Top 40 Oracle SQL Interview Questions and Answers: Your Guide to Success

Preparing for the Oracle SQL interview questions starts with mastering key concepts, practicing common queries, and understanding advanced database operations. Oracle SQL is a critical skill for database professionals, and interviews often test your ability to write efficient queries, optimize performance, and manage complex data structures. You can expect questions about SQL joins, subqueries, data types, and indexing in your interview. This blog discusses some Oracle SQL interview questions commonly asked to help you prepare for your interview and showcase your problem-solving skills.

Oracle SQL Interview Questions and Answers

Getting ready for an Oracle SQL job requires a thorough understanding of SQL concepts, query optimization, and database management. One must be knowledgeable in several SQL operations and their useful applications. Given below are some Oracle SQL interview questions and answers that can help you prepare for the interview from different aspects. 

Basic Oracle SQL Interview Questions for Freshers

Explore essential Oracle SQL topics such as primary and foreign keys, data types, the DISTINCT keyword, and the differences between UNION and UNION ALL. Below are some basic Oracle SQL interview questions. It provides clear explanations and sample answers to help you build a strong foundation in SQL concepts, including queries, joins, subqueries, indexes, and key clauses like WHERE and HAVING.

Q1. What is SQL?

Sample Answer: SQL (Structured Query Language) is a programming language designed for maintaining and altering relational databases. It lets users design and alter database structures in addition to query, enter, update, and delete data.

Q2. What is the difference between CHAR and VARCHAR2 data types in Oracle SQL?

Sample Answer: CHAR is a fixed-length data type, meaning it always reserves the specified length in the database, padding with spaces if necessary. VARCHAR2 is a variable-length data type, which only uses the space required to store the actual data. VARCHAR2 is more space-efficient for storing strings of varying lengths.

Q3. What is a subquery under Oracle SQL?

Sample Answer: A subquery is a search buried inside another query in Oracle. It can be used in select, insert, update, or delete statements to retrieve the data that can be used in the outer query. Subqueries could be correlated or non-correlated.

Q4. What are indexes in Oracle SQL?

Sample Answer: An index is a database object meant to facilitate data retrieval. By building a fast lookup path for rows in a table, they help to reduce the requirement for scanning every row. Indexes can be built with the help of a CREATE INDEX statement.

Q5. Explain the difference between ‘WHERE’ and ‘HAVING’ clauses.

Sample Answer: The WHERE clause filters records before grouping them, while the HAVING clause filters records after they are grouped. WHERE is used with SELECT, UPDATE, and DELETE statements, whereas HAVING is used with GROUP BY.

Q6. In Oracle SQL, what is a primary key?

Sample Answer: A primary key is a column or set of columns that specifically marks every row in a table. It guarantees data integrity using null entries in the main key column and helps to avoid duplication of values.

Q7. What are the data types in Oracle SQL?

Sample Answer: Oracle SQL supports numerical (NUMBER, INT), text (VARCHAR, CHAR), date/time (DATE, TIMESTAMP), and binary (BLOB) data types. These help to specify the kind of data a column can hold.

Q8. What is a foreign key in Oracle SQL? 

Sample Answer: In Oracle SQL, a foreign key is a column referring to the main key of another table that generates a link between two tables. It guarantees integrity by maintaining consistent data between related tables.

Q9. What is the purpose of the DISTINCT keyword?

Sample Answer: The DISTINCT keyword is used in the SELECT statements to return unique (non-duplicate) values in the result set. It eliminates redundant data from the result of a query.

Q10. What is the difference between a CURSOR and a REF CURSOR in Oracle SQL?

Sample Answer: A CURSOR is a static pointer to a result set, defined explicitly in PL/SQL. A REF CURSOR is a dynamic cursor that can be passed between programs and can reference different result sets at runtime.

Oracle SQL Interview Questions and Answers for Mid-Level Candidates 

When preparing for a mid-level Oracle SQL interview, it’s essential to understand that the questions will likely cover a range of topics. This includes INNER JOIN and OUTER JOIN, the use of DENSE_RANK() and RANK(). Below are some Oracle SQL interview questions that are usually asked to mid-level candidates to test their abilities and knowledge: 

Q11. What is the difference between a CURSOR and a REF CURSOR in Oracle SQL?

