For All SQL , Tested in PostgresSQL  : Javedcc@gmail.com

SELECT * FROM <Table_Name> A
WHERE N = (SELECT COUNT(DISTINCT B.Column_Name ) FROM
<Table_Name> B WHERE A.Column_Name >= B.Column_Name)

when ever we use DML commands

Please find the below link which is related to Practical 
PostgreSQL.

 

http://www.commandprompt.com/ppbook

usig commit command

It maintains backups

It is a storage device, like floppy or hard disk 

RAID - Redundant Array of Independant Disks

RAID refers to a hard disk technology which can be used to
speed up data transfer and/or provide disk redundancy
through disk system fault tolerance.
RAID configurations are commonly found in data critical
applications.

RAID serves for two data storage requirements
1]Reliability
2]Performance
->Reliability-It maintains multiple copies of same data in
array of disks to make the storage fault tolerant in case of
disk failure.
->Performance-Different bits of a data component is stored
across an array of disks(bit striping) so that access time
is greatly reduced since single read time is enough to read
all the bits since they are in different disks, thus
boosting the performance.

Higher Data Security: Through the use of redundancy, most
RAID levels provide  protection for the data stored on the
array. This means that the data on the array can  withstand
even the complete failure of one hard disk (or sometimes
more) without any  data loss, and without requiring any data
to be restored from backup. This security feature  is a key
benefit of RAID and probably the aspect that drives the
creation of more RAID  arrays than any other. All RAID
levels  provide some degree of data protection , depending 
on the exact implementation, except RAID level 0 .

For more detail about RAID : http://newadmins.blogspot.com/