Skip to main content

Updating Views

Written by Ian Treasure
Gethyn recently posted on deleting from a view. He used a common situation where several tables are joined into one view. In simpler situations, views can be updated directly. If required, triggers can be used to allow updates to views which join several tables. This post will consider updates to simple views and the use of triggers. It does not consider partitioned updatable views, which are usually used for load balancing. As an alternative to these views partitioned tables should be considered.
First – updating a simple view. Create a base table – call it Employee, with a column to show holidays, in hours. We will then insert a test row

create table Employee
(EmployeeID int,
EmployeeName varchar(30),
Employeecode varchar(30),
MaritalStatus bit,
HolidayHours int)

INSERT Employee(EmployeeID, EmployeeName, Employeecode, MaritalStatus,HolidayHours)
Values (1,'Smith','WR 56 rf 67', '0',160)

Now we create a view, which calculates a column – number of days leave – based on the number of hours in the employee table. There are eight hours per day.

Create view vwEmployees
EmployeeId, EmployeeName,
HolidayHours / 8 As [HolidayDays]
FROM Employee

Now update the view, and change the employee name. This works – there is only one table, and the column exists in both the view and the base table.

update vwEmployees set EmployeeName = 'Jones'
where EmployeeID = 1

Now try to update the view, but this time change the [HolidayDays] column

update vwEmployees set HolidayDays = 45
where EmployeeID = 1

As you would expect, this fails with a message:
Msg 4406, Level 16, State 1, Line 2
Update or insert of view or function 'vwEmployees' failed because it contains a derived or constant field.
This is because there is no storage associated with this column – it is calculated when the view is executed. I will take this further below.
Finally, try the delete command-

delete vwEmployees

Again, this command works OK – you have deleted the row that you created above.
Now, make the [HolidayDays} computed column on the view updateable. Insert a single row of data to the Employee table, and  create a trigger as follows:

INSERT Employee(EmployeeID, EmployeeName, Employeecode, MaritalStatus,HolidayHours)
Values (1,'Smith','WR 56 rf 67', '0',160)

CREATE TRIGGER trg_VWEmployees ON VWEmployees
UPDATE Employee
SET EmployeeName = inserted.employeeName,
HolidayHours = inserted.HolidayDays * 8
FROM inserted
Employee.EmployeeId = inserted.EmployeeID

The following update will now succeed, and sets the HolidayHours column on the Employee table to 168.

update vwEmployees set HolidayDays = 21
where EmployeeID = 1

This approach will also work if you attempt to update a view with several tables joined together. Create an INSTEAD OF trigger and update the individual tables in the trigger.
This approach is potentially useful because it hides details of the application from the user. In this simple example, the application holds leave as hours, but the user may think in terms of days. By creating a view which the user can update, it is possible to hide complexities of the application from the user. The user does not need to understand ho the database is implemented – the view and the trigger reflects the users image of the data.


Popular posts from this blog

SQL Server 2012 and Virtual Service Accounts

This post is written by David Postlethwaite
If you are using SQL Server 2012 you will probably have noticed that the default account for the SQL services has changed from that used in previous versions. With SQL 2005 and 2008 the default account for SQL service and SQL Agent service was “NT Authority\System”. This is one the built in accounts on a Windows machine, managed by the machine and selectable from a dedicated dropdown list

The Network Service account was introduced in Windows 2003 as an alternative to using the LocalSystem account, which has full local system privileges on the local machine, a major security concern.
The Network Service has limited local privileges easing these security concerns but when many services on a machine use the Network Service account it becomes harder to track which service is actually accessing resources and performing actions, because all the services are using the one Network Service account.
Also, this account, by default, has sysadmin per…

Always Encrypted

By David Postlethwaite

Always Encrypted is new features in SQL Server 2016 and it is also available in Azure SQL Database. Here you can encrypt columns in a table with a master key and a certificate so that they will appear as encrypted strings to those who don’t have the required certificate installed on their pc.
Once the certificate is installed on the computer then the unencrypted data can then be seen as normal.

The data passes from database to your application as the encrypted value, only the application with the correct certificate can unencrypt the data so it is secure across the wire. This will go some way to resolving the concern of people worried about putting their sensitive data on a shared server in the cloud such as Microsoft Azure and accessing the data across the Internet.

At the time of writing Always Encrypted is only supported with ADO.NET 4.6, JDBC 6.0 and ODBC 13.1 but expect other driver to become available.

The calling application (including SSMS) must also hav…

New in SQL Server 2017: Graph Databases

David has recorded and published a video of his presentation on SQL Server Graph Database. In his video which you can watch below, David provides an excellent introduction into SQL Server 2017 Graph Databases. In his presentation he looks at Tennis results at tournaments for  his favourite player "The Fed"  Rodger Federer.

David  shows how to set up graph database and work with them in SQL Server 2017.

Graph Database is not new. Other vendors have had graph database capabilities for some time so Microsoft are quite late to the market. In David presentation it appears that Microsoft have done a reasonable job of implementing some of the graph database features but he does point some of the limitations of the Microsoft product too and suggests that it is not ready for production yet but Microsoft seem serious about this feature.

Please watch the video and feel free to leave a comment or feedback - David is delivering a version of this talk on Graph databases in SQL Saturday Ka…