使用IRow::GetColumns (OLE DB)(MSSQL)获取列数据

Fetching column data using IRow::GetColumns (OLE DB)(MSSQL)

本文关键字：MSSQL 获取 数据 OLE IRow GetColumns 使用 DB      更新时间：2023-10-16

在我的应用程序中，我试图从VARCHAR(4000)列读取数据。由于该列是4000字节，所以我有一个足够大的应用程序缓冲区来处理它。但目前我在列中只有10字节的数据。在执行IRow->GetColumns()之后，不确定如何仅复制10字节的数据。我做了以下操作，但是得到了全部4000字节，所以当打印数据时，它是用3990个空格填充的10个字符的实际数据。

retcode = WideCharToMultiByte(CP_UTF8, 0, (WCHAR*)pDBColumnAccess[nCol].pData, -1, (char *)pReadBuf, pDBColumnAccess[nCol].cbDataLen, NULL, NULL);

我认为pDBColumnAccess。cbDataLen只有10，但是它的值是4000。

如何从列中读取确切的字节数?

谢谢。

找到了这个方法，不确定这是实际的解决方案还是只是一个变通方法。

如果在创建表之前将ANSI_PADDING设置为OFF，则不会在实际数据之后填充空白。您可以在表的sp_help中看到属性值"TrimTrailingBlanks"。

谢谢。

您应该设置一个包含DBPART_LENGTH的DBBINDING来获取字段的长度。为了说明这一点，我在SQL Server中使用以下DDL创建了一些VARCHAR数据:

CREATE TABLE NEWS
(
    ID INT NOT NULL,
    ARTICLE NVARCHAR(4000)
);
INSERT INTO NEWS (1, 'Today is a sunny day.');

然后我使用以下基于ATL的c++代码示例来读取ARTICLE字段。如果你在代码中设置了断点，你会看到它会返回

  • 数据。narticllength = 21
  • 数据。文章="今天是个晴天。"

下面是示例代码:

#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <oledb.h>
#include <atlbase.h>
int _tmain(int argc, _TCHAR* argv[])
{
    HRESULT hr = S_OK;
    hr = CoInitializeEx(NULL, COINIT_MULTITHREADED);
    // Connect to SQL Server.
    CComPtr<IDBInitialize> spIDBInitialize;
    hr = spIDBInitialize.CoCreateInstance(OLESTR("SQLNCLI"));
    CComPtr<IDBProperties> spIDBProperties;
    hr = spIDBInitialize->QueryInterface(IID_IDBProperties, (void**) &spIDBProperties);
    CComVariant varDataSource(OLESTR("InsertYourSQLServer"));
    CComVariant varCatalog(_T("InsertYourDatabase"));
    CComVariant varUserID(_T("InsertYourUserName"));
    CComVariant varPassword(_T("InsertYourPassword"));
    DBPROP rgProps[4] =
    {
        { DBPROP_INIT_DATASOURCE, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varDataSource },
        { DBPROP_INIT_CATALOG, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varCatalog },
        { DBPROP_AUTH_USERID, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varUserID },
        { DBPROP_AUTH_PASSWORD, DBPROPOPTIONS_REQUIRED, 0, DB_NULLID, varPassword }
    };
    DBPROPSET propSet = {rgProps, 4, DBPROPSET_DBINIT};
    hr = spIDBProperties->SetProperties(1, &propSet);
    spIDBProperties = NULL;
    hr = spIDBInitialize->Initialize();
    // Execute the query.
    CComPtr<IDBCreateSession> spIDBCreateSession;
    hr = spIDBInitialize->QueryInterface(IID_IDBCreateSession, (void**) &spIDBCreateSession);
    CComPtr<IDBCreateCommand> spIDBCreateCommand;
    hr = spIDBCreateSession->CreateSession(NULL, IID_IDBCreateCommand, (IUnknown**) &spIDBCreateCommand);
    spIDBCreateSession = NULL;
    CComPtr<ICommandText> spICommandText;
    hr = spIDBCreateCommand->CreateCommand(NULL, IID_ICommandText, (IUnknown**) &spICommandText);
    spIDBCreateCommand = NULL;
    hr = spICommandText->SetCommandText(DBGUID_SQL, OLESTR("SELECT ID, ARTICLE FROM NEWS"));
    DBROWCOUNT cRowsAffected = 0;
    CComPtr<IRowset> spIRowset;
    hr = spICommandText->Execute(NULL, IID_IRowset, NULL, &cRowsAffected, (IUnknown**) &spIRowset);
    spICommandText = NULL;
    // Retrieve records.
    HROW hRow = NULL;
    HROW *rghRow = &hRow;
    DBCOUNTITEM cRowsObtained = 0;
    hr = spIRowset->GetNextRows(DB_NULL_HCHAPTER, 0, 1, &cRowsObtained, &rghRow);
    while (hr == S_OK && cRowsObtained == 1)
    {
        // Fetch the ARTICLE field.
        struct
        {
            DBSTATUS dbArticleStatus;
            ULONG nArticleLength;
            char szArticle[4000 + 4];
        } data = {0};
        DBBINDING Binding = {0};
        Binding.iOrdinal    = 2;
        Binding.obValue     = sizeof(DBSTATUS) + sizeof(ULONG);
        Binding.obLength    = sizeof(DBSTATUS);
        Binding.obStatus    = 0;
        Binding.pTypeInfo   = NULL;
        Binding.pObject     = NULL;
        Binding.pBindExt    = NULL;
        Binding.dwPart      = DBPART_STATUS | DBPART_LENGTH | DBPART_VALUE;
        Binding.dwMemOwner  = DBMEMOWNER_CLIENTOWNED;
        Binding.eParamIO    = DBPARAMIO_NOTPARAM;
        Binding.cbMaxLen    = 4000 + 1;
        Binding.wType       = DBTYPE_STR;
        Binding.dwFlags     = 0;
        Binding.bPrecision  = 0;
        Binding.bScale      = 0;
        CComPtr<IAccessor> spIAccessor;
        hr = spIRowset->QueryInterface(IID_IAccessor, (void**) &spIAccessor);
        HACCESSOR hAccessor = NULL;
        hr = spIAccessor->CreateAccessor(DBACCESSOR_ROWDATA, 1, &Binding, 0, &hAccessor, NULL);
        hr = spIRowset->GetData(hRow, hAccessor, &data);
        DBREFCOUNT cRefCount = 0;
        hr = spIAccessor->ReleaseAccessor(hAccessor, &cRefCount);
        spIAccessor = NULL;
        // @@TODO: Do something with data.szArticle and data.nArticleLength
        // ...
        // Fetch next row of data.
        hr = spIRowset->ReleaseRows(1, rghRow, NULL, NULL, NULL);
        cRowsObtained = 0;
        hr = spIRowset->GetNextRows(DB_NULL_HCHAPTER, 0, 1, &cRowsObtained, &rghRow);
    }
    // Release everything
    spIRowset = NULL;
    spIDBInitialize = NULL;
    CoUninitialize();
    return 0;
}
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