vcg13.htm

Visual C++与数据库的连接经典实例

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<I>Data Members</I>

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m_pDatabase

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Contains a pointer to the CDaoDatabase object to which the Querydef is associated, whether the Querydef is saved or not.

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m_pDAOQueryDef

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Contains a pointer to the OLE interface for the underlying DAO Querydef object.

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<I>Construction</I>

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CDaoQueryDef()

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Constructs the CDaoQueryDef object. After the CDaoQueryDef object is created, you must call either Open() or Create().

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Create()

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Creates the underlying DAO Querydef object. The query can be either temporary or saved using a call to the Append() member function.

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Append()

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Appends the Querydef to the database's QueryDefs collection. This will make the Querydef a saved (not temporary) object.

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Open()

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Opens an existing Querydef stored in the database's QueryDefs collection.

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Close()

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Closes the Querydef object.

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Attributes

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CanUpdate()

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Returns a zero result if the database can't be updated.

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GetConnect()

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Obtains the connect string associated with the Querydef.

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GetDateCreated()

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Obtains the query's creation date.

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GetDateLastUpdated()

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Obtains the query's last update date.

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GetName()

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Returns the name of the Querydef.

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GetODBCTimeout()

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Obtains the ODBC timeout value.

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GetRecordsAffected()

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Determines the count of how many records have been affected by an action query.

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GetReturnsRecords()

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Returns zero if the query doesn't return any records.

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GetSQL()

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Returns the SQL string that specifies the query defined by the Querydef.

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GetType()

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Determines what the query type is. The query type could be delete, update, append, make-table, and so on.

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IsOpen()

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Returns zero if the Querydef isn't open or can't be executed.

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SetConnect()

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Sets the connect string for an SQL pass-through query on an ODBC data source.

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SetName()

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Sets the name of the saved query. This new name replaces the name that was in use when the Querydef was created.

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SetODBCTimeout()

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Sets the timeout period to be used by ODBC.

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SetReturnsRecords()

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Specifies that the Querydef will return records.

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SetSQL()

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Sets the SQL string that specifies the query defined by the Querydef.

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<I>Operations</I>

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Execute()

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Executes the query defined by the Querydef object.

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GetFieldCount()

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Determines the number of fields that this Querydef has defined.

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GetFieldInfo()

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Determines information about a field that is specified in the query.

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GetParameterCount()

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Determines the number of parameters defined for the query.

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GetParameterInfo()

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Obtains information about the specified parameter in the query.

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GetParamValue()

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Gets the value of a specified parameter to the query.

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SetParamValue()

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Sets the value of a specified parameter to the query.</FONT>

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<FONT SIZE=4 COLOR="#FF0000"><B><I>CDaoRecordset</I></B></FONT></CENTER></H4>

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<P>The DAO class CDaoRecordset is used to represent a set of records that have been selected from a datasource. CDaoRecordset corresponds directly to the CRecordset object that is used with ODBC.

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<P>The main difference between the DAO recordset and the ODBC recordset is that CDaoRecordset class objects access data through DAO, which is based on OLE, while the CRecordset classes access the database using ODBC.

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<P>A CDaoRecordset object is known as a recordset. A CDaoRecordset object can take one of three forms: a table-type recordset, a dynaset-type recordset, or a snapshot-type recordset:

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<LI>The table-type recordset represents a base table that you can use to examine records, change records, add records to, or delete records from a single database table.

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<LI>The dynaset-type recordset is the result of a query that can have records that are updatable. The dynaset-type recordset comprises a set of records that may be examined, added to, changed, or deleted from an underlying database table or tables. The dynaset-type recordset may contain fields from more than one table in a database if necessary.

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<LI>The snapshot-type recordset contains a static copy of a set of records returned from a query. This static set of records can be used to display information or create graphs or reports. Like a dynaset, a snapshot-type recordset may contain fields from one or more tables in a database.

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<P>The records contained in the recordset are fixed at the time the recordset is opened. Both the table-type recordset and dynaset-type recordset will reflect any changes made to data after the recordset was opened (by either the current process or perhaps by other users). In contrast, the snapshot recordset is static and isn't updated. Many applications use CDaoRecordset directly (usually using classes created using AppWizard). However, you can derive a class from CDaoRecordset if you wish. When you have a recordset object, you can do the following:

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<LI>Use a filter to limit the records selected from the recordset.

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<LI>Scroll through the records either forwards or backwards.

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<LI>With dynaset-type and snapshot-type recordsets, you can use comparison operations to find records. Typical operations include &lt;, &lt;=, =, &gt;=, and &gt;.

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<LI>With table-type recordsets, you can create an index and use the Seek() member function.

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<LI>With dynaset-type and table-type recordsets, you can update the records and specify a locking mode.

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<LI>Sort the recordset.

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<LI>At runtime, you can parameterize the recordset. This will serve to limit the records selected into the recordset.

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<P>To use the CDaoRecordset object, first you must open a database and construct your CDaoRecordset object. A pointer to your CDaoDatabase object must be passed to the constructor. Alternatively, it's possible to create a CDaoRecordset object and have MFC create a temporary CDaoDatabase object.

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<P>After you've created the CDaoRecordset, you must call the Open() member function. It takes a parameter that specifies whether the object is to be a table-type recordset, a dynaset-type recordset, or a snapshot-type recordset. Calling Open() selects data from the database and retrieves the first record.

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<P>After you've created your recordset object, you then can use the object's member functions and data members to scroll through the records and perform operations on them.

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<P>The CDaoRecordset class uses DAO record field exchange (DFX) to support the reading and updating of record fields through type-safe C++ members of your CDaoRecordset or CDaoRecordset-derived class. See the CDaoFieldExchange object in the Visual C++ Books Online for more information.

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<P>Table 13.6 lists the members of the CDaoRecordset class.

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<FONT COLOR="#000080"><B>Table 13.6. The CDaoRecordset class members.</B></FONT></CENTER>

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<I>Member</I>

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<I>Descrip