获取行数据¶
IRowset接口是Rowset对象的基本接口。这个接口提供了顺序获取和管理行数据的方法。用户使用IRowset的方法进行基本的行操作，比如获取和释放行数据，获取列值等等。
当用户创建了Rowset对象，通常第一步是通过IRowsetInfo：：GetProperties读取Rowset的属性，了解Rowset的行为和能力。然后用户可以通过IColumnsInfo或者IColumnsRowset接口获得Rowset的列信息，具体内容为：
Rowset包含的列数
一个DBCOLUMNINFO结构数组，每个元素表示一列
一个包含所有字符串的缓存的指针
收集到Rowset的列信息后，用户可以根据这些信息和应用的需求，构建一个绑定数组作为参数传递给IAccessor：：CreateAccessor，这样可以创建一个Accessor，它代表了用户创建的列信息绑定。用户可以创建多个Accessor来获取不同的列数据。只要Rowset对象还存在，Accessor能够随时创建和释放。
用户通过IRowset::GetNextRows或者IRowsetLocate：：GetRowsAt来获取Rowset中的行数据。这些函数把行数据从数据库读取出来存入OLE DB的行缓存中。用户不能直接访问OLE DB的行缓存。用户必须使用IRowset：：GetData将OLE DB的行缓存复制到用户的缓存中，同样，用户使用IRowsetChange：：SetData来将用户缓存中的要改变的行数据复制到OLE DB的行缓存中。
用户调用GetData，传入一个标识行的句柄，一个标识Accessor的句柄，一个用户分配的缓存指针。GetData根据Accessor所规定的列数据绑定，将行标识句柄标识的行的数据写入用户的缓存。用户可以在同一行上使用不同的Accessor多次调用GetData，也就是说，用户可以获取同一份行数据的不同拷贝。
当用户获取或者更新了行数据以后，它调用ReleaseRows来释放OLE DB的行缓存。当用户对Rowset的对象操作完成后，可调用IAccessor：：ReleaseAccessor来释放acceoosr，然后调用IUnknown：：Release方法来释放Rowset对象本身。当Rowset对象被释放后，所有的行句柄信息和Accessor都将被释放。
GetNextRows方法会顺序的读取Rowset中的所有行数据，不过GetNextRows可以使用skip参数来忽略掉某些行。另外，FindNextRow，Seek，RestartPosition都可以改变GetNextRows的顺序读取方式。
下面举例说明如何从Rowset中提取行数据。
/*
Example shows how to fetch rows from a result set.
*/
void InitializeAndEstablishConnection();
void ProcessResultSet();
#define UNICODE
#define _UNICODE
#define DBINITCONSTANTS
#define INITGUID
#include <stdio.h>
#include <tchar.h>
#include <stddef.h>
#include <windows.h>
#include <iostream.h>
#include <oledb.h>
#include <SQLOLEDB.h>
IDBInitialize* pIDBInitialize = NULL;
IDBProperties* pIDBProperties = NULL;
IDBCreateSession* pIDBCreateSession = NULL;
IDBCreateCommand* pIDBCreateCommand = NULL;
ICommandText* pICommandText = NULL;
IRowset* pIRowset = NULL;
IColumnsInfo* pIColumnsInfo = NULL;
DBCOLUMNINFO* pDBColumnInfo = NULL;
IAccessor* pIAccessor = NULL;
DBPROP InitProperties[4];
DBPROPSET rgInitPropSet[1];
ULONG i, j;
HRESULT hr;
LONG cNumRows = 0;
ULONG lNumCols;
WCHAR* pStringsBuffer;
DBBINDING* pBindings;
ULONG ConsumerBufColOffset = 0;
HACCESSOR hAccessor;
ULONG lNumRowsRetrieved;
HROW hRows[10];
HROW* pRows = &hRows[0];
BYTE* pBuffer;
void main()
{
//Here is the command to execute.
WCHAR* wCmdString
= OLESTR(" SELECT title, price FROM titles WHERE
royalty > 14");
// Call a function to initialize and establish connection.
