Tuesday, December 02, 2008

Enumerating Columns with IDBSchemaRowset

While it's freezing out there (for Lisbon standards, of course) I have been busy working on the schema enumeration code for the OLE DB Client library. My first approach is to enumerate database tables and columns using the IDBSchemaRowset interface. Access to this interface is encapsulated in the CRowsetInterface class that is derived in CTablesRowset and CColumnsRowset classes to enumerate tables and columns respectively. The IDBSchemaRowset interface works by accepting a set of "restrictions" and generates a schema rowset containing the requested data. Schema rowsets are formally defined on the Appendix B of the OLE DB Programmer's Guide, where you can see the returned columns and the accepted "restrictions". SQL Compact implements a subset of these, as described in the BOL.

Using the CTablesRowset is quite easy (and you have seen it at work in a previous sample where it was used to test for the presence of a given table):

CTablesRowset tables(m_session);
CRowset rowset;
hr = tables.Open(NULL, NULL, NULL, NULL, rowset);

The first four NULL parameters mean that you don't want to filter on any of the supported restrictions, so you will get a list of all the tables in the database. SQL Compact does not support table catalogs nor table schemas so the first two parameters are always NULL for this provider. The third restriction is the table name and the fourth is the table type (see the possible values for this in the TABLES schema rowset reference).

When scrolling through the tables rowset, you retrieve the current table name like this:

CString strTable;
rowset.GetValue(3, strTable);

Now that we have the table name, we can enumerate its columns by using the CColumnsRowset and imposing a restriction on the table name (see the COLUMNS schema rowset reference):

CColumnsRowset columns(m_session);
CRowset rsColumns;

hr = columns.Open(NULL, NULL, strTable, NULL, rsColumns);

This retrieves all columns from the given table and you can get the column name from ordinal 4. As you can see from the COLUMNS schema rowset, there are lots of additional schema information about a column, like its OLE DB type, "nullability", size and more.

Although this is a quite convenient way to retrieve a table schema, we can use another OLE DB interface that in a single call returns column and constraint information. On the next post, I will look at ITableCreation.

Sample code:

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