How To Use the ADO OpenSchema Method in Visual Basic

MORE INFORMATION

Here are the three parameters for the OpenSchema method:

• An enumerated value specifying the type of the schema required. Examples are adSchemaTables, adSchemaPrimaryKeys, adSchemaColumns.
• A variant array. The number of elements and the contents in this array depend on the type of schema query to run. You can use this parameter to restrict the number of rows you return in the resultset. However, you cannot limit the number of columns you return using OpenSchema. The criteria Array member indicates the string values to restrict the query results. The number of the array members varies depending on the querytype.
• The third parameter varies depending on the provider you use. It is required only if you set the first parameter to adSchemaProviderSpecific; otherwise, it is not used.

Below are the sample parameters for OpenSchema. Note that the criteria changes with the querytype. The most important thing to remember is that the order of providing the values has to be the same. A list of corresponding criteria for other querytypes is in the online documentation located at the following URL:

   QueryType        Criteria
   =============================

   adSchemaTables   TABLE_CATALOG
                    TABLE_SCHEMA
                    TABLE_NAME
                    TABLE_TYPE
				

Use adSchemaTables to list the tables in a database.

Microsoft Access 97 and Access 2000

To list all of the tables and queries in the Microsoft Access NWind database, simply use the following code:

Set rs = cn.OpenSchema(adSchemaTables)
   While Not rs.EOF
      Debug.Print rs!TABLE_NAME
      rs.MoveNext
   Wend
				

To list only the tables in the Access Nwind database, use:

Set rs = cn.OpenSchema(adSchemaTables, _
         Array(Empty, Empty, Empty, "Table")
				

Use the same syntax, using the OLE DB Provider for ODBC with the Jet ODBC driver and using the Jet OLE DB Providers.

Microsoft SQL Server 6.5 and 7.0

To list all of the tables and views in the Microsoft SQL Server Pubs Database, use:

Set rs = cn.OpenSchema(adSchemaTables)
				

To list just the tables in the Microsoft SQL Server Pubs database, use:

Set rs = cn.OpenSchema(adSchemaTables, _
         Array("Pubs", Empty, Empty, "Table")
				

Use the same syntax using the OLE DB Provider for ODBC with the SQL Server ODBC driver and using the OLE DB Provider for SQL Server.

   QueryType         Criteria
   ===============================

   adSchemaColumns   TABLE_CATALOG
                     TABLE_SCHEMA
                     TABLE_NAME
                     COLUMN_NAME
				

Use adSchemaColumns to list the fields in a table.

Microsoft Access 97 and Access 2000

To list the fields in the Employees table in the Access Nwind database With adSchemaColumns, simply use:

Set rs = cn.OpenSchema(adSchemaColumns,Array(Empty, Empty, "Employees")

While Not rs.EOF
   Debug.Print rs!COLUMN_NAME
   rs.MoveNext
Wend
				

This works using the OLE DB Provider for ODBC with the Jet ODBC Driver and using with the Jet OLE DB Providers.

Microsoft SQL Server 6.5 and 7.0

To list the fields in the Authors table in the SQL Server Pubs database with adSchemaColumns, simply use:

Set rs = cn.OpenSchema(adSchemaColumns, Array("pubs", "dbo", "Authors")
				

Note that TABLE_CATALOG is the database and TABLE_SCHEMA is the table owner. This works using the OLE DB Provider for ODBC with the SQL Server ODBC driver and using the OLE DB Provider for SQL Server.

   QueryType          Criteria
   ================================

   adSchemaIndexes    TABLE_CATALOG
                      TABLE_SCHEMA
                      INDEX_NAME
                      TYPE
                      TABLE_NAME
				

You provide the index name in case of adSchemaIndexes querytype.

Microsoft Access 97 and Access 2000

To list the Indexes in the Employees table in the Access Nwind database With adSchemaIndexes, simply use:

Set rs = cn.OpenSchema(adSchemaIndexes, _
         Array(Empty, Empty, Empty, Empty, "Employees")

While Not rs.EOF
   Debug.Print rs!INDEX_NAME
   rs.MoveNext
Wend
				

This works using the OLE DB Provider for ODBC with the Jet ODBC Driver and using with the Jet OLE DB Providers.

Microsoft SQL Server 6.5 and 7.0

To list the Indexes in the Authors table in the SQL Server Pubs database with adSchemaIndexes, simply use:

Set rs = cn.OpenSchema(adSchemaIndexes, _
         Array("Pubs", "dbo", Empty, Empty, "Authors")
				

This works using the OLE DB Provider for ODBC with the SQL Server ODBC driver and using the OLE DB Provider for SQL Server. The following steps demonstrate the OpenSchema Method.

OpenSchema Method Example

In Visual Basic (VB), select a standard EXE project. Add three Command buttons to the EXE project. From the Project menu, choose References. In the References dialog box, select Microsoft ActiveX Objects Library. This sample uses the Pubs database provided with SQL Server. You need to change the data source name (DSN) to a DSN on your computer. Paste the following code into the GENERAL DECLARATIONS section of the Project:

Note You will need to change UID=<username> and PWD=<strong password> to the correct values before you run this code. Make sure that UID has the appropriate permissions to perform this operation on the database.

   'Open the proper connection.
   Dim cn As New ADODB.Connection
   Dim rs As New ADODB.Recordset

   Private Sub Command1_Click()
   'Getting the information about the columns in a particular table.
      Set rs = cn.OpenSchema(adSchemaColumns, Array("pubs", "dbo", _
             "authors"))
      While Not rs.EOF
          Debug.Print rs!COLUMN_NAME
          rs.MoveNext
      Wend

   End Sub

   Private Sub Command2_Click()
   'Getting the information about the primary key for a table.
      Set rs = cn.OpenSchema(adSchemaPrimaryKeys, Array("pubs", "dbo", _
             "authors"))
      MsgBox rs!COLUMN_NAME
   End Sub

   Private Sub Command3_Click()
   'Getting the information about all the tables.
      Dim criteria(3) As Variant
      criteria(0) = "pubs"
      criteria(1) = Empty
      criteria(2) = Empty
      criteria(3) = "table"
      Set rs = cn.OpenSchema(adSchemaTables, criteria)
      While Not rs.EOF
         Debug.Print rs!TABLE_NAME

         rs.MoveNext
      Wend

   End Sub

   Private Sub Form_Load()
      cn.Open "dsn=pubs;uid=<username>;pwd=<strong password>;"
      'To test with the Native Provider for SQL Server, comment the
      ' line above then uncomment the following line. Modify to use
      ' your server.
      'cn.Open "Provider=SQLOLEDB;Data Source=<servername>;" & _
      '        "User ID=sa;password=;"

   End Sub
				

Run. Click each Command button to test. End.Modify the Form Load event procedure to use the Native Provider for SQL Server. Again test. More information on querytype and Criteria is available in the ADO documentation. The schema information specified in OLE DB is based upon the assumption that the provider supports the concept of a catalog and schema.