Question

I have a table in MS SQL 2005. And would like to do:

update Table
set ID = ID + 1
where ID > 5

And the problem is that ID is primary key and when I do this I have an error, because when this query comes to row with ID 8 it tries to change the value to 9, but there is old row in this table with value 9 and there is constraint violation.

Therefore I would like to control the update query to make sure that it’s executed in the descending order.

So no for ID = 1,2,3,4 and so on, but rather ID = 98574 (or else) and then 98573, 98572 and so on. In this situation there will be no constraint violation.

So how to control order of update execution? Is there a simple way to acomplish this programmatically?

Answer 1

Transact SQL defers constraint checking until the statement finishes.

That’s why this query:

UPDATE  mytable
SET     id = CASE WHEN id = 7 THEN 8 ELSE 7 END
WHERE   id IN (7, 8)

will not fail, though it swaps id‘s 7 and 8.

It seems that some duplicate values are left after your query finishes.