Sql Server - Index on nvarchar field

Question

What is the good approach to keep a nvarchar field unique. I have a field which is storing URLs of MP3 files. The URL length can be anything from 10 characters to 4

Answer 1

You can use a hash function (although theoretically it doesn't guarantee that two different titles will have different hashes, but should be good enough: MD5 Collisions) and then apply the index on that column.

MD5 in SQL Server

Answer 2

You could use CHECKSUM command and put index on column with checksum.

--*** Add extra column to your table that will hold checksum
ALTER TABLE Production.Product
ADD cs_Pname AS CHECKSUM(Name);
GO

--*** Create index on new column
CREATE INDEX Pname_index ON Production.Product (cs_Pname);
GO

Then you can retrieve data fast using following query:

SELECT * 
FROM Production.Product
WHERE CHECKSUM(N'Bearing Ball') = cs_Pname
AND Name = N'Bearing Ball';

Here is the documentation: http://technet.microsoft.com/en-us/library/ms189788.aspx

Answer 3

You could create a hash code of the url and use this integer as a unique index on your db. Beware of converting all characters to lowercase first to ensure that all url are in the same format. Same url will generate equal hash code.