Hashbytes Computed column cannot be persisted because the column is non-deterministic

问题

I am trying to hash the datetime column using HashBytes

alter table dbo.Events 
add HashKey AS hashbytes('MD5', cast(convert(varchar(200), format(datestamp,'yyyy-MM-dd HH:mm:ss.ffffff')) as  varbinary)) persisted

but as it is non-deterministic, I get this error:

Computed column cannot be persisted because the column is non-deterministic.

I managed to get it done by not specifying the format as below

alter table dbo.PolicyEventsFromQueue 
add HashKey AS hashbytes('MD5', cast( datestamp as  varbinary)) persisted

But in SQL Server, when I see the results with the format and without format I am getting different results for the same field value:

Select 
    convert(varchar(200), hashbytes('MD5', cast(convert(varchar(200), format(datestamp, 'yyyy-MM-dd HH:mm:ss.ffffff')) as varbinary)), 1)  
From 
    Events
Where 
    DateStamp ='2016-06-30 12:19:35.257961'

Result:

0xBE06A33FF10644A6D3B38EA134DDB97A

Second query:

select 
    hashbytes('MD5', cast('2016-06-30 12:19:35.257961' as varbinary))

Result:

0xBE06A33FF10644A6D3B38EA134DDB97A

Third query:

Select 
    convert(varchar(200), hashbytes('MD5', cast(DateStamp as varbinary)), 1)  
From 
    Events 
Where 
    DateStamp = '2016-06-30 12:19:35.257961'

Result:

0x3CB5C26B23EB4422515764686DFCAB82

Based on above research what I understood is SQL Server is converting datestamp to another format and then hashing.

But when I get the C# equivalent value for the same date ("2016-06-30 12:19:35.257961") using below function it is not matching with the hash value (0x3CB5C26B23EB4422515764686DFCAB82).

public static byte[] GetMD5Hash(string input)
{
        System.Security.Cryptography.MD5CryptoServiceProvider x = new System.Security.Cryptography.MD5CryptoServiceProvider();
        byte[] bs = System.Text.Encoding.Unicode.GetBytes(input);

        bs = x.ComputeHash(bs);

        return bs;
}

Can anybody hep to match date time format exactly as it taking in SQL Server and C# to make it work with HashBytes.

Note: I need all date including miiliseconds. This question is followup to the below question. It might help you to understand the root problem.

Need C# equivalent for the below SQL HashBytes function

回答1:

I got the solution. I modified HashBytes logic as below to get the required datetime format and also on the C# side, I am using default encode.

Select hashbytes('MD5', convert(varchar(200),(CONVERT(varchar(10),datestamp,126)+' '+CONVERT(VARCHAR(24),datestamp,114)),2))
from Events
Where DateStamp ='2016-06-30 12:19:35.257961'

来源：https://stackoverflow.com/questions/38761149/hashbytes-computed-column-cannot-be-persisted-because-the-column-is-non-determin

标签

sql-server

hashbytes