SQL way to get the MD5 or SHA1 of an entire row

Question

Is there a \"semi-portable\" way to get the md5() or the sha1() of an entire row? (Or better, of an entire group of rows ordered by all their fields, i.e. order by 1,2,3,

Answer 1

You could calculate the hashbytes value for the entire row on an update trigger, I used this as part of an ETL process where previously they were comparing all columns in the tables, the speed increase was huge.

Hashbytes works on varchar, nvarchar, or varbinary datatypes, and I wanted to compare integer keys and text fields, casting everything would have been a nightmare, so I used the FOR XML clause in SQL server as follows:

CREATE TRIGGER get_hash_value ON staging_table
FOR UPDATE, INSERT AS  
UPDATE staging_table
SET sha1_hash = (SELECT hashbytes('sha1', (SELECT col1, col2, col3 FOR XML RAW)))
GO

alternatively, you could calculate the values in a similar way outside of a trigger, if you plan to do many updates on all the rows by using a subquery with the for xml clause also. If going this route, you can even change it to a SELECT *, but not in the trigger, as each time you run it you would be getting a different value because the sha1_hash column would be different each time.

You could modify the select statement to get more than 1 row

Answer 2

In MSSQL -- You can use HashBytes across the entire row by using xml..

SELECT MBT.id,
   hashbytes('MD5',
               (SELECT MBT.*
                FROM (
                      VALUES(NULL))foo(bar)
                FOR xml auto)) AS [Hash]
FROM <Table> AS MBT;

You need the from (values(null))foo(bar) clause to use xml auto, it serves no other purpose..