Top questions with answers asked in MNC on Oracle

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Oracle interview questions along with their answers that might be asked in top multinational companies (MNCs):

  1. What is Oracle Database, and what are its key features?
    • Answer: Oracle Database is a relational database management system (RDBMS) developed and marketed by Oracle Corporation. Its key features include:
      • Scalability: Oracle Database is highly scalable and can handle large volumes of data and thousands of concurrent users, making it suitable for enterprise-level applications.
      • High availability: Oracle Database provides features such as Oracle Real Application Clusters (RAC), Data Guard, and Automatic Storage Management (ASM) for ensuring high availability, data protection, and disaster recovery.
      • Security: Oracle Database offers robust security features, including data encryption, access control, auditing, and fine-grained access privileges, to protect sensitive data and prevent unauthorized access or data breaches.
      • Performance: Oracle Database is optimized for performance, with features such as query optimization, indexing, partitioning, and in-memory processing to deliver fast and efficient data retrieval and processing.
      • Reliability: Oracle Database is known for its reliability and data integrity, with features such as transaction management, rollback and recovery, and data consistency checks to ensure data reliability and durability.
      • Extensibility: Oracle Database supports a wide range of data types, programming languages, APIs, and development tools for building and deploying custom applications and integrating with other systems.
  2. What are the different types of indexes in Oracle Database, and how do they improve query performance?
    • Answer: Oracle Database supports several types of indexes to improve query performance and data access efficiency:
      • B-tree index: B-tree (balanced tree) indexes are the most common type of index in Oracle Database, used for efficient retrieval of data based on equality or range conditions. B-tree indexes store keys in sorted order, allowing for fast lookup and retrieval of data.
      • Bitmap index: Bitmap indexes are specialized indexes used for columns with low cardinality (few distinct values) or boolean columns. Bitmap indexes store bitmap vectors for each distinct value, indicating the presence or absence of rows with that value, making them efficient for queries with multiple predicates or bitmap operations.
      • Function-based index: Function-based indexes are indexes created on expressions or functions of column values, allowing for indexing of computed or transformed data. Function-based indexes can improve query performance for queries involving complex calculations or transformations.
      • Partitioned index: Partitioned indexes are indexes created on partitioned tables, with each index partition corresponding to a table partition. Partitioned indexes can improve query performance by limiting index scans to specific partitions, reducing the amount of data scanned and improving index access efficiency.
      • Reverse key index: Reverse key indexes are indexes created by reversing the bytes of the indexed column values, which can reduce index contention and hot spots in high-concurrency environments. Reverse key indexes can improve query performance by distributing insertions across index blocks more evenly.
      • Bitmap join index: Bitmap join indexes are specialized indexes used for star schema queries involving fact and dimension tables. Bitmap join indexes precompute join operations between fact and dimension tables using bitmap vectors, enabling fast query processing for star queries with multiple join conditions.
  3. What is the difference between DELETE and TRUNCATE in Oracle Database, and when would you use each?
    • Answer: DELETE and TRUNCATE are both SQL statements used for removing data from tables in Oracle Database, but they differ in their behavior and usage:
      • DELETE: The DELETE statement removes rows from a table based on specified conditions, such as a WHERE clause, allowing for selective deletion of data. DELETE operations are logged and can be rolled back using the ROLLBACK statement, and triggers associated with the table are fired for each deleted row. DELETE operations are slower and generate undo and redo logs, consuming more resources compared to TRUNCATE.
      • TRUNCATE: The TRUNCATE statement removes all rows from a table, effectively resetting the table to an empty state. TRUNCATE operations are faster than DELETE operations because they do not generate undo and redo logs, and they release storage space immediately. TRUNCATE operations cannot be rolled back, and triggers associated with the table are not fired. TRUNCATE operations are typically used for bulk deletion of data or resetting tables in data warehousing or staging environments.
  4. What is the purpose of the Oracle Data Dictionary, and how do you query it to retrieve information about database objects?
    • Answer: The Oracle Data Dictionary is a collection of metadata tables and views that store information about database objects, such as tables, views, indexes, constraints, users, roles, and privileges. The Data Dictionary provides a centralized repository for storing and accessing metadata about the database schema, structure, and configuration. To query the Oracle Data Dictionary, you can use SQL queries against predefined views such as:
      • DBA_TABLES: Contains information about tables owned by all users in the database.
      • DBA_VIEWS: Contains information about views owned by all users in the database.
      • DBA_INDEXES: Contains information about indexes owned by all users in the database.
      • DBA_CONSTRAINTS: Contains information about constraints defined on tables in the database.
      • DBA_USERS: Contains information about database users and their privileges.
      • DBA_ROLES: Contains information about database roles and their privileges.
      • DBA_TAB_COLUMNS: Contains information about columns defined in tables in the database. By querying these views with appropriate filtering conditions and joining conditions, you can retrieve detailed information about database objects, their properties, and their relationships.
  5. What are the different types of joins supported by Oracle Database, and how do you use them in SQL queries?
    • Answer: Oracle Database supports several types of joins for combining data from multiple tables in SQL queries:
      • Inner join: An inner join returns only the rows that have matching values in both tables based on the join condition specified in the WHERE clause. Inner joins are commonly used to retrieve related data from multiple tables.
      • Left outer join: A left outer join returns all rows from the left table (the first table specified in the join) and matching rows from the right table (the second table specified in the join) based on the join condition. If no matching rows are found in the right table, NULL values are returned for the columns from the right table.
      • Right outer join: A right outer join returns all rows from the right table and matching rows from the left table based on the join condition. If no matching rows are found in the left table, NULL values are returned for the columns from the left table.
      • Full outer join: A full outer join returns all rows from both tables and combines them based on the join condition. If no matching rows are found in one of the tables, NULL values are returned for the columns from the other table.
      • Cross join: A cross join returns the Cartesian product of the two tables, combining each row from the first table with every row from the second table. Cross joins generate a large result set and are typically used for generating combinations of data from multiple tables. To use joins in SQL queries, you specify the join type (e.g., INNER JOIN, LEFT OUTER JOIN) and the join condition (e.g., ON clause) to define how the tables should be joined and how the rows should be combined.