OpenSchema Method

  • Article

Obtains database schema information from the provider.

Syntax

Set recordset = connection.OpenSchema (QueryType, Criteria, SchemaID)

Return Values

Returns a Recordset object that contains schema information. The Recordset will be opened as a read-only, static cursor. The QueryType determines what columns appear in the Recordset.

Parameters

  • QueryType
    Any SchemaEnum value that represents the type of schema query to run.
  • Criteria
    Optional. An array of query constraints for each QueryType option, as listed in SchemaEnum.
  • SchemaID
    The GUID for a provider-schema query not defined by the OLE DB specification. This parameter is required if QueryType is set to adSchemaProviderSpecific; otherwise, it is not used.

Remarks

The OpenSchema method returns self-descriptive information about the data source, such as what tables are in the data source, the columns in the tables, and the data types supported.

The QueryType argument is a GUID that indicates the columns (schemas) returned. The OLE DB specification has a full list of schemas.

The Criteria argument limits the results of a schema query. Criteria specifies an array of values that must occur in a corresponding subset of columns, called constraint columns, in the resulting Recordset.

The constant adSchemaProviderSpecific is used for the QueryType argument if the provider defines its own nonstandard schema queries outside those listed above. When this constant is used, the SchemaID argument is required to pass the GUID of the schema query to execute. If QueryType is set to adSchemaProviderSpecific but SchemaID is not provided, an error will result.

Providers are not required to support all of the OLE DB standard schema queries. Specifically, only adSchemaTables, adSchemaColumns, and adSchemaProviderTypes are required by the OLE DB specification. However, the provider is not required to support the Criteria constraints listed above for those schema queries.

Remote Data Service Usage   The OpenSchema method is not available on a client-side Connection object.

Note   In Visual Basic, columns that have a four-byte unsigned integer (DBTYPE UI4) in the Recordset returned from the OpenSchema method on the Connection object cannot be compared to other variables. For more information about OLE DB data types, see Chapter 13: Data Types in OLE DB and Appendix A: Data Types of the Microsoft OLE DB Programmer's Reference.

Visual C/C++ Users    When not using client side cursors, retrieving the "ORDINAL_POSITION" of a column schema in ADO returns a variant of type VT_R8 in MDAC 2.7 and later while the type of used in MDAC 2.6 was VT_I4. Programs written for MDAC 2.6 that only look for a variant returned of type VT_I4 would get a zero for every ordinal if run under MDAC 2.7 and later without modification. This change was made because the data type that OLE DB returns is DBTYPE_UI4, and in the signed VT_I4 type there is not enough room to contain all possible values without possibly truncation occurring and thereby causing a loss of data.

See Also

Visual Basic Example | Visual C++ Example | Visual J++ Example

Open Method (ADO Connection) | Open Method (ADO Record) | Open Method (ADO Recordset) | Open Method (ADO Stream) | Appendix A: Providers

Applies To: Connection Object