Top 25+ Oracle SQL Interview Questions With Answers

981

Oracle SQL is a widely used language for managing and querying data in Oracle databases. It plays a key role in storing, retrieving, and organizing data across industries like finance, healthcare, and IT. From running reports to powering backend systems – Oracle SQL is essential for many business operations. Jobs like database developer, SQL analyst, and backend engineer often require strong SQL skills. If you are preparing for such roles, reviewing the most common Oracle SQL interview questions and answers is a smart way to get ready and increases your chances of success.

Fun Fact – Oracle processes over 30 billion transactions per day, highlighting just how critical Oracle SQL is to global business operations.

Basic Level Oracle SQL Interview Questions

Here are some commonly asked interview questions and answers for Oracle SQL to help you prepare for entry-level roles.

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

DELETE removes specific rows using a WHERE clause and can be rolled back. TRUNCATE removes all rows from a table quickly but cannot be rolled back. DELETE fires triggers; TRUNCATE does not. DELETE is DML; TRUNCATE is DDL.

2. Explain the purpose of a primary key in a database table.

A primary key uniquely identifies each record. It doesn’t allow nulls or duplicate values. It helps maintain data accuracy by making sure each row stays unique. It also helps enforce relationships between tables.

3. What is the difference between WHERE and HAVING clauses in SQL?

WHERE filters rows before grouping. It is used with SELECT, UPDATE, or DELETE. HAVING filters after grouping and is used with aggregate functions like SUM, COUNT, or AVG.

4. How does a JOIN differ from a UNION in SQL?

A JOIN combines columns from related tables. A UNION combines results from two queries into one set, vertically. JOIN relates rows; UNION stacks rows.

5. What are indexes and how do they affect query performance?

Indexes speed up data retrieval. They use pointers to find rows faster. While helpful for reads, they slow down writes like INSERT or UPDATE due to maintenance overhead.

6. What are constraints in SQL? Give examples.

Constraints enforce rules on table data. Examples include NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK. They help maintain valid, consistent, and meaningful data across the database.

Note – Basic-level Oracle and SQL interview questions typically cover foundational topics like data types, constraints, joins, filtering, and key differences between commands such as DELETE and TRUNCATE. 

Oracle SQL Interview Questions for Freshers

These Oracle SQL interview questions with answers are designed to help freshers build a strong foundation in SQL basics.

7. What is a foreign key and how is it used?

A foreign key links two tables. It references the primary key of another table. This relationship maintains consistency between related data in different tables.

8. What is normalization and why is it important in database design?

Normalization organizes data to reduce redundancy. It breaks large tables into smaller ones and sets rules for relationships. This structure improves clarity and avoids data duplication.

9. What is a view in SQL and how is it different from a table?

A view is a virtual table created from a SQL query. Unlike a table, it doesn’t store data itself. It shows data from one or more tables in a custom format.

10. How would you retrieve unique records from a table?

I would use the SELECT DISTINCT statement. It filters out duplicate rows from the result. 

For example:

SELECT DISTINCT column_name FROM table_name;

11. What are aggregate functions in SQL? Name a few.

Aggregate functions return a single value from a group of rows. Common ones are SUM(), AVG(), COUNT(), MAX(), and MIN(). I use them for totals, averages, and other summaries.

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

GROUP BY groups rows sharing the same values in columns. It is often used with aggregate functions. It helps me summarize data, like total sales by region or average salary by department.

Oracle SQL Interview Questions for Experienced

Let’s go through some interview questions on Oracle SQL for experienced professionals.

13. How does the Oracle SQL optimizer determine an execution plan?

The optimizer analyzes statistics like table size, indexes, and data distribution. It compares different ways to run a query and picks the one with the lowest estimated cost.

14. What is the difference between RANK and DENSE_RANK functions?

Both assign ranks to rows in a result set. RANK skips numbers when there are ties. DENSE_RANK does not. 

For example, if two rows share rank 1, RANK gives the next one rank 3, but DENSE_RANK gives it 2.

15. How would you troubleshoot a slow-running SQL query in Oracle?

First, I would check the execution plan using EXPLAIN PLAN. Then I look at index usage, joins, and filtering. Sometimes adding indexes or rewriting subqueries improves performance. Stats might need refreshing too.

16. What are bind variables and why are they used in Oracle SQL?

Bind variables are placeholders for values in queries. They help reduce parsing overhead and improve performance. They also protect against SQL injection by separating code from input data.

17. Explain the difference between row-level and statement-level triggers.

Row-level triggers fire once per affected row. They can access data in each row. Statement-level triggers run once for the entire operation, regardless of how many rows are changed.

Oracle SQL Developer Interview Questions

Here are some commonly asked Oracle SQL Developer interview questions to help you prepare for developer-specific roles.

18. What is the difference between a correlated subquery and a non-correlated subquery?

A correlated subquery references columns from the outer query. It runs once per outer row. A non-correlated subquery runs independently and executes only once.

