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Interview Questions and Answers for 'Databases' - 120 question(s) found - Order By Newest

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Frequently asked in Cognizant ( Based on 2 feedback )

Q1. What are Inner , Outer , Left and Right Joins in SQL ?

Ans. Inner join is the intersection of two tables on the condition defined by the where clause i.e will get records from both tables matched by a column.

Outer join is the union of two tables i.e will get all records from both tables and will put null in the columns where related records are not present.

Left Outer join is the left union of two tables i.e all records from the table on the left and values from the right table for related records else null for the columns from right table.

Right Outer join is the right union of two tables i.e all records from the table on the right and values from the left table for related records else null for the columns from left table.

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Frequently asked to fresh graduates.

Q2. 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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Frequently asked to experienced developers. Recently asked in many US interviews.

Q3. What is database deadlock ? How can we avoid them?

Ans. When multiple external resources are trying to access the DB locks and runs into cyclic wait, it may makes the DB unresponsive.

Deadlock can be avoided using variety of measures, Few listed below -

Can make a queue wherein we can verify and order the request to DB.

Less use of cursors as they lock the tables for long time.

Keeping the transaction smaller.

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Q4. What is JDBC? Describe the steps needed to execute a SQL query using JDBC.

Ans. The JDBC is a pure Java API used to execute SQL statements. It provides a set of classes and interfaces that can be used by developers to write database applications.

The steps needed to execute a SQL query using JDBC:

1. Open a connection to the database.
2. Execute a SQL statement.
3. Process th results.
4. Close the connection to the database.

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Q5. What things you would care about to improve the performance of Application if its identified that its DB communication that needs to be improved ?

Ans. 1. Query Optimization ( Query Rewriting , Prepared Statements )

2. Restructuring Indexes.

3. DB Caching Tuning ( if using ORM )

4. Identifying the problems ( if any ) with the ORM Strategy ( If using ORM )

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Q6. If you are given a choice to implement the code to either Insert a Record or Update if already exist, Which approach will you follow ?

1. Insert into the DB Table. If exception occurs, update the existing record.
2. Check if the record exists and update it if it exists, If not insert a new record.

Ans. In first case, there would be 2 DB calls in worst case and 1 in best case. In 2nd approach there will be always 2 DB calls.

Decision on the approach should depend on the following considerations -

1. How costly is the call to DB ? Are we using indices , hibernate etc

If calls to DB are costly , 1st approach should be the choice.

2. Exception Book keeping load upon exception.

The benefit of saving 1st call in approach 1 should be bigger than the Book keeping for the exception.

3. Probability of the exception in first apparoach.

If the DB Table is almost empty, it makes sense to follow Approach 1 as majority of the 1st calls will pass through without exception.

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Q7. What is the use of @GeneratedValue annotation in Hibernate?

Ans. This annotation is added to the auto increment column with the strategy to increment the column value. Usually this is added to the surrogate primary key column and specified with the Database Sequence.

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Q8. What are temp tables ?

Ans. These are the tables that are created temporarily and are deleted once the Stored Procedure is complete.

For example - we may like to pull some info from a table and then do some operations on that data and then store the output in final output table. We can store the intermediary values in a temp table and once we have final output with us, we can just delete it.

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Q9. 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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Q10. 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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Q11. 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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Q12. What is database Normalization ?

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

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Q13. What happens to backups if we delete the RDS instance ?

Amazon Web Services (AWS)

Ans. Automated Backups are deleted automatically whereas Snapshots are preserved. Upon terminating RDS instance , AWS gives a warning about that and requests to create a final snapshot before terminating the instance.

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Q14. What is connection pooling?

Ans. It's a technique to allow multiple clients to make use of a cached set of shared and reusable connection objects providing access to a database or other resource.

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Q15. Difference between Inner and Outer Join ?

Ans. Inner join is the intersection of two tables on a particular columns whereas Outer Join is the Union of two tables.

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Q16. What is a Cursor ?

Ans. It's a facility that allows traversal over the records pulled from a table or combination of tables. Its like iterator in Java.

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Q17. Does SQL allow null values ? Can we use it within Where clause ?

Ans. Yes , we can have null values for columns in SQL. Null value represent that the columns value is unknown or haven't been filled. Yes, We can use it within where clause to get the rows with null values.

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Q18. Write an SQL to find all records having all upper case alphanumeric characters in a field ?

Ans. Select * from Table where field = upper(field)

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Q19. Write an SQL to find all records having all numeric characters in a field ?

Ans. Select * from Table where isnumeric(field) = 1;

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Q20. Do you know of any DB statement that can be used if we would like to insert multiple rows into a table ?

Ans. We can use INSERT ALL.

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Q21. What is the advantage of JPA ?

Ans. Its a specification that guides the implementation of ORM frameworks. Implementations abiding by the specification would mean that one can be replaced with other in an application without much hassle. Only the Features that are added over the specification needs to be taken care of if any such change is made.

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Q22. 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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Q23. 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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Q24. How can we disable a constraint ?

Ans. alter table
table_name
DISABLE constraint
constraint_name;

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Q25. 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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Q26. What should be done for auto generating primary key id in a table ?

Ans. We need to create a sequence.

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Q27. Which constraint cannot be specified as an explicit constraint and should be specified with the column only ?

Ans. NOT NULL

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Q28. How to know the structure of a Table in Oracle ?

Ans. DESC

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Q29. How to know the constraints on a Table in Oracle ?

Ans. SELECT *
FROM user_constraints
WHERE table_name = ''

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Q30. 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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