Question

Given the following simple table structure (SQL Server 2008), I want to be able to maintain uniqueness of my numerical sequence column, but I want to be able to update that value for any given record(s).

CREATE TABLE MYLIST(
      ID int NOT NULL IDENTITY(1, 1)
    , TITLE varchar(50) NOT NULL
    , SEQUENCE int NOT NULL
    , CONSTRAINT pk_mylist_id PRIMARY KEY(ID)
    , CONSTRAINT uq_mylist_sequence UNIQUE(SEQUENCE)
);

My interface allows me to jumble up the order of the items and I will know prior to doing the update which non-overlapping sequence they should all be in, but how do I perform the update without being confronted with a violation of the unique constraint?

For instance, say I have these records:

ID  TITLE   SEQUENCE
1   APPLE   1
2   BANANA  2
3   CHERRY  3

And I want to update their sequence numbers to the following:

ID  TITLE   SEQUENCE
1   APPLE   3
2   BANANA  1
3   CHERRY  2

But really I could be dealing with a couple dozen items. The sequence numbers must not overlap. I’ve thought about trying to use triggers or temporarily disabling the constraint, but that would seem to create more issues. I am using C# and LINQ-to-SQL, but am open to strictly database solutions.

Answer 1

You could assign them the negative of their correct value, then after all updates have occurred, do a final update where you set SEQUENCE = -SEQUENCE.

It is not very efficient, but since you say you only have a couple dozen items, I doubt the impact would be noticeable. I am also assuming that you can use negative numbers as “magic values” to indicate temporarily mis-assigned values.