Sample Answer: A CURSOR is a static pointer to a result set, defined explicitly in PL/SQL. A REF CURSOR is a dynamic cursor that can be passed between programs and can reference different result sets at runtime.

Q12. Explain the application of DENSE_RANK() and RANK() functions.

Sample Answer: The DENSE_RANK() and RANK() functions are both used to assign ranks to rows within a result set, but they handle ties differently. The table below shows the difference in the application for both the functions: 

Function	Application
DENSE_RANK	Gives rows sequential ranks without skipping any rank values. Useful when you have to rank objects in a group but want to avoid breaks in the ranking system. Usually used in situations when all tied rows should have the same rank, but no ranks are skipped.  Helps to rank products, staff, or students effectively without omitting any rank number.
RANK	Gives rows different ranks and skips rank numbers in cases of ties. Perfect for situations when you have to consider ties and reflect gaps in rating. Often used for competitive rankings, where tied values need skipping numbers. Useful for grading financial reports, sporting events, or sales figures where ties are feasible.

Q13. What is the function of a WITH clause in Oracle SQL?

Sample Answer: Often referred to as a common table expression (CTE), the WITH clause lets a user provide temporary result sets accessible within the main query. It simplifies challenging searches and increases query readability.

Q14. How does the NVL feature operate in Oracle SQL?

Sample Answer: NVL lets one substitute a designated value for NULL values. It helps to manage NULL values in searches and guarantees that computations or outputs do not suddenly produce NULL.

Q15. What is the difference between a view and a materialized view?

Sample Answer: A view is a virtual table displaying query results. It is recalculated every time it is accessed and is not kept physically. On the other hand, a materialized view is periodically updated and physically stores the query result.

Q16. What purpose does the ANALYZE command in Oracle SQL serve?

Sample Answer: Tables, indexes, and the health of database objects are evaluated using the ANALYZE command and statistical gathering tools. It offers an understanding of how Oracle maximizes the query execution strategy, leading to higher query performance.

Q17. What is the importance of database normalization in Oracle SQL?

Sample Answer: Database normalization is an important process to reduce redundancy and improve data integrity by organizing data into related tables. It ensures efficient storage and retrieval of data.

Q18. Describe the differences between ‘TRUNCATE’ and ‘DELETE’.

Sample Answer: TRUNCATE speeds things up by removing all rows from a table without recording individual row removals. On the other hand, DELETE removes rows one at a time and can be rolled back, which is a DDL action.

Q19. How can queries in Oracle SQL be optimized?

Sample Answer: Queries in Oracle SQL can be optimized in the following ways:

• Maintaining proper indexing: Create indexes on often-requested columns to enhance query performance and reduce search times.
• Avoid needless joins: Limit the usage of pointless joins in your searches to avoid complexity and reduce data processing volume.
• Guarantee suitable WHERE clause conditions: Make sure the WHERE clause uses efficient conditions to filter data early in the query execution, minimizing the dataset for processing.
• Use the EXPLAIN PLAN command to examine execution plans: Use EXPLAIN PLAN to understand how the database runs a query, enabling you to spot expensive processes or inefficiencies.
• Minimize subqueries: Limit the use of subqueries and nested selects as they can increase complexity and slow down query execution.

Q20. What is a sequence in Oracle SQL? 

Sample Answer: In Oracle SQL, a sequence is a database object that creates a set of distinct numbers, and is often used for auto-incrementing primary key values. With the help of this function, one can also customize sequences for incrementing, cycling, and setting minimum and maximum values.

Oracle SQL Interview Questions and Answers for Experienced Candidates 

This section includes the questions that are asked at an advanced level to test the candidate’s knowledge about Oracle SQL and its concepts of performance tweaking, optimization strategies, and sophisticated searches. Below are some of the commonly asked Oracle SQL interview questions meant to assist experienced professionals in their preparation for the interview: 

Q21. In Oracle SQL, what is the difference between a procedure and a function?

Sample Answer: A procedure acts but does not return a value, while a function returns a single value. Functions can be used in SQL expressions, whereas procedures are generally used for tasks like modifying database objects or performing operations.

Q22. How can you guarantee data consistency in Oracle across transactions?

