Absolute and Relative URLs
A URL specifies the location of a target stored on a local or networked computer. The target can be a file, directory, HTML page, image, program, and so on.
An absolute URL contains all the information necessary to locate a resource.
A relative URL locates a resource using an absolute URL as a starting point. In effect, the "complete URL" of the target is specified by concatenating the absolute and relative URLs.
An absolute URL uses the following format: scheme://server/path/resource
A relative URL typically consists only of the path, and optionally, the resource, but no scheme or server. The following tables define the individual parts of the complete URL format.
Specifies how the resource is to be accessed.
Specifies the name of the computer where the resource is located.
Specifies the sequence of directories leading to the target. If resource is omitted, the target is the last directory in path.
If included, resource is the target, and is typically the name of a file. It may be a simple file, containing a single binary stream of bytes, or a structured document, containing one or more storages and binary streams of bytes.
URL Scheme Registration
If a provider supports URLs, the provider will register one or more URL schemes. Registration means that any URLs using the scheme will automatically invoke the registered provider. For example, the http scheme is registered to the Microsoft OLE DB Provider for Internet Publishing. ADO assumes all URLs prefixed with "http" represent Web folders or files to be used with the Internet Publishing Provider. For information about the schemes registered by your provider, see your provider documentation.
Defining Context with a URL
One function of an open connection, represented by a Connection object, is to restrict subsequent operations to the data source represented by that connection. That is, the connection defines the context for subsequent operations.
With ADO 2.7 or later, an absolute URL can also define a context. For example, when a Record object is opened with an absolute URL, a Connection object is implicitly created to represent the resource specified by the URL.
An absolute URL that defines a context can be specified in the ActiveConnection parameter of the Record object Open method. An absolute URL can also be specified as the value of the "URL=" keyword in the Connection object Open method ConnectionString parameter, and the Recordset object Open method ActiveConnection parameter.
Context can also be defined by opening a Record or Recordset object that represents a directory, because these objects already have an implicitly or explicitly declared Connection object that specifies context.
The context also defines scope—that is, the directory and its subdirectories that can participate in subsequent operations. The Record object has several scoped methods that operate on a directory and all its subdirectories. These methods include CopyRecord, MoveRecord, and DeleteRecord.
Relative URLs as Command Text
You can specify a command to be executed on the data source by typing a string in the CommandText parameter of the Connection object's Execute method, and in the Source parameter of the Recordset object's Open method.
A relative URL can be specified in the CommandText or Source parameter. The relative URL does not actually represent a command, such as an SQL command; it merely specifies the parameters. The context of the active connection must be an absolute URL, and the Option parameter must be set to adCmdTableDirect.
For example, the following code sample shows how to open a Recordset on the Readme25.txt file of the Winnt/system32 directory:
recordset.Open "system32/Readme25.txt", "URL=http://YourServer/Winnt/",,,adCmdTableDirect
The absolute URL in the connection string specifies the server (
YourServer) and the path (
Winnt). This URL also defines the context.
The relative URL in the command text uses the absolute URL as a starting point and specifies the remainder of the path (
system32) and the file to open (
The options field (
adCmdTableDirect) indicates that the command type is a relative URL.
As another example, the following code will open a Recordset on the contents of the
recordset.Open "", "URL=http://YourServer/Winnt/",,,adCmdTableDirect
OLE DB Provider-Supplied URL Schemes
The leading part of a fully-qualified URL is the scheme that is used to access the resource identified by the remainder of the URL. Examples are HTTP (Hypertext Transfer Protocol) and FTP (File Transfer Protocol).
ADO supports OLE DB providers that recognize their own URL schemes. For example, the Microsoft OLE DB Provider for Internet Publishing, which accesses "published" Windows 2000 files, recognizes the existing HTTP scheme.