Ideally you should be using computed columns as per https://rules.ssw.com.au/use-computed-columns-rather-than-denormalized-fields
Many of the databases that SSW works with make use of denormalized fields. We believe this is with good reason. However, several precautions should be taken to ensure that the data held within these fields is reliable. This is particularly the case several applications are updating your denormalized data. To illustrate, let's say that we want to show all Customers with a calculated field totalling their order amount (ie Customer.OrderTotal).
With this example in mind, the main reasons we use denormalized fields are:
Reducing development complexity
A denormalized field can mean that all SELECT queries in the database are simpler. Power users find it easier to use for reporting purposes - without the need for a cube. In our example, we would not need a large view to retrieve the data (as below).
SUM (SalesOrderDetail.OrderQty *
) AS DetailTotal,
INNER JOIN SalesOrderHeader
ON Customer.CustomerID = SalesOrderHeader.CustomerID
INNER JOIN SalesOrderDetail
ON SalesOrderHeader.SalesOrderID =
GROUP BY Customer.CustomerID, Customer.SalesPersonID,
ORDER BY Customer.CustomerID
- Figure: A view to get customer totals when no denormalized fields are used
If we had a denormalized field, the user or developer would simply have run the following query:
Customer.OrderTotal AS DetailTotal
ORDER BY Customer.CustomerID
- Figure: Queries are much simpler with denormalized fields
Note that this is not a particularly complicated example. However, you can see why it can simplify development greatly when working with a large number of tables
Performance is better for read-intensive reports
Particularly when reporting on data with a cube.
When there a multiple tables in a SQL Server view They cannot be updated in one hit - they must be updated one table at a time.
It is a built-in validation device For example, if records are accidentally deleted directly in the database, there is still a validation check for the correct totals. The value of this is mitigated when there is a full audit log on the database
However, there are reasons against using denormalized fields
They have to be maintained and can potentially get out of synch
This can make them unreliable - particularly if several applications are incorrectly updating the denormalized fields. UPDATE, INSERT, DELETEs are more complicated as they have to update the denormalized fields
They can be seen as an unnecessary waste of space
All in all, we choose to still use denormalized fields because they can save development time. We do this with some provisos. In particular, they must be validated correctly to ensure the integrity of the data.
Here is how we ensure that this data is validated:
- Change the description on any denormalized fields to include "Denormalized" in the description - "Denormalized: Sum(OrderTotal) FROM Orders" in description in SQL Server Management Studio.
- Create a view that lists all the denormalized fields in the database - based on the description field.
CREATE VIEW dbo.vwValidateDenormalizedFields
SELECT OBJECT_NAME(id) AS TableName,
COL_NAME(id, smallid) AS ColumnName,
CAST([value] AS VARCHAR(8000)) AS Description,
'procValidate_' + OBJECT_NAME(id) +
'_' + COL_NAME(id, smallid) as
WHERE (name = 'MS_Description') AND
(CAST([value] AS VARCHAR(8000))
- Figure: Standard view for validation of a denormalized field
- Create a stored procedure (based on the above view) that validates whether all denormalized fields have a stored procedure that validates the data within them
CREATE PROCEDURE procValidateDenormalizedFieldValidators
WHERE ValidationProcedureName NOT IN
FROM vwValidateDenormalizedFields AS vw
LEFT JOIN sysobjects
WHERE id IS NOT NULL
- Figure: Standard stored procedure for validation of a denormalized field
If you want to know how to implement denormalized fields, see our rules Do you use triggers for denormalized fields?