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Microsoft has developed technology that enables developers to encapsulate executable code/objects into code components. They can be used from programs developed in the same or any other programming language that supports these kinds of objects. Through the years, this technology has been known by different names: OLE, OLE Automation, COM, DCOM, Automation, ActiveX, COM+ (and the saga continues). SQL Server can initiate code components and access properties and methods encapsulated in them. There is a set of system stored procedures (with the prefix sp_OA ) designed to accomplish such tasks.
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Interaction with the SQL Server Environment
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NOTE: When Microsoft first unveiled this feature in SQL Server, code components were known as OLE Automation objects. For this reason, Microsoft attached the OA prefix to these stored procedure names, and I continue this usage in this section s heading.
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We will demonstrate the use of Automation on a trivial Visual Basic function: 1. Create the DjnToolkit ActiveX DLL project in Visual Basic and then create a DjnTools class. 2. To start with, let s create a trivial method called SpellNumber, which ignores the input value (currency amount) and returns a constant string (see Figure 11-1).
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A COM object created in Visual Basic
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SQL Server 2000 Stored Procedure Programming
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NOTE: Even if you run the object from the Visual Basic IDE (instead of compiling and installing it), you will still be able to access it from Transact-SQL code. This is an important feature for debugging the object.
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The following stored procedure first initiates the COM object using the sp_OACreate system stored procedure. It obtains a token @intObject, which is used from that point to access the class. The sp_OAMethod stored procedure is used to execute class methods. The return value and input parameter of the method are placed at the end of the stored procedure s parameter list. Before the stored procedure is complete, the COM object must be destroyed using sp_OADestroy. If an automation error occurs at any point, sp_OAGetErrorInfo can be used to obtain the source and description of the most recent error.
Alter Procedure prSpellNumber -- demo of use of Automation objects @mnsAmount money, @chvAmount varchar(500) output, @debug int = 0 As set nocount on Declare @intErrorCode int, @intObject int, -- hold object token @bitObjectCreated bit, @chvSource varchar(255), @chvDesc varchar(255) Select @intErrorCode = @@Error If @intErrorCode = 0 exec @intErrorCode = sp_OACreate 'DjnToolkit.DjnTools', @intObject OUTPUT
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If @intErrorCode = 0 Set @bitObjectCreated = 1 else Set @bitObjectCreated = 0 If @intErrorCode = 0 exec @intErrorCode = sp_OAMethod @intObject, 'SpellNumber', @chvAmount OUTPUT, @mnsAmount If @intErrorCode <> 0 begin Raiserror ('Unable to obtain spelling of number', 16, 1) exec sp_OAGetErrorInfo @intObject, @chvSource OUTPUT, @chvDesc OUTPUT Set @chvDesc = 'Error (' + Convert(varchar, @intErrorCode) + ', ' + @chvSource Raiserror (@chvDesc, 16, 1) end if @bitObjectCreated = 1 exec sp_OADestroy @intObject + ') : ' + @chvDesc
return @intErrorCode
Once you are sure that the communications between Transact-SQL and Visual Basic code are working, you can use Visual Basic to write code that converts numbers to text. Since this is not a book about Visual Basic, we will not go into detail on that subject. Instead, we will examine system stored procedures that use OLE Automation in more detail.
SQL Server 2000 Stored Procedure Programming
sp_OACreate
Before any code component can be accessed, it has to be initialized. This stored procedure creates an instance of a code component and returns a reference to it (that is, an objecttoken):
sp_OACreate progid, | clsid, objecttoken OUTPUT[, context]
You can specify the code component using either a programmatic identifier (progid) or a class identifier (clsid). A programmatic identifier (progid) is a string that serves as a name for the code component. It always appears in the form Component.Object. For example, Excel can be referenced as Excel.Application. A class identifier is a unique identifier (GUID) for a class. It appears as a string in the following form:
{nnnnnnnn-nnnn-nnnn-nnnn-nnnnnnnnnnnn}
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