chap10_5.htm

着重介绍基于C++的web编程技术

      </tr>
      <tr>
        <td WIDTH="39%"><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">CRecordset::dynamic</font></td>
        <td WIDTH="61%"><font SIZE="3"><p ALIGN="JUSTIFY">提供比动态集更好的动态特性，大部分</font><font FACE="Times New Roman" SIZE="3">ODBC</font><font SIZE="3">驱动程序不支持这种记录集．</font></td>
      </tr>
      <tr>
        <td WIDTH="39%"><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">CRecordset::forwardOnly</font></td>
        <td WIDTH="61%"><font SIZE="3"><p ALIGN="JUSTIFY">只能前向滚动的只读记录集．</font></td>
      </tr>
    </table>
    <p><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="CENTER"></font><b><font SIZE="3">表</font><font FACE="Times New Roman" SIZE="3">10.4 
    </font><font SIZE="3">创建记录集时的常用选项</font></b></p>
    <table BORDER="1" CELLSPACING="1" CELLPADDING="1" WIDTH="579">
      <tr>
        <td WIDTH="39%"><font SIZE="3"><p ALIGN="JUSTIFY">选项</font></td>
        <td WIDTH="61%"><font SIZE="3"><p ALIGN="JUSTIFY">含义</font></td>
      </tr>
      <tr>
        <td WIDTH="39%"><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">CRecordset::none</font></td>
        <td WIDTH="61%"><font SIZE="3"><p ALIGN="JUSTIFY">无选项（缺省）．</font></td>
      </tr>
      <tr>
        <td WIDTH="39%"><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">CRecordset::appendOnly</font></td>
        <td WIDTH="61%"><font SIZE="3"><p ALIGN="JUSTIFY">不允许修改和删除记录，但可以添加记录．</font></td>
      </tr>
      <tr>
        <td WIDTH="39%"><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">CRecordset::readOnly</font></td>
        <td WIDTH="61%"><font SIZE="3"><p ALIGN="JUSTIFY">记录集是只读的．</font></td>
      </tr>
      <tr>
        <td WIDTH="39%"><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">CRecordset::skipDeletedRecords</font></td>
        <td WIDTH="61%"><font SIZE="3"><p ALIGN="JUSTIFY">有些数据库（如</font><font FACE="Times New Roman" SIZE="3">FoxPro</font><font SIZE="3">）在删除记录时并不真删除，而是做个删除标记，在滚动时将跳过这些被删除的记录．</font></td>
      </tr>
    </table>
    <p><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <blockquote>
      <blockquote>
        <p ALIGN="JUSTIFY">virtual CString GetDefaultSQL( );<br>
        Open</font><font SIZE="3">函数在必要时会调用该函数返回缺省的</font><font FACE="Times New Roman" SIZE="3">SQL</font><font SIZE="3">语句或表名以查询数据源中的记录．一般需要在</font><font FACE="Times New Roman" SIZE="3">CRecordset</font><font SIZE="3">派生类中覆盖该函数并在新版的函数中提供</font><font FACE="Times New Roman" SIZE="3">SQL</font><font SIZE="3">语句或表名．下面是一些返回字符串的例子．<br>
        “</font><font FACE="Times New Roman" SIZE="3">Section</font><font SIZE="3">” </font><font FACE="Times New Roman" SIZE="3">//</font><font SIZE="3">选择</font><font FACE="Times New Roman" SIZE="3">Section</font><font SIZE="3">表中的所有记录到记录集中<br>
        “</font><font FACE="Times New Roman" SIZE="3">Section, Course</font><font SIZE="3">” </font><font FACE="Times New Roman" SIZE="3">//</font><font SIZE="3">合并</font><font FACE="Times New Roman" SIZE="3">Section</font><font SIZE="3">表和</font><font FACE="Times New Roman" SIZE="3">Course</font><font SIZE="3">表的各列到记录集中</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">//</font><font SIZE="3">对</font><font FACE="Times New Roman" SIZE="3">Section</font><font SIZE="3">表中的所有记录按</font><font FACE="Times New Roman" SIZE="3">CourseID</font><font SIZE="3">的升序进行排序，然后建立记录集</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY"></font><font SIZE="3">“</font><font FACE="Times New Roman" SIZE="3">SELECT * FROM Section ORDER BY CourseID ASC</font><font SIZE="3">”</font><font FACE="Times New Roman" SIZE="3"></p>
      </blockquote>