Sample Answer: Oracle guarantees data consistency via ACID characteristics (atomicity, consistency, isolation, durability). COMMIT and ROLLBACK statements are used to regulate transaction completion, while SAVEPOINTS are used to assist in designating intermediate points for ROLLBACK within transactions. 

Q23. What is the purpose of the EXPLAIN PLAN in Oracle SQL?

Sample Answer: The purpose of the ‘EXPLAIN PLAN’ is to show a SQL query’s execution plan. It reveals how Oracle will run the query, therefore enabling performance analysis, bottleneck identification, and query optimization.

Q24. What is the difference between LOCAL and GLOBAL TEMPORARY tables in Oracle? 

Sample Answer: Local temporary tables are removed automatically and exist only for the length of the session or transaction. Whereas GLOBAL TEMPORARY tables exist across sessions and help to maintain structure between sessions. They store data on a per-session basis. 

Q25. How do you approach deadlock in Oracle SQL? 

Sample Answer: A deadlock is experienced when two or more sessions are awaiting data that has been sealed by each other. Oracle automatically detects deadlocks and rolls back one of the transactions involved. One should ensure that the transactions are short, access tables are kept in the same sequence, and tables are correctly indexed to lower the possibility of locking conflicts. All these steps together will help avoid a deadlock. 

Q26. What is the function of Oracle’s flashback technology?

Sample Answer: Flashback technology lets one access former data states, therefore facilitating data recovery and undoing modifications. Data can be accessed at any designated time or transaction using flashback searches, tables, and database capabilities.

Q27. Define partitioning in Oracle SQL.

Sample Answer: Partitioning can be defined as breaking up a big table into smaller, more doable bits. This allows data administration to be simplified and query efficiency to be improved. There are different types of partitioning such as range, list, hash, and composite partitioning, which offer advantages for various usage situations.

Q28. What is the purpose of the MERGE statement in Oracle SQL?

Sample Answer: The MERGE statement is used to perform INSERT, UPDATE, or DELETE operations in a single statement. It is often used for synchronizing two tables by updating existing rows or inserting new ones based on a condition.

Q29. What is the difference between Oracle SQL’s clustered and non-clustered indexes?

Sample Answer: All indexes in Oracle are non-clustered by default, hence they do not naturally store data in the same sequence as the table. Conversely, clustered indexes arrange the data according to the index order, therefore enhancing retrieval times for range searches.

Q30. How can you maximize searches in Oracle SQL?

Sample Answer: To maximize searches in Oracle SQL, the following methods can be used: 

• Use the EXPLAIN PLAN to identify any inefficiencies in operations and understand how Oracle executes a query. 
• Index frequently searched columns to speed up query performance by reducing search time.
• Reduce the use of multiple joins and subqueries to lower complexity and improve performance.
• Avoid SELECT * in queries to retrieve only the necessary information, reducing overhead. 
• Optimize queries by restructuring them for better performance, focusing on logic and indexing.

Pro Tip: Check out our blog on SQL interview questions and answers for experienced professionals, to explore additional concepts and practice them for better preparation.  

Oracle SQL Interview Questions and Answers on Functions and Aggregations 

To be successful in the Oracle SQL interview questions, you need to have knowledge of functions and aggregations for manipulating and summarizing data in Oracle SQL. Below are some questions and answers focusing on these topics to help you prepare effectively for your interview.

Q31. What are the differences between Oracle SQL’s aggregate functions and scalar functions?

Sample Answer: Aggregate functions, such as COUNT(), SUM(), or AVG(), perform computations on a set of values and produce one result. On the other hand, UPPER(), ROUND(), or LENGTH() are scalar functions that run and produce a single value.

Q32. In Oracle SQL, what is the purpose of the COUNT() feature?

Sample Answer: COUNT() function counts the rows in a result set that satisfies certain criteria. It counts all rows, or the rows depending on a particular column or condition (e.g., COUNT(DISTinct column)).

Q33. Explain the use of Oracle SQL’s SUM() function. 

Sample Answer: For the rows satisfying the given condition, the SUM() function compiles the values in a designated column. With numerical data, it is frequently used to compute totals, such as total sales or total expenses.

Q34. What is the purpose of the GROUP BY clause in Oracle SQL? 

