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Obtaining Metadata with Schema Rowsets

Sometimes you need to obtain information about the provider, rowset, table, columns, or other database information without opening the rowset. Data "about" the database structure is called "metadata" and can be retrieved by a number of different methods. One method is to use schema rowsets.

OLE DB Templates provide a set of classes to retrieve schema information; these classes create predefined schema rowsets and are listed in Schema Rowset Classes and Typedef Classes.

Note   If you are using OLAP and some of your rowsets are not supported by the schema rowset classes (for example, you have a variable number of columns), you should consider using CManualAccessor or CDynamicAccessor. You can scroll through the columns and use case statements to handle the possible data types for each column.

The Catalog/Schema Model

ANSI SQL defines a "catalog/schema" model for data stores; OLE DB uses this model. In this model, catalogs (databases) contain schemas, and schemas contain tables.

  • Catalog   A catalog is another name for a database. It is a collection of related schemas. To list the catalogs (databases) belonging to a given data source, use CCatalog. Because many databases have only one catalog, metadata is sometimes simply called "schema information."
  • Schema   A schema is a collection of database objects that are owned or have been created by a particular user. To list the schemas owned by a given user, use CSchemata.

    In Microsoft SQL Server and ODBC 2.x terms, a schema is an owner (for example, "dbo" is a typical schema name). Also, SQL Server stores metadata in a set of tables: one table contains a list of all the tables, and another table contains a list of all the columns. There is no equivalent to a schema in a Microsoft Access database.

  • Table   Tables are collections of columns arranged in specific orders. To list the tables defined in a given catalog (database), and information about those tables, use CTables).

Restrictions

When you query for schema information, you can use restrictions to specify the type of information in which you are interested. You can think of restrictions as a filter or qualifier in a query. For example, in the query:

SELECT * FROM authors where l_name = 'pivo'

l_name is a restriction. This is a very simple example with only one restriction; the schema rowset classes support several restrictions.

The schema rowset typedef classes encapsulate all the OLE DB schema rowsets so that you can access a schema rowset just like any other rowset by instantiating and opening it. For example, the typedef class CColumns is defined as:

CRestrictions<CAccessor<CColumnsInfo>

The CRestrictions class supplies the restriction support. After you create an instance of the schema rowset, call CRestrictions::Open. This method returns a result set based on the restrictions that you specify.

To specify restrictions, refer to Appendix B: Schema Rowsets and look up the rowset that you are using. For example, CColumns corresponds to the COLUMNS Rowset; that topic lists the restriction columns in the COLUMNS rowset: TABLE_CATALOG, TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME. You must follow that order in specifying your restrictions.

So, for example, if you want to restrict by table name, note that TABLE_NAME is the third restriction column, and then call Open, specifying the desired table name as the third restriction parameter, as shown in the following example.

To use schema rowsets

  1. You must include the header file atldbsch.h (of course you need atldbcli.h for consumer support as well).
  2. Instantiate a schema rowset object in the consumer's or the document's header file. If you want table information, declare a CTables object; if you want column information, declare a CColumns object. This example shows how to retrieve the columns in the "authors" table:
    CDataSource ds;
    ds.Open();
    CSession ss;
    ss.Open();
    CColumns ColumnSchemaRowset;
    // TABLE_NAME is the third restriction column, so
    // specify "authors" as the third restriction parameter:
    hr = ColumnSchemaRowset.Open(ss, NULL, NULL, "authors");
    hr = ColumnSchemaRowset.MoveFirst();
    while (hr == S_OK)
    {
       hr = ColumnSchemaRowset.MoveNext();
    }
    
  3. To fetch the information, access the appropriate data member of the schema rowset object, for example, ColumnSchemaRowset.m_szColumnName. This corresponds to COLUMN_NAME. To see which OLE DB column each data member corresponds to, see CColumns.

For the reference of the schema rowset typedef classes provided in the OLE DB Templates, see Schema Rowset Classes and Typedef Classes

For detailed information on OLE DB schema rowsets, including restriction columns, see Appendix B: Schema Rowsets in the OLE DB Programmer's Reference.

See the CatDB and DBViewer samples for more complex examples of how to use schema rowset classes.

For information on provider support for schema rowsets, see Supporting Schema Rowsets.

See Also

Using Accessors

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