    </blockquote>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">上面的例子说明，通过合理地安排</font><font FACE="Times New Roman" SIZE="3">SQL</font><font SIZE="3">语句和表名，</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">函数可以十分灵活地查询数据源中的记录．用户可以合并多个表的字段，也可以只选择记录中的某些字段，还可以对记录进行过滤和排序．</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">上一小节说过，在建立记录集时，</font><font FACE="Times New Roman" SIZE="3">CRecordset</font><font SIZE="3">会构造一个</font><font FACE="Times New Roman" SIZE="3">SELECT</font><font SIZE="3">语句来查询数据源．如果在调用</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">时只提供了表名，那么</font><font FACE="Times New Roman" SIZE="3">SELECT</font><font SIZE="3">语句还缺少选择列参数</font><font FACE="Times New Roman" SIZE="3">rfx-field-list</font><font SIZE="3">（参见</font><font FACE="Times New Roman" SIZE="3">10.5.3</font><font SIZE="3">）．框架规定，如果只提供了表名，则选择列的信息从</font><font FACE="Times New Roman" SIZE="3">DoFieldExchange</font><font SIZE="3">中的</font><font FACE="Times New Roman" SIZE="3">RFX</font><font SIZE="3">语句里提取．例如，如果在调用</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">时只提供了＂</font><font FACE="Times New Roman" SIZE="3">Section</font><font SIZE="3">＂表名，那么将会构造如下一个</font><font FACE="Times New Roman" SIZE="3">SELECT</font><font SIZE="3">语句：</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">SELECT CourseID,SectionNo,InstructorID,RoomNo, Schedule,Capacity FROM 
    Section</p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">建立记录集后，用户可以随时调用</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">成员函数来重新查询和建立记录集．</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">有两个重要用途：</p>
    <ul>
      <li><p ALIGN="JUSTIFY">使记录集能反映用户对数据源的改变（参见<font FACE="Times New Roman" SIZE="3">10.5.1</font>）．</p>
      </li>
      <li><p ALIGN="JUSTIFY">按照新的过滤或排序方法查询记录并重新建立记录集．</font></p>
      </li>
    </ul>
    <p><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">在调用</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">之前，可调用</font><font FACE="Times New Roman" SIZE="3">CanRestart</font><font SIZE="3">来判断记录集是否支持</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">操作．要记住</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">只能在成功调用</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">后调用，所以程序应调用</font><font FACE="Times New Roman" SIZE="3">IsOpen</font><font SIZE="3">来判断记录集是否已建立．函数的声明为</p>
    <blockquote>
      <blockquote>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">virtual BOOL Requery( 
        );throw( CDBException, CMemoryException );<br>
        </font><font SIZE="3">返回</font><font FACE="Times New Roman" SIZE="3">TRUE</font><font SIZE="3">表明记录集建立成功，否则返回</font><font FACE="Times New Roman" SIZE="3">FALSE</font><font SIZE="3">．若函数内部出错则产生异常．</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">BOOL CanRestart( ) const; 
        //</font><font SIZE="3">若支持</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">则返回</font><font FACE="Times New Roman" SIZE="3">TRUE</p>
        <p ALIGN="JUSTIFY">BOOL IsOpen( ) const; //</font><font SIZE="3">若记录集已建立则返回</font><font FACE="Times New Roman" SIZE="3">TRUE</p>
      </blockquote>
    </blockquote>
    <p ALIGN="JUSTIFY">CRecordset</font><font SIZE="3">类有两个公共数据成员</font><font FACE="Times New Roman" SIZE="3">m_strFilter</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">m_strSort</font><font SIZE="3">用来设置对记录的过滤和排序．在调用</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">或</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">前，如果在这两个数据成员中指定了过滤或排序，那么</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">Requery</font><font SIZE="3">将按这两个数据成员指定的过滤和排序来查询数据源．</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">成员</font><font FACE="Times New Roman" SIZE="3">m_strFilter</font><font SIZE="3">用于指定过滤器．</font><font FACE="Times New Roman" SIZE="3">m_strFilter</font><font SIZE="3">实际上包含了</font><font FACE="Times New Roman" SIZE="3">SQL</font><font SIZE="3">的</font><font FACE="Times New Roman" SIZE="3">WHERE</font><font SIZE="3">子句的内容，但它不含</font><font FACE="Times New Roman" SIZE="3">WHERE</font><font SIZE="3">关键字．