updated sql q&a

7/30/2019 updated sql Q&A

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SQL QUESTIONS AND ANSWERS

1. What is the difference between TRUNCATE and DELETE commands?

2. Which system table contains information on constraints on all the tables

created?3. Explain normalization?

4. Types of SQL functions

5. WHAT OPERATOR PERFORMS PATTERN MATCHING?

6. What is database?

7. How can i hide a particular table name of our schema?

8. How do I eliminate the duplicate rows?

9. How do I display row number with records?

10. Find out nth highest salary from EMP table11. Display Odd/ Even number of records

12. WHAT IS A INLINE VIEW

13. Which date function returns number value?

14. What are the more common pseudo-columns?

15. What is a relational database management system?

16. State the difference between a primary key and foreign key?

17. What is a synonym?

18. What is a view?

19. What is a schema?

20. What is a join, explain the types of joins?

21. What command is used to get back the privileges offered by the

GRANT command?

22. What command is used to create a table by copying the structure of

another table?

23. How can variables be passed to a SQL routine?

24. What is the use of the DROP option in the ALTER TABLE command?

25. What is a Cartesian product?

26. What is the usage of SAVEPOINTS, COMMIT and ROLLBACK?

27. What is difference between CHAR and VARCHAR2? What is the

maximum SIZE allowed for each type?

28. How many LONG columns are allowed in a table? Is it possible to use

LONG columns in WHERE clause or ORDER BY?

29. Can a view be updated/inserted/deleted? If Yes under what

conditions?

30. What are the advantages of VIEW?31. How can I find the total number of records in a table?

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32. How can you compare a part of the name rather than the entire name?

33. What is GROUP BY?

34. How can I change my Oracle password?

**************************************ANSWERS************************

***********

1. What is the difference between TRUNCATE and DELETE

commands?

TRUNCATE DELETEIt is a DDL statement It is DML statement

Truncate command will remove all

the rows from a table, leaving the

table empty.

Using Delete command we can

remove specific rows from a table

using where clause.Once table is truncated the data is

lost and data cannot be rollback.

Data can be rollback before issuing

COMMITIt is an auto commit statement. It is not an auto commit statement.

In case of TRUNCATE Trigger doesn't

get fired

But in DML commands like DELETE.

Trigger get firedIt free up the memory space

Truncate is faster than delete

SYNTAX SYNTAX

TRUNCATE TABLE EMPLOYEES; DELETE FROM EMPLOYEESWHERE EMPLOYEE_ID = 100;

2. Which system table contains information on constraints on all

the tables created?

USER_CONSTRAINTS table contains the information on

constraints created on all the tables.

SYNTAX

SELECT CONSTRAINT_NAME, CONSTRAINT_TYPE, TABLE_NAME

FROM USER_CONSTRAINTS

WHERE TABLE_NAME='EMPLOYEES';

OR

SELECT * FROM USER_CONSTRAINTS;

3. Difference between decode and case?

Decode

It specify the single column

Can compare only discrete values

Cannot process null

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Case

It specify the multiple columns

Can handle range values (between, )

Processing time is faster when compared to Decode Can process null

All the expressions (columns) must be of the same date

types.

4. Types of functions

Types of functions

Single row functions

Multiple row functions

I.Case manipulation functions 1. Group

Functions

II.character manipulation functions

III. Date functions

IV. Data type conversion

V.Number Functions

VI. General Functions

Single row functions

CASE MANIPULATION FUNCTIONS

Lower - select lower(last_name) from employees; O/P

- king

Upper - select upper(last_name) from employees; O/P

- KING

Inticap - select inticatp(last_name) from employees;

O/P King

CHARACTER MANIPULATION FUNCTIONS

SUBSTR - select substr(last_name,1,3) from employees;

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INSTR - select instr(last_name,n) from employees;

LENGTH -select length(last_name) from employees;

CONCAT - select concat(first_name,last_name) from

employees; (only 2 arguement)

REPLACE - select replace (last_name,a,123) from employees;

(a replace with 123)

REVERSE - select reverse(last_name) from employees;

LPAD / RPAD - select lpad(last_name,20,#) from

employees;

LTRIM / RTRIM - select ltrim(last_name,k) from employees;

DATE FUNCTIONS

MONTHS_BETWEEN - select months_between(sysdate,01-jan-

10) from dual;

(only months_between date functions returns anumber)

ADD_MONTHS - select add_months(sysdate,3) from dual;

NEXT_DAY - select next_day(sysdate,Friday) from dual;

LAST_DAY - select last_day (sysdate) from dual;

DATA TYPE CONVERSION

TO-DATE

TO-CHAR

TO_NUMBER

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NUMBER FUNCTIONS

ROUND

TRUNC

MOD

General functions

NVL - it accepts 2 arguments. If the 1st argument is null

it return 2nd arguments value. Else it returns the 1st

arguments value.NVL2 - it accepts 3 arguments. 1st argument is null it

return 3rd arguments value. Else it returns the 2nd

arguments value.

