CDaoRecordset::Open
You must call this member function to retrieve the records for the recordset.
virtual void Open(
int nOpenType = AFX_DAO_USE_DEFAULT_TYPE,
LPCTSTR lpszSQL = NULL,
int nOptions = 0
);
virtual void Open(
CDaoTableDef* pTableDef,
int nOpenType = dbOpenTable,
int nOptions = 0
);
virtual void Open(
CDaoQueryDef* pQueryDef,
int nOpenType = dbOpenDynaset,
int nOptions = 0
);
Parameters
nOpenType
One of the following values:dbOpenDynaset A dynaset-type recordset with bidirectional scrolling. This is the default.
dbOpenTable A table-type recordset with bidirectional scrolling.
dbOpenSnapshot A snapshot-type recordset with bidirectional scrolling.
lpszSQL
A string pointer containing one of the following:A NULL pointer.
The name of one or more tabledefs and/or querydefs (comma-separated).
A SQL SELECT statement (optionally with a SQL WHERE or ORDER BY clause).
A pass-through query.
nOptions
One or more of the options listed below. The default value is 0. Possible values are as follows:dbAppendOnly You can only append new records (dynaset-type recordset only). This option means literally that records may only be appended. The MFC ODBC database classes have an append-only option that allows records to be retrieved and appended.
dbForwardOnly The recordset is a forward-only scrolling snapshot.
dbSeeChanges Generate an exception if another user is changing data you are editing.
dbDenyWrite Other users cannot modify or add records.
dbDenyRead Other users cannot view records (table-type recordset only).
dbReadOnly You can only view records; other users can modify them.
dbInconsistent Inconsistent updates are allowed (dynaset-type recordset only).
dbConsistent Only consistent updates are allowed (dynaset-type recordset only).
备注
The constants dbConsistent and dbInconsistent are mutually exclusive. You can use one or the other, but not both in a given instance of Open.
pTableDef
A pointer to a CDaoTableDef object. This version is valid only for table-type recordsets. When using this option, the CDaoDatabase pointer used to construct the CDaoRecordset is not used; rather, the database in which the tabledef resides is used.pQueryDef
A pointer to a CDaoQueryDef object. This version is valid only for dynaset-type and snapshot-type recordsets. When using this option, the CDaoDatabase pointer used to construct the CDaoRecordset is not used; rather, the database in which the querydef resides is used.
Remarks
Before calling Open, you must construct the recordset object. There are several ways to do this:
When you construct the recordset object, pass a pointer to a CDaoDatabase object that is already open.
When you construct the recordset object, pass a pointer to a CDaoDatabase object that is not open. The recordset opens a CDaoDatabase object, but will not close it when the recordset object closes.
When you construct the recordset object, pass a NULL pointer. The recordset object calls GetDefaultDBName to get the name of the Microsoft Access .MDB file to open. The recordset then opens a CDaoDatabase object and keeps it open as long as the recordset is open. When you call Close on the recordset, the CDaoDatabase object is also closed.
备注
When the recordset opens the CDaoDatabase object, it opens the data source with nonexclusive access.
For the version of Open that uses the lpszSQL parameter, once the recordset is open you can retrieve records in one of several ways. The first option is to have DFX functions in your DoFieldExchange. The second option is to use dynamic binding by calling the GetFieldValue member function. These options can be implemented separately or in combination. If they are combined, you will have to pass in the SQL statement yourself on the call to Open.
When you use the second version of Open where you pass in a CDaoTableDef object, the resulting columns will be available for you to bind via DoFieldExchange and the DFX mechanism, and/or bind dynamically via GetFieldValue.
备注
You can only call Open using a CDaoTableDef object for table-type recordsets.
When you use the third version of Open where you pass in a CDaoQueryDef object, that query will be executed, and the resulting columns will be available for you to bind via DoFieldExchange and the DFX mechanism, and/or bind dynamically via GetFieldValue.
备注
You can only call Open using a CDaoQueryDef object for dynaset-type and snapshot-type recordsets.
For the first version of Open that uses the lpszSQL parameter, records are selected 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. |
|
A comma-separated list of one or more tabledefs and/or querydef names. |
All columns represented in the DoFieldExchange. |
"Customer" |
SELECT column-list FROM table-list |
The specified columns from the specified tabledef(s) and/or querydef(s). |
"SELECT CustId, CustName FROM Customer" |
The usual procedure is to pass NULL to Open; in that case, Open calls GetDefaultSQL, an overridable member function that ClassWizard generates when creating a CDaoRecordset-derived class. This value gives the tabledef(s) and/or querydef 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.
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, set m_strSort or m_strFilter 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.
If you call Open on a dynaset-type or snapshot-type recordset, or if the data source refers to a SQL statement or a tabledef that represents an attached table, you cannot use dbOpenTable for the type argument; if you do, MFC throws an exception. To determine whether a tabledef object represents an attached table, create a CDaoTableDef object and call its GetConnect member function.
Use the dbSeeChanges flag if you wish to trap changes made by another user or another program on your machine when you are editing or deleting the same record. For example, if two users start editing the same record, the first user to call the Update member function succeeds. When Update is called by the second user, a CDaoException is thrown. Similarly, if the second user tries to call Delete to delete the record, and it has already been changed by the first user, a CDaoException occurs.
Typically, if the user gets this CDaoException while updating, your code should refresh the contents of the fields and retrieve the newly modified values. If the exception occurs in the process of deleting, your code could display the new record data to the user and a message indicating that the data has recently changed. At this point, your code can request a confirmation that the user still wants to delete the record.
提示
Use the forward-only scrolling option (dbForwardOnly) to improve performance when your application makes a single pass through a recordset opened from an ODBC data source.
For related information, see the topic "OpenRecordset Method" in DAO Help.
Requirements
Header: afxdao.h