Sample Answer: In a SELECT statement, the GROUP BY clause is employed to aggregate data from multiple rows and arrange the results by one or more columns or expressions. Usually, this is used with aggregate operations like COUNT(), SUM(), or AVG(), to create summaries of group data.

Q35. What is the difference between the HAVING clause and WHERE clause in Oracle SQL?

Sample Answer: WHERE filters rows before grouping, while HAVING filters results after grouping. HAVING is used with GROUP BY to filter groups, whereas WHERE is used to filter individual rows before they are grouped.

Q36. What is the purpose of the ROUND() feature in Oracle SQL?

Sample Answer: The ROUND() function rounds a numerical value to a predefined decimal place count. It is widely used to format data, including monetary amounts, to a consistent level of accuracy, for example, ROUND(price, 2).

Q37. What is the function of the DISTINCT keyword in Oracle SQL?

Sample Answer: From a result set, the DISTINCT keyword removes duplicate values. It returns just unique values for a given column or group of columns in the query output.

Q38. In Oracle SQL, how can you use the CONCAT() feature?

Sample Answer: The CONCAT() feature is used to merge two strings into one single string. It takes two string arguments and returns the concatenated result. For example, CONCAT(‘Hello’, ‘ World’) returns “Hello World.”

Q39. What is the difference between the RANK() and DENSE_RANK() functions in Oracle SQL?

Sample Answer: Both RANK() and DENSE_RANK() functions are used to assign ranks to rows based on specific sorting criteria. However, they treat rows with equal values differently when assigning ranks. The table below explains the difference between the two functions: 

Function	Difference
RANK ()	Skips rank numbers in situations of ties. If there are two rows tied for rank 1, it assigns the next row rank three, therefore excluding rank 2. Provides breaks in the ranking order when ties arise. It is useful for situations when the ranking system must represent the count of different rankings.
DENSE_RANK()	It does not skip rank numbers in cases of ties If there are two rows tied for rank 1, it will have the next row ranked 2, thus continuing without gaps. Guarantees consecutive ranking free from numerical skipping, even in cases of ties. Suitable when you must keep a small and consistent rank sequence even with ties.

Q40. What is the effect of the COALESCE() function in Oracle SQL?

Sample Answer: COALESCE() returns the first non-NULL expression in a list. In searches, it is commonly used to manage NULL values since it offers a default value should a NULL arise in a field.

Tips to Prepare for Oracle SQL Interview

Preparing for the Oracle SQL interview questions requires a good knowledge of SQL questions, functions, and database management concepts. By understanding the important subjects, including joins, subqueries, and performance optimization, you will be able to ace the interview. Here are some tips that can help you prepare for your Oracle SQL interview: 

• Know the Foundations Thoroughly: Make sure you understand all fundamental ideas, such as SQL searches, database design, indexing, and normalizing. A strong foundation will help you answer complex questions with ease. 
• Work on Solving Problems: Work on a range of practice tasks about SQL questions, joins, subqueries, and data manipulation. This will increase your accuracy and quickness, thereby preparing you for any type of question asked. 
• Understand the Interview Format: Research the company’s interview format to understand whether they emphasize coding issues, theoretical questions, or both.  Knowing the structure helps you prepare better for the kind of questions you might face.
• Communicate Clearly: Be prepared to describe your thought process clearly while solving difficult scenarios. Candidates who can clearly and logically present their answers are much sought for by interviewers. Before the interview, you can also get comfortable by talking about your method of approaching problems.
• Review Real-World Scenarios: Practice real-world scenarios that may involve SQL problems and situations. Knowing how SQL fits into bigger database management systems and applications will enable you to connect technical knowledge to useful contexts.

Conclusion 

To excel in an Oracle-related role, you should have a thorough understanding of the kind of Oracle SQL interview questions that you might encounter. A solid understanding of SQL concepts—such as joins, subqueries, and performance optimization—is essential for success in this field. Regularly practicing these fundamental questions will sharpen your problem-solving skills and deepen your technical expertise. By combining technical knowledge with strong communication skills, you’ll be well-equipped to tackle any challenge and stand out as a candidate. 

Check out our blog on how to get a job in Oracle, to get a better understanding of the application process and tips for success.  

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