Question { 8661 }

what is null value?

Answer

Null value is that value which is neither zero or space or
any character.This value is defined by Oracle server, and
hence cannot be described.Also every null value generated
by server is unique than another..hence we cannot compare
it with character, numeric or date functions.

Question { 12774 }

what is the difference between rollback & commit?
can a foreign key has null value?

Answer

With commit, all the changes, which are made using insert,
update or delete SQL statments, are save permanently in the
table(ie DB).Whereas, if one want to undo the changes made
by DML statments, then before issuing commit, one can use
rollback statment.Rollback statment has no effect, if used
after commit.
Yes foreign key can have null value.

Question { TCS, 19150 }

what is the difernece between named and anonymous pl/sql
blocks??

Answer

Anonymous block those pl/sql blocks which are not stored in
your DB as an objects, whereas named pl/sql blocks are
those blocks which are stored in DB as an objects.Due to
storage in DB, these named pl/sql blocks can be called from
another named or anonymous blocks, or from any form also...