Whenever we delete records then they are stored in data 
dictionary and can be retrieved later ie by roll back.
but when we truncate then the memory used by the records 
which is to be truncated is released and it cannot be 
rolled back.

when truncate command is issued, all the rows from the 
table are deleted and the memory space occupied by the 
table in the tablespace is also released whereas when 
delete command is issued, only all the rows are deleted 
fronm the table

Truncate will remove the watermark from the table, but the 
delete will not.

Delete command will remove the data values ( all the 
records of table) and space and table structure will be 
ramain. We cannot free the space and remove the table 
structure using Delete command.
Truncate command will free the memory and removes table 
structure from data dictionary.

ramumcato@yahoo.com

we can go upto some number of records through delete while
in truncate we have to unoccupied all records.

Deletes perform normal DML. That is, they take locks on 
rows, they generate redo  and they require segments in the 
UNDO tablespace. Deletes clear records out of blocks 
carefully. If a mistake is made a rollback can be issued to 
restore the records prior to a commit. 
   Truncates are DDL and truncate moves the High Water Mark 
of the table back to zero. No row-level locks are taken, no 
redo or rollback is generated.so they have the same effect 
as a delete, but without all the overhead. Just one slight 
problem: a truncate is a DDL command, so you can't roll it 
back if you decide you made a mistake. (It's also true that 
you can't selectively truncate -no "WHERE" clause is 
permitted, unlike with deletes, of course).

You cannot TRUNCATE a table that has any foreign key
constraints.  You will have to remove the contraints,
TRUNCATE the table, and reapply the contraints.

You can DELETE any row that will not violate a constraint,
while leaving the foreign key or any other contraint in place.

DELETE: DML, Manual Commit, Till not committed can be 
rollback, can be applied for both row level as well as 
table level, can be used in trigger, doesn't release memory.

TRUNCATE: DDL, auto commit, cann't rollback, applied table 
level, cann't be used in trigger, releases memory.

In both cases structure remains.

the delete command log all the stmts in the transaction log 
and the truncate command dont log the stmts.

Delete is associated with rollback segments like U can undo 
it before committing and U have a choice of deleting a 
specific row or rows where as

Truncate is not associated with rollback segments ie U cant 
undo it and U cant delete a row/rows of ur choice