使用C ++和SQL Native Client的SQLGetData问题

我有一个使用SQL Native Client连接到MS SQL Server 2000的C ++应用程序。

我正在尝试从TEXT列检索结果，该列包含的数据比最初分配给它的缓冲区提供的数据多。为了澄清我的问题，我将概述我在做什么（下面的代码）：

分配1024字节使用的缓冲区 使用SQLBindColumn将缓冲区绑定到列 使用SQLExecute执行SELECT查询 使用SQLFetch遍历结果 SQLFetch无法将整个结果返回到我的缓冲区：我想使用SQLGetData检索整个列值 上面的操作顺序提出了一个问题：SQLGetData在驱动程序中的绑定列上不起作用。

一个有效的解决方案是使用SQL_DATA_AT_EXEC标志，如下面的代码所示。

起始代码：

#include <windows.h>

#include <sql.h>
#include <sqlext.h>
#include <sqltypes.h> 
#include <sqlncli.h>

#include <cstdio>
#include <string>

const int MAX_CHAR = 1024;

bool test_retcode( RETCODE my_code, const char* my_message )
{
    bool success = ( my_code == SQL_SUCCESS_WITH_INFO || my_code == SQL_SUCCESS );
    if ( ! success )
    {
        printf( "%s", my_message );
    }

    return success;
}

int main ( )
{
    SQLHENV EnvironmentHandle;

    RETCODE retcode = SQLAllocHandle( SQL_HANDLE_ENV, SQL_NULL_HANDLE, &EnvironmentHandle );
    test_retcode( retcode, "SQLAllocHandle(Env) failed!" );

    retcode = SQLSetEnvAttr( EnvironmentHandle, SQL_ATTR_ODBC_VERSION, (SQLPOINTER) SQL_OV_ODBC3, SQL_IS_INTEGER );
    test_retcode( retcode, "SQLSetEnvAttr(ODBC version) Failed" );

    SQLHDBC ConnHandle;
    retcode = SQLAllocHandle( SQL_HANDLE_DBC, EnvironmentHandle, &ConnHandle );
    test_retcode( retcode, "Could not allocate MS SQL 2000 connection handle." );

    SQLSMALLINT driver_out_length;
    retcode = SQLDriverConnect( ConnHandle,
        NULL, // we're not interested in spawning a window
        (SQLCHAR*) "DRIVER={SQL Native Client};SERVER=localhost;UID=username;PWD=password;Database=Test;",
        SQL_NTS,
        NULL,
        0,
        &driver_out_length,
        SQL_DRIVER_NOPROMPT );
    test_retcode( retcode, "SQLConnect() Failed" );

    SQLHSTMT StatementHandle;
    retcode = SQLAllocHandle(SQL_HANDLE_STMT, ConnHandle, &StatementHandle);
    test_retcode( retcode, "Failed to allocate SQL Statement handle." );

    char* buffer = new char[ MAX_CHAR ];
    SQLINTEGER length = MAX_CHAR - 1;

    // -- Bind Block
    retcode = SQLBindCol( StatementHandle, 
        1,
        SQL_C_CHAR,
        (SQLPOINTER) NULL,
        (SQLINTEGER) SQL_DATA_AT_EXEC,
        &length );

    test_retcode( retcode, "Failed to bind column." );
    // -- End Bind Block

    retcode = SQLExecDirect( StatementHandle, (SQLCHAR*) "SELECT VeryLong FROM LongData", SQL_NTS );
    test_retcode( retcode, "SQLExecDirect failed." );

    // -- Fetch Block
    retcode = SQLFetch( StatementHandle );
    if ( retcode != SQL_SUCCESS && retcode != SQL_SUCCESS_WITH_INFO && retcode != SQL_NO_DATA )
    {
        printf( "Problem fetching row.\n" );
        return 9;
    }

    printf( "Fetched data.  length: %d\n", length );
    // -- End Fetch Block


    bool sql_success;
    std::string data;
    RETCODE r2;
    do
    {
        r2 = SQLGetData( StatementHandle, 1, SQL_C_CHAR, buffer, MAX_CHAR, &length );

        if ( sql_success = test_retcode( r2, "SQLGetData failed." ) )
        {
            data.append( buffer );
        }
        else
        {
            char* err_msg = new char[ MAX_CHAR ];
            SQLSMALLINT req = 1;
            SQLCHAR state[6];
            SQLINTEGER error;
            SQLINTEGER output_length;

            int sql_state = SQLGetDiagRec( SQL_HANDLE_STMT, StatementHandle, req, state, &error, (SQLCHAR*) err_msg, (SQLINTEGER) MAX_CHAR, (SQLSMALLINT*) &output_length );

            // state is: 07009, error_msg: "[Microsoft][SQL Native Client]Invalid Descriptor Index"
            printf( "%s\n", err_msg );

            delete err_msg;

            return 9;
        }
    } 
    while ( sql_success && r2 != SQL_SUCCESS );

    printf( "Done.\n" );

    return 0;
}