1. Using Hash Fields in SQL Server | TechRepublic
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Using Hash Fields in SQL
Server
By Tim Chapman
December 18, 2008, 8:33 AM PST
Takeaway: From auditing data changes to capturing data for loading a data warehouse, hash
fields have a ton of practical uses. In today’s article, database consultant Tim Chapman shows
how you can use built-in SQL Server functions to create your own hash fields.
A hash field is a calculated numeric field based on the value(s) contained in one or more fields.
This calculated value is great for detecting data changes, which can be put to use in auditing
strategies or datawarehouse ETL (extraction, transformation, loading) operations.
Creating a hash field in SQL Server is easy. The database engine has a couple of built-in
functions that can be used to generate the hashed value. Two of these functions are
CHECKSUM() and BINARY_CHECKSUM(). CHECKSUM() is intended to be used to build hash
indexes while BINARY_CHECKSUM() is great for detected data changes. I’ll be using the
BINARY_CHECKSUM funtion for the purposes of todays writing.
Using the hash field
In the following example I’ll show you how you can take advantage of the BINARY_CHECKSUM()
function. First, I’ll create the SalesHistory table and load some data into it.
CREATE TABLE [dbo].[SalesHistory]
(
SaleID int IDENTITY(1,1),
Product varchar(10) NULL,
SaleDate datetime NULL,
StatusID TINYINT NULL,
SalePrice money NULL
)
GO
SET NOCOUNT ON
http://www.techrepublic.com/blog/datacenter/using-hash-fields-in-sql-server/489[08/29/2012 3:48:29 PM]

2. Using Hash Fields in SQL Server | TechRepublic
BEGIN TRANSACTION
DECLARE @i INT
SET @i = 1
WHILE (@i <=500)
BEGIN
INSERT INTO [SalesHistory](Product, SaleDate, SalePrice, StatusID)
VALUES (’Computer’, DATEADD(ww, @i, ‘3/11/1919′),
DATEPART(ms, GETDATE()) + (@i + 57), 1)
INSERT INTO [SalesHistory](Product, SaleDate, SalePrice, StatusID)
VALUES(’BigScreen’, DATEADD(ww, @i, ‘3/11/1927′),
DATEPART(ms, GETDATE()) + (@i + 13),5)
INSERT INTO [SalesHistory](Product, SaleDate, SalePrice, StatusID)
VALUES(’PoolTable’, DATEADD(ww, @i, ‘3/11/1908′),
DATEPART(ms, GETDATE()) + (@i + 29),8)
SET @i = @i + 1
END
COMMIT TRANSACTION
ALTER TABLE SalesHistory
ADD CONSTRAINT pk_SalesHistory_SaleID PRIMARY KEY CLUSTERED (SaleID)
Once my table is created, I can add a calculated field to the SalesHistory table using the
BINARY_CHECKSUM() function.
ALTER TABLE SalesHistory
ADD HashField AS BINARY_CHECKSUM(SaleID, StatusID, Product)
The HashField I create above creates a checksum on the SaleID, StatusID, and Product fields.
The resulting value is a signed integer value based on the values in the SaleID, StatusID, and
Product fields. As the values in these fields the HashField value will change also. I can then use
this HashField value to keep track of changes to specific data fields, which makes it great for
capturing changes for loading a data warehouse.
Capturing these changes can be as easy as comparing each hash value to a previously stored
hash value to determine changed rows to as complicated to finding row data changes and then
http://www.techrepublic.com/blog/datacenter/using-hash-fields-in-sql-server/489[08/29/2012 3:48:29 PM]

3. Using Hash Fields in SQL Server | TechRepublic
routines to find the changed values.
Regardless of how you use it, using hash values in SQL Server is a great tool.
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3 Join the conversation! Add Your Opinion
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RE: Using Hash Fields in SQL Server 0
edbeo@... 19th Jun 2009 Votes
Hi, I hope you can help me with this question, How can I convert a hash value to
sqltext? I have a SQL Server 2000 SP4 Standard ed.
Thank you!
View in thread
Whereever you like as far as I can gather 0
Tony Hopkinson 8th Jan 2009 Votes
In the data warehousing strategy You'd have the last update to it store the hash, and
then compare it with the operational DB, to see if there's any change, and may be
which field changed. It's going... Read Whole Comment +
View in thread
Can you elaborate on a typical data checking process 0
victor.gutzler@... 7th Jan 2009 Votes
You wrote "Capturing these changes can be as easy as comparing each hash value
to a previously stored hash value to determine changed rows to as complicated to
finding row data changes and then... Read Whole Comment +
View in thread
http://www.techrepublic.com/blog/datacenter/using-hash-fields-in-sql-server/489[08/29/2012 3:48:29 PM]

4. Using Hash Fields in SQL Server | TechRepublic
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http://www.techrepublic.com/blog/datacenter/using-hash-fields-in-sql-server/489[08/29/2012 3:48:29 PM]