InitializeAndEstablishConnection();
//Create a session object.
if(FAILED(pIDBInitialize->QueryInterface(
IID_IDBCreateSession,
(void**) &pIDBCreateSession)))
{
cout << "Failed to obtain IDBCreateSession interface.\n";
}
if(FAILED(pIDBCreateSession->CreateSession(
NULL,
IID_IDBCreateCommand,
(IUnknown**)
&pIDBCreateCommand)))
{
cout << "pIDBCreateSession->CreateSession failed.\n";
}
//Access the ICommandText interface.
if(FAILED(pIDBCreateCommand->CreateCommand(
NULL,
IID_ICommandText,
(IUnknown**)
&pICommandText)))
{
cout << "Failed to access ICommand interface.\n";
}
//Use SetCommandText() to specify the command text.
if(FAILED(pICommandText->SetCommandText(DBGUID_DBSQL,
wCmdString)))
{
cout << "Failed to set command text.\n";
}
//Execute the command.
if(FAILED(hr = pICommandText->Execute(NULL,
IID_IRowset,
NULL,
&cNumRows,
(IUnknown **) &pIRowset)))
{
cout << "Failed to execute command.\n";
}
//Process the result set.
ProcessResultSet();
pIRowset->Release();
//Free up memory.
pICommandText->Release();
pIDBCreateCommand->Release();
pIDBCreateSession->Release();
if(FAILED(pIDBInitialize->Uninitialize()))
{
/*Uninitialize is not required, but it fails if an interface
has not been released. This can be used for debugging.
cout << "Problem uninitializing.\n"; */
} //endif.
pIDBInitialize->Release();
//Release the COM library.
CoUninitialize();
};
//--------------------------------------------------------------------
void InitializeAndEstablishConnection()
{
//Initialize the COM library.
CoInitialize(NULL);
//Obtain access to the SHENTONGOLEDB provider.
hr = CoCreateInstance(CLSID_OSCAROLEDB,
NULL,
CLSCTX_INPROC_SERVER,
IID_IDBInitialize,
(void **) &pIDBInitialize);
if(FAILED(hr))
{
printf("Failed to get IDBInitialize interface.\n");
} //end if
/*
Initialize the property values needed
to establish the connection.
*/
for(i = 0; i < 4; i++)
VariantInit(&InitProperties[i].vValue);
//Server name.
InitProperties[0].dwPropertyID = DBPROP_INIT_DATASOURCE;
InitProperties[0].vValue.vt = VT_BSTR;
InitProperties[0].vValue.bstrVal=
SysAllocString(L"testdb");
InitProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[0].colid = DB_NULLID;
//Database.
InitProperties[1].dwPropertyID = DBPROP_INIT_LOCATION;
InitProperties[1].vValue.vt = VT_BSTR;
InitProperties[1].vValue.bstrVal= SysAllocString(L"192.9.200.10");
InitProperties[1].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[1].colid = DB_NULLID;
//Username (login).
InitProperties[2].dwPropertyID = DBPROP_AUTH_USERID;
InitProperties[2].vValue.vt = VT_BSTR;
InitProperties[2].vValue.bstrVal= SysAllocString(L"login");
InitProperties[2].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[2].colid = DB_NULLID;
//Password.
InitProperties[3].dwPropertyID = DBPROP_AUTH_PASSWORD;
InitProperties[3].vValue.vt = VT_BSTR;
InitProperties[3].vValue.bstrVal= SysAllocString(L"Password");
InitProperties[3].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[3].colid = DB_NULLID;
/*
Now that the properties are set, construct the DBPROPSET structure
(rgInitPropSet). The DBPROPSET structure is used to pass an array
of DBPROP structures (InitProperties) to the SetProperties method.
*/
rgInitPropSet[0].guidPropertySet = DBPROPSET_DBINIT;
rgInitPropSet[0].cProperties = 4;
rgInitPropSet[0].rgProperties = InitProperties;
//Set initialization properties.
hr = pIDBInitialize->QueryInterface(IID_IDBProperties,
(void **)&pIDBProperties);
if(FAILED(hr))
{
cout << "Failed to get IDBProperties interface.\n";
}
hr = pIDBProperties->SetProperties(1, rgInitPropSet);
if(FAILED(hr))
{
cout << "Failed to set initialization properties.\n";
} //end if
pIDBProperties->Release();
//Now establish the connection to the data source.
if(FAILED(pIDBInitialize->Initialize()))
{
cout << "Problem in establishing connection to the data
source.\n";
}
} //end of InitializeAndEstablishConnection.
