Fetching Columns Using IRow::GetColumns (or IRow::Open) and ISequentialStream

Data creazione: 14 aprile 2006

Large data can be bound or retrieved using the ISequentialStream interface. For bound columns, the status flag DBSTATUS_S_TRUNCATED indicates that the data is truncated.

The complete sample code is in this file FetchColumns_A.cpp. You can download an archive containing the sample from the SQL Server Downloads page on MSDN.

This sample was developed using Microsoft Visual C++ 2005.

The code below creates the sample table used by the application.

USE AdventureWorks
GO

IF EXISTS (SELECT name FROM sysobjects WHERE name = 'MyTable')
     DROP TABLE MyTable
GO

CREATE TABLE MyTable
(
     col1  int,
     col2  varchar(50),
     col3  char(50),
     col4  datetime,
     col5  float,
     col6  money,
     col7  sql_variant,
     col8  binary(50),
     col9  text,
     col10 image
)
GO

/* Enter data. */
INSERT INTO MyTable
values
(
     10,
     'abcdefghijklmnopqrstuvwxyz',
     'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
     '11/1/1999 11:52 AM',
     3.14,
     99.95,
     CONVERT(nchar(50), N'AbCdEfGhIjKlMnOpQrStUvWxYz'),
     0x123456789,
     REPLICATE('AAAAABBBBB', 500),
     REPLICATE(0x123456789, 500)
)
GO

Nota sulla protezione:
When possible, use Windows Authentication. If Windows Authentication is not available, prompt users to enter their credentials at run time. Avoid storing credentials in a file. If you must persist credentials, you should encrypt them with the Win32 crypto API.

To fetch columns using IRow::GetColumns (or IRow::Open) and ISequentialStream

1. Establish a connection to the data source.

2. Execute the command (in this example, ICommandExecute::Execute() is called with IID_IRow).

3. Fetch the column data using IRow::Open() or IRow::GetColumns().

   • IRow::Open() can be used to open an ISequentialStream on the row. Specify DBGUID_STREAM to indicate that the column contains a stream of binary data (IStream or ISequentialStream can then be used to read the data from the column).
   • If IRow::GetColumns() is used, the pData element of DBCOLUMNACCESS structure is set to point to a stream object.

4. Use ISequentialStream::Read() repeatedly to read the specified number of bytes into the consumer buffer.

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