Absolute and Relative URLs

A URL specifies the location of a target stored on a local or networked computer, such as a file, directory, HTML page, image, program, and so on. In this discussion, an absolute URL is of the form:

scheme://server/path/resource

where:

scheme: Specifies how the resource is to be accessed.
server: Specifies the name of the computer where the resource is located.
path: Specifies the sequence of directories leading to the target. If resource is omitted, the target is the last directory in path.
resource: 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.

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. A relative URL typically consists only of the path, and optionally, the resource, but no scheme or server.

URL Scheme Registration

If a provider supports URLs, it will register for one or more URL schemes. This means that any URLs using this 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 2.5, an absolute URL may 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 may be specified in the ActiveConnection parameter of the Record object Open method. An absolute URL may also be specified as the value of the new "URL=" keyword in the Connection object Open method ConnectionString parameter, and the Recordset object Open method ActiveConnection parameter.

Context may also be defined with an open Record or Recordset object that represents a directory because these objects already have an implicitly or explicitly declared Connection object that specifies context.

Scoped Operations

The context simultaneously defines a scope—that is, the directory and its subdirectories that may participate in subsequent operations. The Record object has several scoped methods, including CopyRecord, MoveRecord, and DeleteRecord, that operate on a directory and all its subdirectories.

Relative URLs as Command Text

A string specifying a command to be executed on the data source may be specified in the Connection object Execute method CommandText parameter, and the Recordset object Open method Source parameter.

A relative URL may be specified in the CommandText or Source parameter. The relative URL does not actually specify a command (such as an SQL command); it is merely specified in those parameters. In addition, the context of the active connection must be an absolute URL, and the Option parameter must be set to adCmdTableDirect.

For example, a Recordset could be opened on the Readme25.txt file of the Winnt/system32 directory like this:

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 (Readme25.txt).

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 Winnt directory:

recordset.Open "", "URL=http://YourServer/Winnt/",,,adCmdTableDirect

OLE DB Provider-Supplied URL Schemes

The leading part of a fully-qualified URL is the scheme used to access the resource identified by the remainder of the URL. Examples are HTTP (HyperText Transfer Protocol) and FTP (File Transfer Protocol).