//--------------------------------------------------------------------
//Retrieve and display data resulting from a query.
void ProcessResultSet()
{
//Obtain access to the IColumnInfo interface, from the Rowset
object.
hr = pIRowset->QueryInterface(IID_IColumnsInfo,
(void **)&pIColumnsInfo);
if(FAILED(hr))
{
cout << "Failed to get IColumnsInfo interface.\n";
} //end if
//Retrieve the column information.
pIColumnsInfo->GetColumnInfo(&lNumCols,
&pDBColumnInfo,
&pStringsBuffer);
//Free the column information interface.
pIColumnsInfo->Release();
//Create a DBBINDING array.
pBindings = new DBBINDING[lNumCols];
//Using the ColumnInfo structure, fill out the pBindings array.
for(j=0; j<lNumCols; j++) {
pBindings[j].iOrdinal = j+1;
pBindings[j].obValue = ConsumerBufColOffset;
pBindings[j].pTypeInfo = NULL;
pBindings[j].pObject = NULL;
pBindings[j].pBindExt = NULL;
pBindings[j].dwPart = DBPART_VALUE;
pBindings[j].dwMemOwner =
DBMEMOWNER_CLIENTOWNED;
pBindings[j].eParamIO = DBPARAMIO_NOTPARAM;
pBindings[j].cbMaxLen = pDBColumnInfo[j].ulColumnSize;
pBindings[j].dwFlags = 0;
pBindings[j].wType = pDBColumnInfo[j].wType;
pBindings[j].bPrecision = pDBColumnInfo[j].bPrecision;
pBindings[j].bScale = pDBColumnInfo[j].bScale;
//Compute the next buffer offset.
ConsumerBufColOffset = ConsumerBufColOffset +
pDBColumnInfo[j].ulColumnSize;
};
//Get the IAccessor interface.
hr = pIRowset->QueryInterface(IID_IAccessor, (void **)
&pIAccessor);
if(FAILED(hr))
{
cout << "Failed to obtain IAccessor interface.\n";
}
//Create an accessor from the set of bindings (pBindings).
pIAccessor->CreateAccessor(DBACCESSOR_ROWDATA,
lNumCols,
pBindings,
0,
&hAccessor,
NULL);
//Print column names.
for(j=0; j<lNumCols; j++) {
printf("%-40S", pDBColumnInfo[j].pwszName);
};
//Get a set of 10 rows.
pIRowset->GetNextRows(
NULL,
0,
10,
&lNumRowsRetrieved,
&pRows);
//Allocate space for the row buffer.
pBuffer = new BYTE[ConsumerBufColOffset];
//Display the rows.
while(lNumRowsRetrieved > 0) {
//For each row, print the column data.
for(j=0; j<lNumRowsRetrieved; j++) {
//Clear the buffer.
memset(pBuffer, 0, ConsumerBufColOffset);
//Get the row data values.
pIRowset->GetData(hRows[j], hAccessor, pBuffer);
//Print title and price values.
printf("%-40s%f\n", &pBuffer[pBindings[0].obValue],
(FLOAT) Buffer[pBindings[0].obValue]);
}; //for.
//Release the rows retrieved.
pIRowset->ReleaseRows(lNumRowsRetrieved,
hRows,
NULL,
NULL,
NULL);
//Get the next set of 10 rows.
pIRowset->GetNextRows(NULL,
0,
10,
&lNumRowsRetrieved,
&pRows);
}; //while lNumRowsRetrieved > 0.
//Free up all allocated memory.
delete [] pBuffer;
pIAccessor->ReleaseAccessor(hAccessor, NULL);
pIAccessor->Release();
delete [] pBindings;
} //ProcessResultSet.