CRecordset::Open

Статья
10/16/2008

Opens the recordset by retrieving the table or performing the query that the recordset represents.

virtual BOOL Open( 
   UINT nOpenType = AFX_DB_USE_DEFAULT_TYPE, 
   LPCTSTR lpszSQL = NULL, 
   DWORD dwOptions = none  
);

Параметры

nOpenType
Accept the default value, AFX_DB_USE_DEFAULT_TYPE, or use one of the following values from the enum OpenType:
- CRecordset::dynaset A recordset with bi-directional scrolling. The membership and ordering of the records are determined when the recordset is opened, but changes made by other users to the data values are visible following a fetch operation. Dynasets are also known as keyset-driven recordsets.
- CRecordset::snapshot A static recordset with bi-directional scrolling. The membership and ordering of the records are determined when the recordset is opened; the data values are determined when the records are fetched. Changes made by other users are not visible until the recordset is closed and then reopened.
- CRecordset::dynamic A recordset with bi-directional scrolling. Changes made by other users to the membership, ordering, and data values are visible following a fetch operation. Note that many ODBC drivers do not support this type of recordset.
- CRecordset::forwardOnly A read-only recordset with only forward scrolling.
  
  For CRecordset, the default value is CRecordset::snapshot. The default-value mechanism allows the Visual C++ wizards to interact with both ODBC CRecordset and DAO CDaoRecordset, which have different defaults.
For more information about these recordset types, see the article Recordset (ODBC). For related information, see the article "Using Block and Scrollable Cursors" in the Windows SDK.

Внимание!

If the requested type is not supported, the framework throws an exception.

Внимание!
If the requested type is not supported, the framework throws an exception.

lpszSQL
A string pointer containing one of the following:

A NULL pointer.
The name of a table.
A SQL SELECT statement (optionally with a SQL WHERE or ORDER BY clause).
A CALL statement specifying the name of a predefined query (stored procedure). Be careful that you do not insert whitespace between the curly brace and the CALL keyword.

For more information about this string, see the table and the discussion of ClassWizard's role under Remarks.

Примечание.
The order of the columns in your result set must match the order of the RFX or Bulk RFX function calls in your DoFieldExchange or DoBulkFieldExchange function override.

dwOptions
A bitmask which can specify a combination of the values listed below. Some of these are mutually exclusive. The default value is none.
- CRecordset::none No options set. This parameter value is mutually exclusive with all other values. By default, the recordset can be updated with Edit or Delete and allows appending new records with AddNew. Updatability depends on the data source as well as on the nOpenType option you specify. Optimization for bulk additions is not available. Bulk row fetching will not be implemented. Deleted records will not be skipped during recordset navigation. Bookmarks are not available. Automatic dirty field checking is implemented.
- CRecordset::appendOnly Do not allow Edit or Delete on the recordset. Allow AddNew only. This option is mutually exclusive with CRecordset::readOnly.
- CRecordset::readOnly Open the recordset as read-only. This option is mutually exclusive with CRecordset::appendOnly.
- CRecordset::optimizeBulkAdd Use a prepared SQL statement to optimize adding many records at one time. Applies only if you are not using the ODBC API function SQLSetPos to update the recordset. The first update determines which fields are marked dirty. This option is mutually exclusive with CRecordset::useMultiRowFetch.
- CRecordset::useMultiRowFetch Implement bulk row fetching to allow multiple rows to be retrieved in a single fetch operation. This is an advanced feature designed to improve performance; however, bulk record field exchange is not supported by ClassWizard. This option is mutually exclusive with CRecordset::optimizeBulkAdd. Note that if you specify CRecordset::useMultiRowFetch, then the option CRecordset::noDirtyFieldCheck will be turned on automatically (double buffering will not be available); on forward-only recordsets, the option CRecordset::useExtendedFetch will be turned on automatically. For more information about bulk row fetching, see the article Recordset: Fetching Records in Bulk (ODBC).
- CRecordset::skipDeletedRecords Skip all deleted records when navigating through the recordset. This will slow performance in certain relative fetches. This option is not valid on forward-only recordsets. If you call Move with the nRows parameter set to 0, and the CRecordset::skipDeletedRecords option set, Move will assert. Note that CRecordset::skipDeletedRecords is similar to driver packing, which means that deleted rows are removed from the recordset. However, if your driver packs records, then it will skip only those records that you delete; it will not skip records deleted by other users while the recordset is open. CRecordset::skipDeletedRecords will skip rows deleted by other users.
- CRecordset::useBookmarks May use bookmarks on the recordset, if supported. Bookmarks slow data retrieval but improve performance for data navigation. Not valid on forward-only recordsets. For more information, see the article Recordset: Bookmarks and Absolute Positions (ODBC).
- CRecordset::noDirtyFieldCheck Turn off automatic dirty field checking (double buffering). This will improve performance; however, you must manually mark fields as dirty by calling the SetFieldDirty and SetFieldNull member functions.Note that double buffering in class CRecordset is similar to double buffering in class CDaoRecordset. However, in CRecordset, you cannot enable double buffering on individual fields; you either enable it for all fields or disable it for all fields. Note that if you specify the option CRecordset::useMultiRowFetch, then CRecordset::noDirtyFieldCheck will be turned on automatically; however, SetFieldDirty and SetFieldNull cannot be used on recordsets that implement bulk row fetching.
- CRecordset::executeDirect Do not use a prepared SQL statement. For improved performance, specify this option if the Requery member function will never be called.
- CRecordset::useExtendedFetch Implement SQLExtendedFetch instead of SQLFetch. This is designed for implementing bulk row fetching on forward-only recordsets. If you specify the option CRecordset::useMultiRowFetch on a forward-only recordset, then CRecordset::useExtendedFetch will be turned on automatically.
- CRecordset::userAllocMultiRowBuffers The user will allocate storage buffers for the data. Use this option in conjunction with CRecordset::useMultiRowFetch if you want to allocate your own storage; otherwise, the framework will automatically allocate the necessary storage. For more information, see the article Recordset: Fetching Records in Bulk (ODBC). Note that specifying CRecordset::userAllocMultiRowBuffers without specifying CRecordset::useMultiRowFetch will result in a failed assertion.

