SQL Server唯一约束问题

Question

How to create a unique constraint on a varchar(max) field in visual studio, visually.

the problem is when i try it:

manage indexes and keys > add > columns

I can only chose the bigint columns, but not any of the varchar(max) ones.

Do I maybe have to use check constraints ?

If yes, what to put in the expression?

Thnx for the info

Answer 1

You cannot put a unique constraint on a VARCHAR(MAX) column (which could be up to 2 GB of text!!). You just simply cannot.

The unique constraint is enforced by a unique index in the background, and SQL Server has a 900 byte limit on index entries. You also cannot put a unique constraint on a VARCHAR(2000) field for that reason.

You'll need to find another way to achieve what you're trying to do. You could eg calculate the length and something like a checksum over your text and put a unique constraint on those length and checksum columns.

Answer 2

Even if this were possible, it would be a bad idea.

1) There is another way. Find some other data to use as your unique column

2) If you ABSOLUTELY HAVE TO use the varchar(Max). Maybe hash it on insert/update and add a hash column?

Answer 3

One way to do this would be to add a column for a hash that is calculated whenever you insert or update the column and put a unique index on that. While hash collisions do happen, it is extremely unlikely.

You could use this T-SQL keyword:

http://msdn.microsoft.com/en-us/library/ms174415.aspx