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Database - Interview Questions and Answers for 'Oracle' - 18 question(s) found - Order By Newest

Frequently asked to fresh graduates.

Q1. What is ACID ?

Ans. ACID stands for Atomicity, Consistency, Isolation, Durability is a set of properties of database transactions.

Atomicity means all or nothing. i.e parts of a transaction shouldn't commit if any one of them fails. Either the whole transaction should succeed or it should be complete rollback.

Consistency means that any transaction should lead database from one stabe state to another.

Isolation means that the execution of transaction results in a system state that would be obtained if transactions were executed serially.

Durability means that when a transaction is committed it forms the permanent state of database.

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_{Like Discuss Correct / Improve database acid Asked in 7 Companies Intermediate}

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Q2. What is a Database Trigger ?

Ans. A trigger is a special kind of stored procedure that automatically gets executed upon an event in the database server.

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Q3. Have you heard about the external table feature of Oracle ?

Ans. The external tables feature is a complement to existing SQL Loader functionality. It enables to access data in external sources as if it were in a table in the database. We have used it few times for replicating tables across different database systems.

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Q4. What is a self Join and give an example of a self Join ?

or

What is self Join and What is it's purpose ?

Ans. When a Table Join itself , it's a Self Join. For example - we like to know the pair of department names where first dept has lesser employees than the later.

Select D1.name , D2.name from Dept D1, Dept D2 where D1.employee_count < D2.employee_count

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Q5. What is database Normalization ?

Ans. https://en.wikipedia.org/wiki/Database_normalization

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Q6. What things you will look for if you get following exception while making DB call ?

table or view does not exist

Ans. First will check if the table or view actually exist in the DB

If it does , Will make sure to see that the application has rights on the schema that holds the respective Table.

Will then make sure that we have prefixed the schema with the table name while accessing it.

Will then make sure that its not DB Cache that's causing it as the table DDL might have been created recently.

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Q7. What's the benefit for specifying constraints like Not Null , Primary Key explicitly instead of specifying it against the column ?

Ans. In case we specify them explicitly we can have control over constraint name as otherwise they will be system generated. It provides an ease in case we plan to drop the constraint in future.

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Q8. How can we disable a constraint ?

Ans. alter table
table_name
DISABLE constraint
constraint_name;

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Q9. Can we have foreign key reference to a non primary key column ?

Ans. Yes, but the respective key in the foreign table should be declared Unique.

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Q10. What should be done for auto generating primary key id in a table ?

Ans. We need to create a sequence.

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Q11. Which constraint cannot be specified as an explicit constraint and should be specified with the column only ?

Ans. NOT NULL

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Q12. How to know the structure of a Table in Oracle ?

Ans. DESC

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Q13. How to know the constraints on a Table in Oracle ?

Ans. SELECT *
FROM user_constraints
WHERE table_name = ''

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Q14. How to know the default column value of a Table in Oracle ?

Ans. Select DATA_DEFAULT
from DBA_TAB_COLUMNS
where TABLE_NAME = '' and COLUMN_NAME='';

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Q15. Difference between JDBC and Hibernate ?

Ans. JDBC is a standard Java Api for Database communication whereas Hibernate is an ORM framework that uses JDBC to connect with Database.

Hibernate is another layer of object table mapping over JDBC that provide a database independent, object oriented and an efficient way of database communication.

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Q16. What is the difference between Oracle , Cassandra and Redis ?

Ans. Oracle is a relational database. Cassandra is a noSQL Database whereas Redis is an inmemory cache.

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Q17. What is the order in which triggers gets fired ?

Ans. There is no specific order in which database fires the triggers if they are for the same operation. If we need to have a specific order for their execution, we should have them all in a stored procedure and the procedure should get executed upon the event.

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Q18. What is the difference between JDBC, Hibernate and ODBC ?

^{This question was recently asked at 'Oracle Argentina'.This question is still unanswered. Can you please provide an answer.}

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