SQL: Updating a user ordered sequence of items within a group - sql

Question

A table for storing items, in a particular order, associated with a container. Separate ak_* constraints involving item_id and seq ensure a container contains distinct items and the sequence of those items is distinct.
CREATE TABLE [container_items] (
[container_item_id] INT IDENTITY (1, 1) NOT NULL,
[container_id] INT NOT NULL,
[item_id] INT NOT NULL,
[seq] INT NOT NULL,
CONSTRAINT [pk_container_item] PRIMARY KEY CLUSTERED ([container_item_id] ASC),
CONSTRAINT [ak_container_item_seq] UNIQUE NONCLUSTERED ([container_id] ASC, [seq] ASC),
CONSTRAINT [ak_container_item_item] UNIQUE NONCLUSTERED ([container_id] ASC, [item_id] ASC),
CONSTRAINT [fk_container_item_item] FOREIGN KEY ([item_id]) REFERENCES [items] ([item_id]),
CONSTRAINT [fk_container_item_container] FOREIGN KEY ([container_id]) REFERENCES [containers] ([container_id])
);
Suppose for container_id=1 the original data is
container_item_id, container_id, item_id, seq
1,1,1,1
2,1,3,2
3,1,10,3
4,1,8,4
and some client app for reordering says the new sequence for the item_ids is
8,1
10,2
3,3
1,4
The ak_* constraints make it impossible to update the data base table directly. For instance, trying update in this manner:
update container
container_items
set item_id=8, seq=1
where container_item_id = 1
fails
Violation of UNIQUE KEY constraint ak_container_item_item. Cannot insert duplicate key in object 'container_items'. The duplicate key value is (1, 8).
The statement has been terminated.
Q: Is it worth the effort to find an algorithm that would reuse existing container_item_id records when the seq order is changed ?
A non-reusing approach would be to delete the existing records for survery_id=1 and then append the new sequenced item_ids as new records.

Answer 1

You can encapsulate the whole operation in a simple atomic transaction. Also, you need a at least one 'aux' value, I a couple of aux values with simple * -1 operation in this sample:
begin transaction tx1;
set transaction isolation level serializable;
update container
survey_items
set seq=-1*seq #<-- set aux values
where container_id = 1;
update container
survey_items
set seq=1
where container_id = 1 and item_id = 8;
update container
survey_items
set seq=2
where container_id = 1 and item_id = 10;
#and so on
commit;
Notice than you can work at repeatable read with the same guaranties because no phantoms are made.