在T-SQL中为字符串实现GetHashCode

Question

I want to create a scalar function in T-SQL that would be analogous to those, say, in Java.

The standard implementation in imperative languages is:

int hash = 0;
for (int i = 0; i < length; i++)
{
    hash = 31*hash + value[i];
}
return hash;

I'm not really that good in tsql to implement this, and from my POV writing imperative-style code in t-sql is something that should be avoided. I guess this could be done using CTE? Please =)

Also, can I make it always be positivie, ie when results exceed integrer max, then it flows over 0 rather than integer min? Let's assume that number of possible arguments (number of that special classes in my solution) is not really large. Let's say it will never exceed 1000, so I'm sure one can avoid collisions here even with uint.

---

PS: If someone's interesed what do I need this for, than I can explain, and probably you can suggest a better solution. I have a table with integer identity column and a varchar column 'TypeFullName' - thats a full name of a class in our C# solution.

And I need to write a script that will manually set ID's as a function-dependence of TypeFullName (yes, turn on the SET IDENTITY INSERT option). So that I can compute ID, if I know the type name. I know that sounds like a system with bad design, and it probably is, but believe me, I just have to do this now)

Thank you!

Answer 1

Read my article on CheckSum versus Hashbytes. ( http://craftydba.com/?p=3005 )

They are two built in SQL Server functions that will generate you a hash key given a value. One is more unique than another.

If you still have questions, just ask.

Sincerely

John

www.craftydba.com

PS:

You are losing precision when casting to a int or big int. Just save it as a GUID (16 byte hex).

Answer 2

I've found a solution in the internet and slightly updated it to limit output to positives:

begin
declare @h bigint
set @h = 0
select @h = (@h*31 + ascii(substring(@str,X.pos,1)))%4294967296
   from (select top(len(@str)) 
             row_number() over (order by getdate()) as pos 
           from sys.all_objects) as X
if @h >= 2147483647 set @h = @h - 2147483647
return convert(int, @h)
end;

That select top from sys.all_objects is really hacky, but ((( at least it works.