How to update ordered list having a constraint

Question

hi friends,
I have to update a column for all the records in the table with a unique contraint
ex-- select rec_no from tablename;
9
8
7
6
5
now i have to update this like
select rec_no from tablename;
8
7
6
5
4
but when i am doing update tablename set rec_no=rec_no-1
i am getting unique constraint error
plz help me

Answer 1

dj wrote:
but when i am doing update tablename set rec_no=rec_no-1
i am getting unique constraint error
No you don't. Show us.
Oracle ensures "statement level constraint" consistency. If you do this in one update statement (which is what you state above), then Oracle will not give you a constraint error.
For if it did, then we all would have a serious problem: whether or not the update succeeds would then depend on the order in which Oracle (ie. the execution plan) happens to update the rows. And we can't have such 'undeterministic' behavior (similar to what we would have when Oracle did not provide us with the mutating table error...).
So just try it, or show us:
create table uk_test
(i number not null unique)
insert into uk_test values(4);
insert into uk_test values(3);
insert into uk_test values(2);
commit;Now try both of these:
update (select * from uk_Test order by i desc) set i=i-1;
update (select * from uk_Test order by i) set i=i-1;They both succeed. Oracle allows temporary constraint violations during/inside the execution of one statement. There is no need to set this constraint to deferred here.
I talk about statement level constraint consistency here too:
http://rulegen.blogspot.nl/2012/01/statement-level-constraint-validation.html