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I have a varchar(max) column where users will populate a pre-given key from another system. I want to enable that the users are forced to enter a unique key which means no duplicate values on that specific column. Should I change the collation or is there some sort of check constraint that I can add?

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People are entering > 900 bytes by hand? This does not sound like a "key" to me... –  Aaron Bertrand Feb 25 '13 at 2:18

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up vote 4 down vote accepted

Ordinarily, you'd just create a unique constraint on that column. But you can't put a unique constraint on a varchar(max) column. That could be close to 2GB per index entry.

In SQL Server, the maximum size of an index key is 900 bytes. I believe SQL Server enforces unique constraints by using a unique index, so I'd expect that limitation to apply to your problem, too.

In any case, I think this is an XY problem.

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I don't think you can have a unique constraint on a varchar(max). It's up to VARCHAR(8000). –  ypercube Feb 24 '13 at 13:44
    
@ypercube: Yeah, I realized it was a varchar(max) column right after I posted, and I was looking for the limits. Thanks. –  Mike Sherrill 'Cat Recall' Feb 24 '13 at 13:48

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