19. How would you implement a running total in Oracle SQL?

I use the SUM() function with the OVER() clause. It looks like this:

SELECT salary, SUM(salary) OVER (ORDER BY hire_date) AS running_total  

FROM employees;  

This adds each row’s value to the sum of previous rows.

20. What is the MERGE statement used for in Oracle SQL?

MERGE performs insert, update, or delete in one statement. It is useful for syncing two tables. It checks if a match exists and applies the required action.

21. How do you use the EXPLAIN PLAN to analyze query performance?

I run EXPLAIN PLAN FOR your_query; then view the plan with SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);. It shows how Oracle executes the query and highlights potential slow operations.

Oracle SQL Query Interview Questions

Here are frequently asked Oracle SQL query interview questions that test your ability to write and optimize SQL statements.

22. Write a query to find employees who earn more than their managers.

SELECT e.employee_id, e.salary  

FROM employees e  

JOIN employees m ON e.manager_id = m.employee_id  

WHERE e.salary > m.salary;

23. How would you write a query to find duplicate rows in a table?

SELECT column1, COUNT(*)  

FROM table_name  

GROUP BY column1  

HAVING COUNT(*) > 1;

This finds values in column1 that appear more than once.

24. Write a query to return the top 3 products by total sales revenue.

SELECT product_id, SUM(revenue) AS total  

FROM sales  

GROUP BY product_id  

ORDER BY total DESC  

FETCH FIRST 3 ROWS ONLY;

25. How do you pivot rows into columns in Oracle SQL?

Use the PIVOT operator (Oracle 11g+). 

Example:

SELECT *  

FROM sales  

PIVOT (SUM(amount) FOR month IN (1 AS Jan, 2 AS Feb, 3 AS Mar));

It turns month values into columns.

Oracle PL SQL Interview Questions

Here are some important Oracle PL SQL interview questions and answers.

26. What is the difference between a procedure and a function in PL/SQL?

A function returns a value; a procedure does not. Functions can be used in SQL statements, while procedures are called explicitly. Both can take parameters and include logic.

27. What is a trigger in PL/SQL and when would you use it?

A trigger is a block that runs automatically on an event like INSERT, UPDATE, or DELETE. I use it to track changes, enforce rules, or update audit tables.

28. How is exception handling implemented in PL/SQL?

PL/SQL uses BEGIN…EXCEPTION…END blocks. I catch errors using specific exceptions like NO_DATA_FOUND or TOO_MANY_ROWS. I can also define custom exceptions with RAISE.

29. What is the purpose of using cursors in PL/SQL?

Cursors let me fetch query results row by row. They are useful when I need to process each row individually, especially in complex business logic that loops through data.

Also Read - Top 30+ PL/SQL Interview Questions and Answers

Oracle SQL PL SQL Interview Questions

Here are top interview questions on Oracle SQL and PL SQL with answers to help you prepare for both query-based and procedural programming roles.

30. How would you manage transactions in PL/SQL using COMMIT, ROLLBACK, and SAVEPOINT?

I use COMMIT to make changes permanent. ROLLBACK undoes changes since the last commit. SAVEPOINT lets me mark a point to roll back to without undoing the whole transaction.

31. What are the key differences between SQL and PL/SQL?

SQL is used for data queries and updates. PL/SQL adds procedural features like loops and conditions. PL/SQL supports blocks, functions, and error handling. SQL runs one statement; PL/SQL runs multiple as a block.

32. Explain the use of packages in Oracle PL/SQL.

Packages group related procedures, functions, variables, and cursors. The package spec defines the interface. The body holds the logic. I use them to organize code and reduce recompilation.

33. How would you schedule and run a PL/SQL block using DBMS_SCHEDULER?

I use DBMS_SCHEDULER.CREATE_JOB. It lets me define the PL/SQL block, schedule time, and set recurrence. I activate the job using ENABLE. It is useful for automating tasks like reports or cleanup scripts.

Also Read - Top 50+ SQL Interview Questions and Answers

Tips to Prepare for Oracle SQL Interview

Preparing for an Oracle and SQL interview questions requires strong basics and confidence in solving real-world query problems. Here are some tips you can follow –

• Review core SQL concepts like joins, subqueries, and aggregate functions
• Practice writing queries by hand to improve recall
• Understand execution plans and indexing
• Solve scenario-based tasks using mock tables
• Go through Oracle SQL questions for practice from recent interviews
• Learn how to read and debug SQL output correctly
• Practice explaining your logic clearly during mock interviews

Also Read - Top 50+ Oracle Interview Questions and Answers

Wrapping Up

With these 25+ Oracle SQL interview questions and answers, you will have a better chance of success. Focus on both writing and explaining queries clearly. Practice regularly and stay updated with new features in Oracle.

Looking for Oracle SQL jobs? Find top IT openings on Hirist and get the right role that fits your skills and career goals.

FAQs