Instance: It refers to the Oracle process that accesses the database. An instance works on the database but first it has to mount it. An instance can mount atmost one database but a database can be mounted by multiple instances.

Database: It refers to the set of files where the metadata and the application data is stored. This is the operational data.

SID: An SID identifies an instance.

The Exists clause:

The exists clause is used to check for the existence of rows in the subquery. Based on whether the rows exist, the outer query would be executed. If the subquery does not return any rows then the outer query would not be executed.

For instance:

  1. select 1 from table t2; => This would return as many rows as there are in the table "t2" . If we have the following query:
    select count(*) from table t1 where exists (select 1 from table t2); => The result would be the row count of the table t1 if there t2 is not empty and has some rows.
  2. select count(*) from table t1 where exists (select 1 from table t2 where =; => This is an example of a corelated subquery.

An interesting question:

Suppose there is a table "Emp(id, salary)" and you need to return the top 3 employees w.r.t salary. You could do this:

select * from (select * from Emp E order by salary desc) where rownum < 4;



Nelson’s ode to sweetheart

In the dawn, you came and dazzled

with the beauty of your smile

the lips pressed and the hearts weld

the aura of these moments I compile


love unremitting, entwined in passion

our souls in tight embrace 

travelled from nation to nation 

time extended but losing trace 


heart beseeched to not adjourn

migrate I must for it is winter

take on wings away from this sojourn

warm is the heart that is torn asunder

Nelson’s quatrains

Gaze at the spring I fully engage

forget the winter that comes and stays

why to drink in haste

prepare for summer in these long cold days


Visiting my soul I ask thee

when its not yours to keep

why ask the humming bird which flower is sweet

for it will never fly on its wings back to the cage in glee