Oracle technical interview questions 2025. Top 31+ Questions. Final Part.

In an Oracle database, the control file serves several critical purposes:

1. Database Identification: Contains essential information about the database, such as the database name and unique identifier (DBID), which helps identify the database on the system.

2. File Locations: Stores the locations of the database’s data files and redo log files, allowing Oracle to locate and manage these files during database operations.

3. Checkpoint Information: Tracks checkpoint data, which helps synchronize the data files and redo log files, enabling faster and more consistent recovery in case of system failures.

4. Recovery Management: Contains information on backup and recovery, including archive log history and RMAN backup metadata, making it essential for database recovery.

5. Status and Metadata: Maintains the current state of the database, including timestamp information, log history, and the SCN (System Change Number), used for consistency across database instances

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)What is a nested table?

Solution: In Oracle, a nested table is a table within a table, allowing you to store a set of values (like an array) as a single column in another table. This lets you model one-to-many relationships directly within a table structure.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Differentiate between a hot and cold backup in Oracle and tell about their benefits.

Solution:

Hot Backup

Definition: A hot backup, also known as an online backup, is performed while the database is actively running and accessible to users.

Benefits:

1. Minimal Downtime: Since the database remains operational, users can continue to access and modify data during the backup process, resulting in minimal disruption.

2. Continuous Availability: Ideal for environments requiring high availability, such as production systems, as users do not experience any interruptions in service.

3. Incremental Backups: Allows for incremental backups to be taken, where only the changes since the last backup are saved, reducing storage space and backup time.

Cold Backup

Definition: A cold backup, also known as an offline backup, is performed when the database is completely shut down and inaccessible to users.

Benefits:

1. Consistency: Ensures a consistent backup since no changes are being made during the backup process, which helps in maintaining data integrity.

2. Simplicity: Easier to manage as there are no active transactions or open connections to consider, making the backup process straightforward.

3. Less Resource Intensive: Requires fewer system resources since the database is not actively processing user transactions, leading to potentially faster backup times.

Both backup types are essential in different scenarios, and the choice depends on the specific requirements of the database environment.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)What is the relationship between database, tablespace and data file?

Solution:

The relationship between a database, tablespace, and data file in an Oracle database can be summarized as follows:

1. Database: This is the highest-level structure that encompasses all the data and database objects. It includes the overall organization of data and the management of various components like tables, indexes, views, and schemas.

2. Tablespace: A tablespace is a logical storage unit within a database that groups related data files. It acts as a container for various types of data, allowing for efficient management of storage and allocation of space for database objects (like tables and indexes). A database can contain multiple tablespaces, each serving different purposes or containing different types of data.

3. Data File: A data file is a physical file on the disk that stores the actual data for a tablespace. Each tablespace consists of one or more data files, and the data in these files corresponds to the database objects contained within that tablespace. Data files are the lowest-level component in this hierarchy and directly store the raw data used by the database.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Define COALESCE function?

Solution: The COALESCE function in SQL returns the first non-null expression from a list of arguments. It is commonly used to handle null values in queries, allowing you to provide default values when certain data points are absent.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Explain the ANALYZE command in Oracle.

Solution:

The ANALYZE command in Oracle is used to gather statistics about database objects, such as tables, indexes, and partitions. This information helps the Oracle optimizer make informed decisions about the most efficient way to execute SQL queries. The command can perform various tasks, including:

1. Gathering Statistics: Collects data on the number of rows, blocks, and distinct values, which are used to estimate the cost of query execution plans.

2. Checking for Integrity: Validates the integrity of the specified database objects.

3. Identifying Storage Characteristics: Helps in identifying the storage and usage characteristics of the object.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Tell about the various constraints in Oracle.

Solution:

Here are the various types of constraints in Oracle:

1. Primary Key Constraint:

   • Uniquely identifies each record in a table.

   • Ensures that no two rows have the same primary key value and that it cannot be null.