NULLIF - it accepts 2 arguments. If the both arguments

are equal it returns null. Else it returns the 1st arguments

value.COALESCE - it accepts N number of arguments. It

returns the 1st not null value.

Multiple row functions

Group functions

MAX

MIN

COUNT

SUM

AVG

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5. WHAT OPERATOR PERFORMS PATTERN MATCHING? We have to use the LIKE operators in the where clause to perform

the pattern matching.

SYNTAX

1.For finding first character pattern matching :-

Select *

From employees

Where last_name likea%;

2. For finding last character pattern matching :-

Select *

From employees

Where last_name like%a;

3.For finding the letter in any part of the last_name:-

Select *

From employees

Where last_name like%a%

6. What is database?

A database is the collection of information where we can store the

date, manipulate the date and retrieve the stored data.

Database is designed so that we can store and manage the

enterprise information

7. How can I hide a particular table name of our schema

Using SYNONYMNS we can hide a particular table name of our

schema.

SYNTAX

CREATE SYNONYMN EMP for EMPLOYEES;

After creating the above synonymn we can access the data of EMPLOYEES

table using EMP as table name as below

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SELECT * from EMP;

We can also give alternate name to the objects as well.

8. How do I eliminate the duplicate rows ?

Using DISTINCT keyword we can eliminate duplicate records.

SYNTAX

SELECT DISTINCT DEPARTMENT_ID

FROM EMPLOYEES;

9.How do I display row number with records?

Using ROWNUM keyword we can display row number with records.

Rownum is a pseudocolumn.

After issuing a selectstatement, oracle does is to assign an increasing

(starting with 1, increased by 1) number to each row returned.

SYNTAX

SELECT ROWNUM,EMPLOYEE_ID

FROM EMPLOYEES;

10.Find out nth highest salary from emp table

SYNTAX

select salary from

(select salary,rownum RK from

(select salary from employees

order by salary desc))

where RK=n;

11.Display Odd/ Even number of records

We can use the MOD functions is number functions

SYNTAX

ODD:

select * from employees

where (rowid,1) in

(select rowid, mod(rownum,2) from employees)

Alternative query


where mod(employee_id,2)=0;

EVEN:


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where (rowid,0) in

(select rowid, mod(rownum,2) from employees)

Alternative query


where mod(employee_id,2)=1;

12. WHAT IS A INLINE VIEW

An inline view is a SELECT statement in the FROM-clause of another

SELECT statement.

In-line views are commonly used simplify complex queries by removing

join operations and condensing several separate queries into a singlequery.

SYNTAX

SELECT SALARY FROM

(SELECT SALARY , ROWNUN RK FROM

(SELECT SALARY FROM EMPLOYEES ORDER BY SALARY DESC) )

WHERE RK=4

13. Which date function returns number value?

MONTHS_BETWEEN date function returns number value.

In that date function, we can able to find the months between the defined

date by the user.

SYNTAX

select months_between (sysdate,'01-jan-10') from dual;

date1 date2

The date1 is later than date2 Result will be in Positive.The date1 is earlier than date2 Result will be in Negative.

The defined date format should be in DD-MM-YY. If we insert the other

date format, the oracle server will through the error.

14. What are the more common pseudo-columns?

A. ROWNUM

Rownum is a pseudo column. It numbers the records in a result set.

The first record that meets the where criteria in a select statement is

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given rownum=1, and every subsequent record meeting that same criteria

increases rownum.

B. ROWID

A rowid is a pseudo column (like versions_xid), that uniquely identifies a

row within a table, but not within a database. It is possible for two rows of

two different tables stored in the same cluster to have the same rowid.

C.SYSDATE

Sysdate is a pseudo column, Itll show the system date.

D.SYSTIMESTAMP

Systimestamp is a pseudo column, Itll show the system date with time

E.USER

USER is a pseudo column. It returns the name of the user currentlyconnected

D.UID

UID is a pseudo column,itll display the user ID. For every user the oracle

server has produces the unique ID for every user.

