FIX: Default Schema Rowsets Do Not Return NULL Columns

When you retrieve a schema rowset from an OLE DB provider created using the ATL OLE DB Provider Wizard, the TABLE_CATALOG and TABLE_SCHEMA columns return a status of DBSTATUS_S_OK and a zero length DBTYPE_WSTR value. According to the OLE DB specification 2.0. If a provider does not support the IDBInfo interface (and a default ATL OLE DB Provider Wizard generated OLE DB provider does not), then the provider must return DBSTATUS_S_ISNULL for all TABLE_CATALOG and TABLE_SCHEMA columns in schema rowsets.

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

This bug was corrected in Visual Studio 6.0 Service Pack 3. For more information about Visual Studio service packs, please see the following articles in the Microsoft Knowledge Base:

194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why  (http://support.microsoft.com/kb/194022/EN-US/ )

194295 HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed  (http://support.microsoft.com/kb/194295/EN-US/ )

The following information is taken from the OLE DB Programmer's Reference, Appendix B Schema Rowsets.

"If a provider returns non-NULL CATALOG or SCHEMA column values in schema rowsets, it must support IDBInfo to describe how a fully qualified name is assembled. A provider that does not support IDBInfo must return NULL values for CATALOG and SCHEMA columns in all schema rowsets."

Since an ATL OLE DB Provider Wizard generated OLE DB provider does not support the IDBInfo interface, it does not follow the OLE DB specification when it returns zero length strings in the TABLE_CATALOG and TABLE_SCHEMA schema rowset columns.

You can test a particular ATL OLE DB Provider as seen below, by using the RowsetViewer tool that ships with the Microsoft Data Access SDK 2.0.

1. Open RowsetViewer.
2. Point to Select File and click Full Connect.
3. Select your ATL OLE DB Provider Wizard generated OLE DB provider in the Provider drop down menu and click OK.
4. Point to Select Session and click IDBSchemaRowset and GetRowset.
5. Select DBSCHEMA_TABLES and click OK.
6. Double-click the single returned row and check the status flags for the TABLE_CATALOG and TABLE_SCHEMA columns.

If the status flags are set to DBSTATUS_S_ISNULL, the Provider is displaying the correct behavior. If the status flags are set to DBSTATUS_S_OK, the Provider is not following the OLE DB 2.0 specification.

The Microsoft Data Access SDK 2.1 Online Documentation: Microsoft OLE DB; OLE DB Programmer's Reference; Part 3 Appendixes; Appendix B Schema Rowsets.