2. Foreign Key Constraint:

   • Establishes a relationship between two tables by linking a column in one table (the foreign key) to the primary key of another table.

   • Ensures that the foreign key values match values in the referenced table or are null.

3. Unique Constraint:

   • Ensures that all values in a column (or a combination of columns) are unique across the table.

   • Unlike a primary key, it allows null values (unless specified otherwise).

4. Check Constraint:

   • Ensures that all values in a column satisfy a specific condition defined by a logical expression.

   • Useful for enforcing business rules, such as ensuring age is greater than zero.

5. Not Null Constraint:

   • Ensures that a column cannot contain null values, enforcing that a value must always be present in that column.

6. Default Constraint:

   • Specifies a default value for a column when no value is provided during an insert operation.

   • Helps to ensure that certain columns have a predefined value if not explicitly set.

7. Composite Key Constraint:

   • A combination of two or more columns that uniquely identify a record in a table.

   • Can be used as a primary or unique key constraint.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)How are comments represented in Oracle?

Solution:

In Oracle, comments can be represented in two ways:

1. Single-line comments: Preceded by --. Everything following -- on that line is considered a comment.

2. Multi-line comments: Enclosed between /* and */. Everything within these markers is considered a comment, regardless of line breaks.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Differentiate between SQL and iSQL*Plus?

Solution:

SQL

• Definition: SQL (Structured Query Language) is a standard programming language used for managing and manipulating relational databases. It includes commands for querying data, inserting, updating, and deleting records.

• Usage: Primarily used for writing database queries and commands directly to interact with the database.

• Environment: Can be executed in various environments, including SQL*Plus, database applications, programming languages, and database management tools.

• Features: Focuses on data manipulation and definition, allowing users to create, read, update, and delete data.

iSQL*Plus

• Definition: iSQLPlus is a web-based version of SQLPlus, allowing users to execute SQL and PL/SQL commands through a web browser.

• Usage: Designed for interactive SQL sessions and reports via a web interface, making it accessible from anywhere with internet access.

• Environment: Runs in a web browser and provides a graphical user interface (GUI) for executing SQL commands and scripts.

• Features: Offers additional features such as web-based reporting, scripting capabilities, and session management in a more user-friendly environment compared to the command-line SQL*Plus.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)What is DML?

Solution: DML, or Data Manipulation Language, is a subset of SQL used for managing and manipulating data in a database. It includes commands such as INSERT, UPDATE, DELETE, and SELECT, allowing users to add, modify, remove, and retrieve data from database tables.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Describe the Redo Log file mirroring?

Solution:

Redo log file mirroring is a feature in Oracle databases that enhances data protection and recovery. It involves maintaining two copies of redo log files—one primary copy and one or more mirrored copies.

Key Points:

1. Redundancy: By having multiple copies of redo logs, the database can continue to function even if one copy becomes corrupt or is lost, ensuring higher availability and reliability.

2. Automatic Management: Oracle automatically manages the mirroring process, allowing for real-time writing of redo information to both primary and mirrored log files.

3. Recovery: In the event of a failure, the mirrored redo log files can be used to recover committed transactions, minimizing the risk of data loss.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)What is a View?

Solution: A view in Oracle is a virtual table that is based on the result of a SQL query. It presents data from one or more tables in a specific format or structure without storing the data itself. Views can simplify complex queries, provide a layer of security by restricting access to certain data, and can be used to present data in a way that is more meaningful for users.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)In SQL queries, what is the purpose of the double ampersand (&&)? Could you give an example?

Solution:

In SQL queries, the double ampersand (&&) is used as a substitution variable in SQL*Plus and similar environments. It allows users to prompt for input values, which can be reused within the same session.

Example:

SELECT * FROM employees WHERE department_id = &&dept_id;

When this query is run, the user will be prompted to enter a value for dept_id. If the user inputs 10, the query will effectively become:

SELECT * FROM employees WHERE department_id = 10;

The value entered will be remembered and can be used in subsequent occurrences of &&dept_id within the same session.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Describe the save point in the Oracle database.

