unique & update

Hi All

Im into SQL trees trying to work with CELKO way to do it:
http://www.intelligententerprise.com/001020/celko.shtml

The problem is that if I try to add a new descendent which is not the most
right sibling I get UNIQUE error.

This occur ( if im right ) when the update try to update the lft column
which is UNIQUE

Example of table:

CREATE TABLE areas (
lft INT UNIQUE
);

I inserted 3 rows with value: 1, 2, 3

now im trying to update:
update areas set lft = lft +1;

Then I get the UNIQUE problem.

Is there a way to do UNIQUE check after the whole table got updated ?
If not, any ideas to walk around the problem ?

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