15. What is a relational database management system?

RDBMS

A database management software system that organizes data into a

series of records that are stored in linked tables. This provides the ability

to relate different records, fields and tables, and aids data access and data

transformation.

A relational Databse is collections of relation (or) two

dimensional tables. The organization wants to store the information

about all the employees in your company. In the relational

databases the informational can store in the different tables. All the

table have relations

16. State the difference between a primary key and foreign key?

PRIMARY KEY FOREIGN KEY

Uniquely identifies each row of the table It is referencial identifier

Itll not allow the null values Itll allows the null values

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Itll not allow the duplicate values Itll allows the duplicate values

Only one primary key allow in a table Each table can have more than one

foreign keyPrimary key is unique foreign key reference as Primary key in

another table

Oracle server enforce the uniqueness by

creating a unique index on the primary

key column

Oracle server doesnt enforce such

unique index for the foreign key

column.

17. What is a synonym?

SYNONYM is an alternative name for an object. The

advantages are

a. creating easy reference to a table.

b. shorter length object name.

SYNTAX

To create synonym

CREATE SYNONYM SEMP for EMPLOYEES;

To drop synonymDrop synonym SEMP;

To create the public synonym

Create public synonym dept for deparments;

The public synonym is created so that the all users can

access the synonyms. The database administrator can create

the public synonym. And the removal of public synonym can

be done only by the database administrator.

Drop public synonym dept;

To display all the synonyms created by the user

Select * from user_synonyms;

18. What is a view?

A VIEW is a virtual table.

View logicaly a subset of data from two or more table.

Views contains no data of its own

Its a window through which the data from table can view.Types of Views VIEWS

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SIMPLE VIEWS COMPLEX VIEWSONE Number of tables One or moreNO Contains Functions YESNO Contains groups of

data

YES

YES DML operations

through view

Not always

SYNTAX

To create the view

Create or replace view [View_name]

AS

Select statement

To drop the view

Drop view [view_name]

WITH READ ONLY keyword

Create or replace view [View_name]

AS

Select statement

With read only

By adding with read only keyword, the DML operators are

restricted in the particular view.

Advantages

It is used to restrict data access

It makes complex query easy

Views can hide the complexity of data; for example a view could appear as

Sales2000 or Sales2001, transparently partitioning the actual underlying

table

Views take very little space to store; the database contains only the

definition of a view, not a copy of all the data it presents.

19. What is a schema?

A SCHEMA is a collection of database objects.

A schema is owned by a database user and has the same name as

that user.

Schema objects are logical structures created by users to contain,or reference, their data.

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Schema objects include structures like tables, views, and indexes.

20. What is a join, explain the types of joins?

Selecting data from 2 or more tables is called as joins.

To link the two table, first we have to find out the column

which is exist in the both the table

Types of joins

1. Natural joins

2. Cross joins

3. Outer joins

4. Equi joins

5. Nonequijoins

6. Self joins

NATURAL JOINS

Natural joins clause is based on all columns in the two tables

that have the same name.

The selected rows frim the two tables that have equal values

in all matched column.

The natural joins automatically select the column which is

common in the both tables. But the column name in the both

table should be the same and data types as well

SYNTAX

Select column_name1,column_name2

From table1

NATURAL JOIN table2

CROSS JOINS

The cross join clause produces the cross product of two

tables.

SYNTAX

Select column_name1,column_name2

From table1

CROSS JOIN table2

OUTER JOINS

Selecting matched, Un-matched records is called as outerjoins.

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Types of outer joins

1. left outer joins

2. Right outer joins

3. Full outer joins

Left Outer Joins

A joins between two tables that returns the matched rows as

well as the unmatched rows from the left table is called as

left outer join

SYNTAX

select e.department_id,

d.department_name,

from departments d left outer join employees eon (e.department_id = d.department_id);

Right Outer Joins

A joins between two tables that returns the matched rows as

well as the unmatched rows from the Right table is called as

Right outer join

SYNTAX


d.department_name,

from departments d Right outer join employees e

on (e.department_id = d.department_id);

Full Outer joins

A join between that returns the matched rows and the

unmatched rows from the both left and right tables is called

as full outer joins.

SYNTAX


d.department_name,

from departments d full outer join employees e

on (e.department_id = d.department_id);

21. What command is used to get back the privileges offered by

the GRANT command?

REVOKE command is used to get back the privileges offered

by the GRANT command.

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The SQL command revoke allows to take away System

privileges and object privileges from users and roles

22. What command is used to create a table by copying the

structure of another table?

SYNTAX

CREATE TABLE EMP AS

SELECT * FROM EMPLOYEES

WHERE 1=2 Invalid Condition

If we doesnt give the invalid condition in the where clause

the whole data will copy to the new table (EMP table). Forcopying the structure alone we have to give one invalid

condtion in the where clause.

23. How can variables be passed to a SQL routine?

using the & symbol a variables can be passed to a SQL

routine.

It is a SQLPLUS command.

Entering the data in a row

SYNTAX

Insert into t1 values(&a,&b);

We can assign the both the values every time.

Insert into t1 values (&&a,&b);

We can fix the a value as constant and b value will change

24. What is the use of the DROP option in the ALTER TABLE

command?

The use of the DROP option in the ALTER TABLE command is to dropa specific COLUMN.

SYNTAX

ALTER TABLE TABLE_NAME

DROP COLUMN COLUMN_NAME

25. What is a Cartesian product?

A Cartesian product is a result set which contains all the possible

combinations of each row in each table included in a query.

A Cartesian product is almost always an incorrect result.

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When a join condition is invalid or omitted completely the result is

known as Cartesian product.

By using the cross join , the Cartesian product will produce.

To avoid the Cartesian product, insert the valid join condition.

26. What is the usage of SAVEPOINTS, COMMIT, ROLLBACK?

COMMIT is used to confirm the changes done whenever a

DML(INSERT/UPDATE/DELETE) statement is issued on a table.

SYNTAX

COMMIT;

ROLLBACK is used to undo the changes done issued by a

DML(INSERT/UPDATE/DELETE) statement command.

SYNTAX

ROLLBACK;

Note:- we can undo the changes unless a COMMIT is not issued.

SAVEPOINT

We specify the savepoint after the current transaction with specified

name.

We can rollback to that particular point by rollback to

savepoint_name

Example:-

Insert1

Update1

Savepoint A First savepoint

Insert 2Update2

Savepoint B second savepoint

If we gave the rollback command as

ROLLBACK TO A;

The rollback operators execute upto the savepoint A. The

update2 and insert2 will not be committed until we gave

commit command.

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27. What is difference between CHAR and VARCHAR2? What is

the maximum SIZE allowed for each type?

CHAR VARCHARThe char is a fixed-length character

data type.

The varchar is a variable-length

character data type.The storage size of the char value is

equal to the maximum size for this

column.

The storage size of the varchar

value is the actual length of the

data entered, not the maximum size

for this column.A char can be used when the data

entries in a column are expected to

be the same size.

A varchar can be used when the

data entries in a column are

expected to vary considerably in

size.Maximum size is 2000 and

(minimum/default) is 1

Maximum size is 4000 and

(minimum/default) is 1

28. How many LONG columns are allowed in a table? Is it possible

to use LONG columns in WHERE clause or ORDER BY?

Only One LONG column is allowed in each table.

Long column cannot be included in group by clause (or) order by

clause.

Long column is not copied when a table is created using sub-query.

No constraints can be defined on a long column.

29. Can a view be updated/inserted/deleted? If Yes under what

conditions?

You cannot add the data through a view, if the view contains the

following.

Group Functions

Group by Clause

DISTINCT Keyword

Pseudo column ROWNUM keyword

Columns defined by expressions

NOT NULL column in the base table that are not selected by view.

30. What are the advantages of VIEW?

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To restrict data access

To make complex queries easy

To provide data independence

To present different views of same data

31. How can I find the total number of records in a table?

To find the number of records in a table we can use the

COUNT(*) function.

SYNTAX

SELECT COUNT(*) FROM EMPLOYEES;

32. How can you compare a part of the name rather than the

entire name?

By using LIKE operator in the where clause we can compare

the part of the name.

Example:-

Select employee_id,last_name from employees

Where last_name like %A;

33.What is GROUP BY?

The GROUP BY statement is used in conjunction with the

aggregate functions to group the result-set by one or more

columns.

The GROUP BY clause can be used in a SELECT statement to

collect data across multiple records and group the results byone or more columns.

If the arguement / Expression is added in the select

statement with the group function. That expression will

include with the group by clause in the statement.

If you fail to include the expression in the GROUP BY clause,

the ORACLE server through the error.

The GROUP BY column doesnt have to be in the select list.

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SYNTAX

SELECT EMPLOYEE_ID,FIRST_NAME,MAX(SALARY) FROM EMPLOYEES

WHERE SALARY >15000

GROUP BY EMPLOYEE_ID,FIRST_NAME;

34. How can I change my Oracle password?

ALTER user hr identified by admin;

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