensive restoration, Incremental Backups are more efficient and consume less storage space. 

37) What are Materialised Views? 

In Oracle databases, a Materialised View is a precomputed table that stores the results of a complex query. Unlike regular views that simply provide a virtual representation of data, Materialised Views physically store the data, allowing for quicker data retrieval and improved performance.  

These views are especially useful when dealing with large datasets or complex joins, as they eliminate the need to repeatedly execute resource-intensive queries. By storing the computed data, Materialised Views reduce the workload on the database server and enhance response times, making them a valuable tool for optimising query performance in Oracle database systems. 

38) What is the difference between an ‘INNER JOIN’ and a ‘LEFT JOIN’? 

An ‘INNER JOIN’ retrieves only the matching records from both tables, filtering out non-matching ones. This creates a tighter link between the tables and is useful when you want to focus solely on shared data. On the other hand, a ‘LEFT JOIN’ retrieves all records from the left table and the matching records from the right table. This inclusive approach ensures that even non-matching entries from the left table are displayed, maintaining context and aiding in data analysis. 

39) How can you improve query performance in Oracle? 

To enhance query performance in Oracle, an effective approach is optimising the database schema by employing appropriate indexing techniques. Indexes expedite data retrieval, reducing the need for full-table scans. Moreover, refining SQL queries through intelligent coding, minimising the use of unnecessary joins, and avoiding Wildcard Character usage can significantly boost performance.  

Utilising Materialised Views to precompute query results and leveraging query optimisation tools such as the Oracle Query Optimiser can further optimise Execution Plans. Lastly, maintaining regular database maintenance tasks, like updating statistics and managing fragmentation, ensures consistent and efficient query performance, enhancing the overall system's responsiveness. 

40) Explain the purpose of the Oracle Data Pump utility. 

The Oracle Data Pump utility offers a fast and efficient means of exporting and importing large volumes of data, making it essential for tasks like database migration, system upgrades, and data consolidation. This utility provides enhanced control over the export and import processes, allowing users to specify data subsets, compression options, and parallel execution to optimise performance.  

By streamlining data transfer, the Oracle Data Pump utility empowers administrators to ensure seamless data transitions while minimising downtime and maintaining data integrity. It stands as a pivotal tool in managing Oracle database environments with precision and efficiency.  

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Conclusion 

Preparation is the key to success in any Oracle interview. Whether you're facing basic, intermediate, or advanced questions, having a solid understanding of Oracle's database concepts is essential. By mastering these top Oracle Interview Questions and Answers, the readers will give themselves the best chance of landing their dream jobs.  

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