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Database - Interview Questions and Answers for 'At' - 123 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 ?Database2017-01-09 13:07:55

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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   Like         Discuss         Correct / Improve     sql  inner join  outer join  right join  left join     Asked in 24 Companies      basic        frequent


Very frequently asked in HCL Tech ( Based of 4 inputs )
  Q2. Write a Program to reverse a string iteratively and recursivelyCore Java

Ans. Using String method -

new StringBuffer(str).reverse().toString();

Iterative -

Strategy - Loop through each character of a String from last to first and append the character to StringBuilder / StringBuffer

public static String getReverseString(String str){
StringBuffer strBuffer = new StringBuffer(str.length);
for(int counter=str.length -1 ; counter>=0;counter--){
strBuffer.append(str.charAt(counter));
}
return strBuffer;
}

Recursive -

Strategy - Call the method with substring starting from 2nd character recursively till we have just 1 character.

public static String getReverseString(String str){
if(str.length <= 1){
return str;
}
return (getReverseString(str.subString(1)) + str.charAt(0);
}

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   Like         Discuss         Correct / Improve     string  StringBuffer  recursion  for loop  control statements  loop statement  stringbuffer.append  java.lang.String  java.lang.StringBuffer  String Manipulation     Asked in 8 Companies        frequent


 Q3. What is JDBC? Describe the steps needed to execute a SQL query using JDBC.Database
Anonymous
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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   Like         Discuss         Correct / Improve     java   jdbc   db connectivity     Asked in 2 Companies      basic        frequent


Frequently asked to experienced developers. Recently asked in many US interviews.
  Q4. What is database deadlock ? How can we avoid them?Database
Admin
info@buggybread.com
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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   Like         Discuss         Correct / Improve     java   database   architecture     Asked in 7 Companies      expert        frequent


 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 ?Solution
Admin
info@buggybread.com
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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   Like         Discuss         Correct / Improve     java   db   database   hibernate   orm   at&t   overstock.com   performance improvement   architecture   technical lead   architect      intermediate


 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.
Solution
admin
info@buggybread.com
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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   Like         Discuss         Correct / Improve     database   insert-update   db exceution plan   db strategy   design   architecture   technical lead


Frequently asked to fresh graduates.
 Q7. What is ACID ?Database2016-11-04 08:09:44

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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 Q8. What is the use of @GeneratedValue annotation in Hibernate?Hibernate
Admin
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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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  Q9. What is a self Join and give an example of a self Join ?

or

What is self Join and What is it's purpose ?
Database2016-06-30 20:13:35

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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   Like         Discuss         Correct / Improve     sql  joins  self join  self-join  Pair of employee names with first having lesser salary than later  Pair of department names where first dept has lesser employees than the later     Asked in 26 Companies      basic        frequent


 Q10. What happens to backups if we delete the RDS instance ?Amazon Web Services (AWS)2019-01-25 14:35:07

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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   Like         Discuss         Correct / Improve     aws rds  aws database  amazon cloud database


 Q11. What is connection pooling?Database
Admin
info@buggybread.com
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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   Like         Discuss         Correct / Improve     java   jdbc   db connectivity   connection pooling   architecture     Asked in 3 Companies      basic        frequent


 Q12. Difference between Inner and Outer Join ?Database
Admin
info@buggybread.com
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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   Like         Discuss         Correct / Improve     sql   joins   inner join   outer join     Asked in 3 Companies      basic        frequent


 Q13. What is a Cursor ?Database
Admin
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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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 Q14. What are temp tables ?
Admin
info@buggybread.com
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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 Q15. Does SQL allow null values ? Can we use it within Where clause ?
Admin
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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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 Q16. Write an SQL to find all records having all upper case alphanumeric characters in a field ?
admin
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Ans. Select * from Table where field = upper(field)

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 Q17. Write an SQL to find all records having all numeric characters in a field ?
admin
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Ans. Select * from Table where isnumeric(field) = 1;

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 Q18. Do you know of any DB statement that can be used if we would like to insert multiple rows into a table ? Database
Admin
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Ans. We can use INSERT ALL.

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   Like         Discuss         Correct / Improve     database   insert   dml statement   sql


 Q19. What is the advantage of JPA ?Database
Admin
info@buggybread.com
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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 Q20. What things you will look for if you get following exception while making DB call ?

table or view does not exist
Database
Admin
info@buggybread.com
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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 Q21. What's the benefit for specifying constraints like Not Null , Primary Key explicitly instead of specifying it against the column ?Database
Admin
info@buggybread.com
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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 Q22. How can we disable a constraint ?Database
Admin
info@buggybread.com
Ans. alter table
table_name
DISABLE constraint
constraint_name;

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 Q23. Can we have foreign key reference to a non primary key column ?Database
Admin
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Ans. Yes, but the respective key in the foreign table should be declared Unique.

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   Like         Discuss         Correct / Improve     sql   database   oracle     Asked in 2 Companies


 Q24. What should be done for auto generating primary key id in a table ?Database
admin
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Ans. We need to create a sequence.

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 Q25. Which constraint cannot be specified as an explicit constraint and should be specified with the column only ?Database
Admin
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Ans. NOT NULL

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   Like         Discuss         Correct / Improve     sql   oracle   database   ddl   constraint   table creation


 Q26. How to know the structure of a Table in Oracle ?Database
Admin
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Ans. DESC

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 Q27. How to know the constraints on a Table in Oracle ?Database
Admin
info@buggybread.com
Ans. SELECT *
FROM user_constraints
WHERE table_name = ''

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 Q28. How to know the default column value of a Table in Oracle ?Database
Admin
info@buggybread.com
Ans. Select DATA_DEFAULT
from DBA_TAB_COLUMNS
where TABLE_NAME = '' and COLUMN_NAME='';

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 Q29. What are the advantages of using distributed database management system ?Database2016-11-30 16:05:24

Ans. Reliability and Continuity if some of the sites goes down.Easy Scaling up and Down as sites can be added or removed without impacting business continuity.

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 Q30. When do we generally get this database error - integrity constraint violated - parent key not found ?2016-07-12 08:40:57

Ans. This error most likely will be thrown during an insert statement, while inserting a value within a foreign key column which doesnt exist within the Parent column. For example - Trying to add a dept number reference within a Employee Table when the Dept doesnt exist in the Dept Table.

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