Возвращаемое значение

Nonzero if the CRecordset object was successfully opened; otherwise 0 if CDatabase::Open (if called) returns 0.

Заметки

You must call this member function to run the query defined by the recordset. Before calling Open, you must construct the recordset object.

This recordset's connection to the data source depends on how you construct the recordset before calling Open. If you pass a CDatabase object to the recordset constructor that has not been connected to the data source, this member function uses GetDefaultConnect to attempt to open the database object. If you pass NULL to the recordset constructor, the constructor constructs a CDatabase object for you, and Open attempts to connect the database object. For details on closing the recordset and the connection under these varying circumstances, see Close.

Примечание.
Access to a data source through a CRecordset object is always shared. Unlike the CDaoRecordset class, you cannot use a CRecordset object to open a data source with exclusive access.

When you call Open, a query, usually a SQL SELECT statement, selects records based on criteria shown in the following table.

Value of the lpszSQL parameter	Records selected are determined by	Example
NULL	The string returned by GetDefaultSQL.
SQL table name	All columns of the table-list in DoFieldExchange or DoBulkFieldExchange.	"Customer"
Predefined query (stored procedure) name	The columns the query is defined to return.	"{call OverDueAccts}"
SELECT column-list FROM table-list	The specified columns from the specified table(s).	"SELECT CustId, CustName FROM Customer"

Внимание!
Be careful that you do not insert extra whitespace in your SQL string. For example, if you insert whitespace between the curly brace and the CALL keyword, MFC will misinterpret the SQL string as a table name and incorporate it into a SELECT statement, which will result in an exception being thrown. Similarly, if your predefined query uses an output parameter, do not insert whitespace between the curly brace and the '?' symbol. Finally, you must not insert whitespace before the curly brace in a CALL statement or before the SELECT keyword in a SELECT statment.

Be careful that you do not insert extra whitespace in your SQL string. For example, if you insert whitespace between the curly brace and the CALL keyword, MFC will misinterpret the SQL string as a table name and incorporate it into a SELECT statement, which will result in an exception being thrown. Similarly, if your predefined query uses an output parameter, do not insert whitespace between the curly brace and the '?' symbol. Finally, you must not insert whitespace before the curly brace in a CALL statement or before the SELECT keyword in a SELECT statment.

The usual procedure is to pass NULL to Open; in this case, Open calls GetDefaultSQL. If you are using a derived CRecordset class, GetDefualtSQL gives the table name(s) you specified in ClassWizard. You can instead specify other information in the lpszSQL parameter.

Whatever you pass, Open constructs a final SQL string for the query (the string may have SQL WHERE and ORDER BY clauses appended to the lpszSQL string you passed) and then executes the query. You can examine the constructed string by calling GetSQL after calling Open. For additional details about how the recordset constructs a SQL statement and selects records, see the article Recordset: How Recordsets Select Records (ODBC).

The field data members of your recordset class are bound to the columns of the data selected. If any records are returned, the first record becomes the current record.

If you want to set options for the recordset, such as a filter or sort, specify these after you construct the recordset object but before you call Open. If you want to refresh the records in the recordset after the recordset is already open, call Requery.

For more information, including additional examples, see the articles Recordset (ODBC), Recordset: How Recordsets Select Records (ODBC), and Recordset: Creating and Closing Recordsets (ODBC).

Исключения

This method can throw exceptions of type CDBException* and CMemoryException*.

Пример

The following code examples show different forms of the Open call.

// rsSnap, rsLName, and rsDefault are CRecordset or CRecordset-derived 
// objects

// Open rs using the default SQL statement, implement bookmarks, and turn 
// off automatic dirty field checking
rsSnap.Open(CRecordset::snapshot, NULL, CRecordset::useBookmarks | 
   CRecordset::noDirtyFieldCheck);

// Pass a complete SELECT statement and open as a dynaset
rsLName.Open(CRecordset::dynaset, _T("Select L_Name from Customer"));

// Accept all defaults
rsDefault.Open();