Interview Questions and Answers for 'Sql' | Search Interview Question

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Q11. Do you know of any DB statement that can be used if we would like to insert multiple rows into a table ?

Database

Ans. We can use INSERT ALL.

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Q12. What things you will look for if you get following exception while making DB call ?

table or view does not exist

Database

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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Q13. What's the benefit for specifying constraints like Not Null , Primary Key explicitly instead of specifying it against the column ?

Database

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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Q14. How can we disable a constraint ?

Database

Ans. alter table
table_name
DISABLE constraint
constraint_name;

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Q15. Can we have foreign key reference to a non primary key column ?

Database

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

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Q16. What should be done for auto generating primary key id in a table ?

Database

Ans. We need to create a sequence.

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Q17. Which constraint cannot be specified as an explicit constraint and should be specified with the column only ?

Database

Ans. NOT NULL

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Q18. How to know the structure of a Table in Oracle ?

Database

Ans. DESC

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Q19. How to know the constraints on a Table in Oracle ?

Database

Ans. SELECT *
FROM user_constraints
WHERE table_name = ''

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Q20. How to know the default column value of a Table in Oracle ?

Database

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

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Q21. Difference between relational databases and NoSQL databases ?

Database

Ans. https://www.mongodb.com/scale/nosql-vs-relational-databases

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Q22. With a Table employee having columns Id,Name,ManagerEmployeeId, Write a SQL to find the Manager with maximum number of employees ?

Database

Ans. Select Name from EMPLOYEE where ID in (Select ManagerEmployeeId from EMPLOYEE
Group By ManagerEmployeeId
order by count(Id)
LIMIT 1)

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Q23. What do you make out of this error

org.hibernate.exception.ConstraintViolationException: could not insert: [<Entity>]
Caused by: java.sql.SQLIntegrityConstraintViolationException: ORA-00001: unique constraint (<Constraint Name>) violated

and how would you go about debugging it ?

Database

Ans. Application is unable to insert a record as it violates a unique constraint.

The exception states the constraint and Table can be located by the Entity mapping. So I will go to the DB and will first check to which all columns the unique constraint applies. And then I will go and check the code and logs to see how come the duplicate column values were attempted to be inserted when they were not supposed to be.

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Q24. If Table A has 100 records and Table B has 50 records , how many records will the equi join of Table generate ?

SQL

Ans. It depends on the join condition.

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Q25. If Table A has 100 records and Table B has 50 records , how many records will the left join of Table A with Table B generate ?

SQL

Ans. 100 records

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Q26. If Table A has 100 records and Table B has 50 records , how many records will the right join of Table A with Table B generate ?

SQL

Ans. 50 records

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Q27. If Table A has 0 records and Table B has 50 records , how many records will the equi join of Table generate ?

SQL

Ans. 0 records

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Q28. Write a SQL to remove records with a duplicate field in a Table ?

Database

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

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Q29. How different is the Search algorithm when we query Cassandra Tables and When we query Oracle Tables ?

Cassandra

Ans. Relational Database uses linear search if we don't query using indices. Performance for linear search is O(n). If indices have been used , relational database uses binary search. Performance for binary search is O(Log n).

Casandra uses hash search. Performance for Hash Search is O(1).

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Q30. Which of the following databases - Cassandra / Oracle - provides more flexibility about querying the database ?

Database

Ans. Oracle as it is not modeled for a particular query or set of queries.

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Q31. Which is the best NoSql Database for Performance ?

NoSql

Ans. Possible Answer - doument base - monogodb

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Q32. Write a SQL to find 2nd Max value in a Table.

Database

Ans. SELECT MIN(FIELD) FROM (SELECT TOP 2 FIELD FROM TABLE ORDER BY FIELD DESC)

or

SELECT MAX(FIELD) FROM TABLE WHERE FIELD NOT IN (SELECT MAX(FIELD) FROM TABLE)

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Q33. Write a SQL to find max salary by Dept

SQL

Ans. SELECT MAX(SALARY),DEPT FROM EMPLOYEE GROUP BY DEPT

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Q34. Write a SQL to display all duplicate names from Employee Table

SQL

Ans. SELECT * FROM EMPLOYEE A WHERE A.ROW_ID IN (SELECT MAX(ROW_ID) FROM EMPLOYEE b WHERE A.NAME = B.NAME )

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Q35. Write an SQL to get Employees who are also Managers

SQL

Ans. SELECT A.NAME FROM EMPLOYEE A, EMPLOYEE B WHERE A.ID = B.MANAGER_ID

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Q36. What is Left Outer Join ?

Database

Ans. It returns all the rows form the left table and matches the rows in both tables.

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Q37. Describe SQL Joins

Database

Ans. Oracle having following types of joins these are also called 8i joins. There are:

1. Equi (or) Inner join
2. Non Equi join
3.Self join
4.Outer join

Oracle also support the 9i (or) ANSI standard joins.There are:

1.Inner join
2.Left outer join
3.Right outer join
4.Full outer join
5.Natural join

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Q38. What are different in memory data structures that are used in No Sql Database ?

NoSql Database

^{This question is still unanswered. Can you please provide an answer.}

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Q39. What are different types of No Sql Databases ?

NoSql

Ans. graph based
column based
document based

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Q40. Difference between Left and Inner Join ?

SQL

^{This question was recently asked at 'Canopy Tax,Cogent Infotech Corp'.This question is still unanswered. Can you please provide an answer.}

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