Generate a hash for a set of rows in sql server

Question

Is there any way in SQL Server 2012 to generate a hash of a set of rows and columns?

I want to generate a hash, store it on the parent record. The when an update co

Answer 1

Another approach:

-- compute a single hash value for all rows of a table
begin

    set nocount on;

    -- init hash variable
    declare @tblhash varchar(40);
    set @tblhash = 'start';

    -- compute a single hash value
    select @tblhash = sys.fn_varbintohexsubstring(0, hashbytes('sha1',(convert(varbinary(max),@tblhash+
    (select sys.fn_varbintohexsubstring(0,hashbytes('sha1',(convert(varbinary(max),
    -- replace 'select *' if you want only specific columns to be included in the hash calculation
    -- [target table] is the name of the table to calc the hash from
    -- [row_id] is the primary key column within the target table
    -- modify those in the next lines to suit your needs:
    (select * from [target_table] obj2 where obj2.[row_id]=obj1.[row_id] for xml raw)
    ))),1,0))
    ))),1,0)
    from [target_table] obj1;

    set nocount off;

    -- return result
    select @tblhash as hashvalue;

end;

Answer 2

For single row hashes:

select HASHBYTES('md5', Name + Description + AnotherColumn)
FROM MyChildTable WHERE ParentId = 2

for table checksum:

select sum(checksum(Name + Description + AnotherColumn)*1.0)
FROM MyChildTable WHERE ParentId = 2

Answer 3

You can use the CHECKSUM_AGG aggregate. it is made for that purpose.

Answer 4

select HashBytes('md5',convert(varbinary(max),(SELECT * FROM MyChildTable WHERE ParentId = 2 FOR XML AUTO)))

but HashBytes is limited to 8000 bytes only... you can make a function to get de Md5 for every 8000 bytes....