First published on MSDN on Jun 06, 2006
SQL Server 2005 allows creating of User Defined Aggregate in any of the .NET languages such as C# or VB. For simple cases like SUM or MAX you probably want to use built-in aggregates, however there are cases where build-ins are insufficient. In such cases people used to put the business logic on a client on a middle tier. With the new version of SQL Server you can have this logic on a server.
Let’s say company XYZ wants to come up with a way of calculating a bonus for their employees. XYZ uses NWIND database (NWIND database can be downloaded from
http://msdn2.microsoft.com/en-us/library/ms143221.aspx
). XYZ wants to have a business rule such that the bonus is never greater than 200% of the salary and each regular sale adds 1% to the bonus and each sale to Germany adds 3% to the bonus.
With the new Sql Server 2005 you can write your own aggregates in C# (or any .NET compatible language). Here is the aggregate.
[Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedAggregate(Format.Native)]
public struct Bonus
{
private int m_nRegularSales;
private int m_nGermanSales;
public void Init()
{
m_nRegularSales = 0;
m_nGermanSales = 0;
}
public void Accumulate(SqlString Country)
{
if (Country == "Germany")
{
++m_nGermanSales;
}
else
{
++m_nRegularSales;
}
}
public void Merge(Bonus Group)
{
m_nRegularSales += Group.m_nRegularSales;
m_nGermanSales += Group.m_nGermanSales;
}
public int Terminate()
{
return Math.Min(200, (m_nRegularSales + 3 * m_nGermanSales));
}
}
And here is a T-SQL query that uses this aggregate to calculate bonus for each employee.
select
Employees.FirstName, Employees.LastName, dbo.Bonus(Orders.ShipCountry)
from
Employees join Orders on Employees.EmployeeId = Orders.EmployeeId
group by
Employees.EmployeeId, Employees.FirstName, Employees.LastName