Solution: A savepoint in an Oracle database is a marker within a transaction that allows you to set a point to which you can later roll back, without affecting the entire transaction. This enables partial undoing of changes made after the savepoint while keeping the previous changes intact. Savepoints are useful for managing complex transactions and improving error recovery.

Example:

SAVEPOINT my_savepoint;
— Some DML operations
ROLLBACK TO my_savepoint; — Reverts to the state at the savepoint

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)What are Aggregate functions in Oracle?

Solution:

Aggregate functions in Oracle are built-in functions that perform calculations on a set of values and return a single summarizing value. They are commonly used in SQL queries to summarize data.

Common Aggregate Functions:

1. SUM(): Calculates the total of a numeric column.

2. AVG(): Computes the average value of a numeric column.

3. COUNT(): Counts the number of rows or non-null values in a column.

4. MAX(): Finds the maximum value in a column.

5. MIN(): Retrieves the minimum value in a column.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Differentiate between PL SQL and SQL?

Solution:

SQL (Structured Query Language)

• Definition: SQL is a standard language used for querying and manipulating relational databases. It is used to perform operations like retrieving, inserting, updating, and deleting data.

• Nature: Declarative language, meaning it focuses on what to do (e.g., fetch data) rather than how to do it.

• Functionality: Primarily used for data manipulation and definition; cannot perform procedural operations like loops or conditionals.

• Execution: Executes one statement at a time; each SQL command is independent.

PL/SQL (Procedural Language/SQL)

• Definition: PL/SQL is an extension of SQL developed by Oracle that incorporates procedural programming features, enabling the use of variables, loops, and conditionals.

• Nature: Procedural language, meaning it allows for control structures and programming constructs to define the flow of execution.

• Functionality: Supports complex programming logic, including procedures, functions, packages, and triggers. Allows for error handling and batch processing.

• Execution: Executes multiple SQL statements as a block; can include both SQL and procedural code.

Q)How are pictures stored onto a database?

Solution:

Pictures are stored in a database using BLOBs (Binary Large Objects), which are data types designed to hold large binary data such as images, audio, or video. When an image is uploaded, it is converted into a binary format and stored in a BLOB column of a database table. Alternatively, a reference (such as a file path or URL) to the image can be stored instead of the image itself.

Example:

CREATE TABLE images (
id NUMBER PRIMARY KEY,
image_data BLOB
);

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)Write code to display row numbers with records.

Solution: SELECT ROWNUM AS row_number, employee_id, employee_name
FROM employees;

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)What are Temporal data types?

Solution:

Temporal data types in Oracle are used to store date and time information, allowing for precise handling of time-related data. They include:

1. DATE: Stores date and time down to the second.

2. TIMESTAMP: Extends DATE by including fractional seconds.

3. TIMESTAMP WITH TIME ZONE: Includes time zone information along with the timestamp.

4. TIMESTAMP WITH LOCAL TIME ZONE: Similar to TIMESTAMP WITH TIME ZONE but adjusts the time to the local time zone of the user.

Q)What is the definition of a transaction? Describe the most common errors that can occur during the execution of any transaction.

Solution:

A transaction is a sequence of one or more SQL operations that are executed as a single unit of work. It must either complete entirely or not at all, ensuring data integrity. Transactions follow the ACID properties (Atomicity, Consistency, Isolation, Durability).

Common Errors During Transaction Execution:

1. Deadlock: Occurs when two or more transactions are waiting for each other to release locks, causing a standstill.

2. Timeout: Happens when a transaction takes too long to complete, leading to it being automatically rolled back.

3. Integrity Constraint Violations: Arises when a transaction attempts to perform an operation that violates data integrity rules (e.g., foreign key constraints).

4. Resource Unavailability: Occurs when necessary resources (like locks or memory) are not available, preventing the transaction from executing.

Oracle technical interview questions 2025/Oracle technical interview most important questions for 2025

Q)What is NULL value in Oracle?

Solution: