Oracle Interview Questions And Answers Part - 21

How does one stop and start the OMS? (for DBA)
Use the following command sequence to stop and start the OMS (Oracle Management Server):
oemctl start oms
oemctl status oms sysman/oem_temp
oemctl stop oms sysman/oem_temp
Windows NT/2000 users can just stop and start the required services. The default OEM administrator is "sysman" with a password of "oem_temp".
NOTE: Use command oemctrl instead of oemctl for Oracle 8i and below.

What is an Integrity Constraint ?
Integrity constraint is a rule that restricts values to a column in a table.

How does one create a repository? (for DBA)
For OEM v2 and above, start the Oracle Enterprise Manager Configuration Assistant (emca on Unix) to create and configure the management server and repository. Remember to setup a backup for the repository database after creating it.

If a View on a single base table is manipulated will the changes be reflected on the base table ?
If changes are made to the tables which are base tables of a view will the changes be reference on the view.

The following describes means to create a OEM V1.x (very old!!!) repository on WindowsNT:

. Create a tablespace that would hold the repository data. A size between 200- 250 MB would be ideal. Let us call it Dummy_Space.
. Create an Oracle user who would own this repository. Assign DBA, SNMPAgent, Exp_Full_database, Imp_Full_database roles to this user. Lets call this user Dummy_user. Assign Dummy_Space as the default tablespace.
. Create an operating system user with the same name as the Oracle username. I.e. Dummy_User. Add 'Log on as a batch job' under advanced rights in User manager.
. Fire up Enterprise manager and log in as Dummy_User and enter the password. This would trigger the creation of the repository. From now on, Enterprise manager is ready to accept jobs.

What is a database link ?
Database Link is a named path through which a remote database can be accessed.

How does one list one's databases in the OEM Console? (for DBA)
Follow these steps to discover databases and other services from the OEM Console:
1. Ensure the GLOBAL_DBNAME parameter is set for all databases in your LISTENER.ORA file (optional). These names will be listed in the OEM Console. Please note that names entered are case sensitive. A portion of a listener.ora file:
(SID_NAME = ...
2. Start the Oracle Intelligent Agent on the machine you want to discover. See section "How does one start the Oracle Intelligent Agent?".
3. Start the OEM Console, navigate to menu "Navigator/ Discover Nodes". The OEM Discovery Wizard will guide you through the process of discovering your databases and other services.

What is CYCLE/NO CYCLE in a Sequence ?
CYCLE specifies that the sequence continues to generate values after reaching either maximum or minimum value. After pan ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum.NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value.

What is correlated sub-query ?
Correlated sub query is a sub query which has reference to the main query.

What are the data types allowed in a table ?

What is difference between CHAR and VARCHAR2 ? What is the maximum SIZE allowed for each type ?
CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank spaces. For CHAR it is 255 and 2000 for VARCHAR2.

Can a view be updated/inserted/deleted? If Yes under what conditions ?
A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible.

What are the different types of Coordinations of the Master with the Detail block?

Use the ADD_GROUP_COLUMN function to add a column to a record group that was created at design time?

Use the ADD_GROUP_ROW procedure to add a row to a static record group?

maxvalue.sql Select the Nth Highest value from a table?
select level, max('col_name') from my_table where level = '&n' connect by prior ('col_name') > 'col_name')
group by level;
Given a table called emp with the following columns:
-- id number
-- name varchar2(20)
-- sal number
-- For the second highest salary:
-- select level, max(sal) from emp
-- where level=2
-- connect by prior sal > sal
-- group by level

Find out nth highest salary from emp table?
SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP B WHERE a.sal<=b.sal); For E.g.:- Enter value for n: 2 SAL --------- 3700 Suppose a customer table is having different columns like customer no, payments.What will be the query to select top three max payments?
SELECT customer_no, payments from customer C1
WHERE 3<=(SELECT COUNT(*) from customer C2 WHERE C1.payment <= C2.payment) How you will avoid your query from using indexes?
Where emp_no+' '=12345;
i.e you have to concatenate the column name with space within codes in the where condition.
SELECT /*+ FULL(a) */ ename, emp_no from emp
where emp_no=1234;
i.e using HINTS

What utility is used to create a physical backup?
Either rman or alter tablespace begin backup will do..

What are the Back ground processes in Oracle and what are they.
This is one of the most frequently asked question.There are basically 9 Processes but in a general system we need to mention the first five background processes.They do the house keeping activities for the Oracle and are common in any system.
The various background processes in oracle are
a) Data Base Writer(DBWR) :: Data Base Writer Writes Modified blocks from Database buffer cache to Data Files.This is required since the data is not written whenever a transaction is committed.
b)LogWriter(LGWR) :: LogWriter writes the redo log entries to disk. Redo Log data is generated in redo log buffer of SGA. As transaction commits and log buffer fills, LGWR writes log entries into a online redo log file.
c) System Monitor(SMON) :: The System Monitor performs instance recovery at instance startup. This is useful for recovery from system failure
d)Process Monitor(PMON) :: The Process Monitor performs process recovery when user Process fails. Pmon Clears and Frees resources that process was using.
e) CheckPoint(CKPT) :: At Specified times, all modified database buffers in SGA are written to data files by DBWR at Checkpoints and Updating all data files and control files of database to indicate the most recent checkpoint
f)Archieves(ARCH) :: The Archiver copies online redo log files to archival storal when they are busy.
g) Recoveror(RECO) :: The Recoveror is used to resolve the distributed transaction in network
h) Dispatcher (Dnnn) :: The Dispatcher is useful in Multi Threaded Architecture
i) Lckn :: We can have upto 10 lock processes for inter instance locking in parallel sql.

How many types of Sql Statements are there in Oracle
There are basically 6 types of sql statments.They are
a) Data Definition Language(DDL) :: The DDL statements define and maintain objects and drop objects.
b) Data Manipulation Language(DML) :: The DML statements manipulate database data.
c) Transaction Control Statements :: Manage change by DML
d) Session Control :: Used to control the properties of current session enabling and disabling roles and changing .e.g. :: Alter Statements, Set Role
e) System Control Statements :: Change Properties of Oracle Instance .e.g.:: Alter System
f) Embedded Sql :: Incorporate DDL, DML and T.C.S in Programming Language.e.g:: Using the Sql Statements in languages such as 'C', Open, Fetch, execute and close

What is a Transaction in Oracle
A transaction is a Logical unit of work that compromises one or more SQL Statements executed by a single User. According to ANSI, a transaction begins with first executable statement and ends when it is explicitly committed or rolled back.

Key Words Used in Oracle
The Key words that are used in Oracle are ::
a) Committing :: A transaction is said to be committed when the transaction makes permanent changes resulting from the SQL statements.
b) Rollback :: A transaction that retracts any of the changes resulting from SQL statements in Transaction.
c) SavePoint :: For long transactions that contain many SQL statements, intermediate markers or savepoints are declared. Savepoints can be used to divide a transaction into smaller points.
d) Rolling Forward :: Process of applying redo log during recovery is called rolling forward.
e) Cursor :: A cursor is a handle ( name or a pointer) for the memory associated with a specific stamen. A cursor is basically an area allocated by Oracle for executing the Sql Statement. Oracle uses an implicit cursor statement for Single row query and Uses Explicit cursor for a multi row query.
f) System Global Area(SGA) :: The SGA is a shared memory region allocated by the Oracle that contains Data and control information for one Oracle Instance. It consists of Database Buffer Cache and Redo log Buffer.
g) Program Global Area (PGA) :: The PGA is a memory buffer that contains data and control information for server process.
g) Database Buffer Cache :: Database Buffer of SGA stores the most recently used blocks of database data. The set of database buffers in an instance is called Database Buffer Cache.
h) Redo log Buffer :: Redo log Buffer of SGA stores all the redo log entries.
i) Redo Log Files :: Redo log files are set of files that protect altered database data in memory that has not been written to Data Files. They are basically used for backup when a database crashes.
j) Process :: A Process is a 'thread of control' or mechanism in Operating System that executes series of steps.

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