使用</font><font FACE="Times New Roman" SIZE="3">m_strFilter</font><font SIZE="3">的一个例子为：</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">m_pSet-&gt;m_strFilter=</font><font SIZE="3">“</font><font FACE="Times New Roman" SIZE="3">CourseID=</font><font SIZE="3">‘</font><font FACE="Times New Roman" SIZE="3">MATH101</font><font SIZE="3">’”</font><font FACE="Times New Roman" SIZE="3">; //</font><font SIZE="3">只选择</font><font FACE="Times New Roman" SIZE="3">CourseID</font><font SIZE="3">为</font><font FACE="Times New Roman" SIZE="3">MATH101</font><font SIZE="3">的记录</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">if(m_pSet-&gt;Open(CRecordset::snapshot, </font><font SIZE="3">“</font><font FACE="Times New Roman" SIZE="3">Section</font><font SIZE="3">”</font><font FACE="Times New Roman" SIZE="3">))</p>
    <p ALIGN="JUSTIFY"><b>. . . . . .</b></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">成员</font><font FACE="Times New Roman" SIZE="3">m_strSort</font><font SIZE="3">用于指定排序．</font><font FACE="Times New Roman" SIZE="3">m_strSort</font><font SIZE="3">实际上包含了</font><font FACE="Times New Roman" SIZE="3">ORDER BY</font><font SIZE="3">子句的内容，但它不含</font><font FACE="Times New Roman" SIZE="3">ORDER BY</font><font SIZE="3">关键字．</font><font FACE="Times New Roman" SIZE="3">m_strSort</font><font SIZE="3">的一个例子为</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">m_pSet-&gt;m_strSort=</font><font SIZE="3">“</font><font FACE="Times New Roman" SIZE="3">CourseID DESC</font><font SIZE="3">”</font><font FACE="Times New Roman" SIZE="3">; //</font><font SIZE="3">按</font><font FACE="Times New Roman" SIZE="3">CourseID</font><font SIZE="3">的降序排列记录</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">m_pSet-&gt;Open();</p>
    <p ALIGN="JUSTIFY"><b>. . . . . .</b></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">事实上，</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">函数在构造</font><font FACE="Times New Roman" SIZE="3">SELECT</font><font SIZE="3">语句时，会把</font><font FACE="Times New Roman" SIZE="3">m_strFilter</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">m_strSort</font><font SIZE="3">的内容放入</font><font FACE="Times New Roman" SIZE="3">SELECT</font><font SIZE="3">语句的</font><font FACE="Times New Roman" SIZE="3">WHERE</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">ORDER BY</font><font SIZE="3">子句中．如果在</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">的</font><font FACE="Times New Roman" SIZE="3">lpszSQL</font><font SIZE="3">参数中已包括了</font><font FACE="Times New Roman" SIZE="3">WHERE</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">ORDER BY</font><font SIZE="3">子句，那么</font><font FACE="Times New Roman" SIZE="3">m_strFilter</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">m_strSort</font><font SIZE="3">必需为空．</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">调用无参数成员函数</font><font FACE="Times New Roman" SIZE="3">Close</font><font SIZE="3">可以关闭记录集．在调用了</font><font FACE="Times New Roman" SIZE="3">Close</font><font SIZE="3">函数后，程序可以再次调用</font><font FACE="Times New Roman" SIZE="3">Open</font><font SIZE="3">建立新的记录集．</font><font FACE="Times New Roman" SIZE="3">CRecordset</font><font SIZE="3">的析构函数会调用</font><font FACE="Times New Roman" SIZE="3">Close</font><font SIZE="3">函数，所以当删除</font><font FACE="Times New Roman" SIZE="3">CRecordset</font><font SIZE="3">对象时记录集也随之关闭。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p></font><b><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font></b><font color="#3973DE" FACE="Times New Roman" SIZE="3">10.5.5 
    </font><font SIZE="3" color="#3973DE">滚动记录</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">CRecordset</font><font SIZE="3">提供了几个成员函数用来在记录集中滚动，如下所示．当用这些函数滚动到一个新记录时，框架会自动地把新记录的内容拷贝到域数据成员中．</p>
    <blockquote>
      <blockquote>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">void MoveNext( ); //</font><font SIZE="3">前进一个记录</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">void MovePrev( ); //</font><font SIZE="3">后退一个记录</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">void MoveFirst( ); //</font><font SIZE="3">滚动到记录集中的第一个记录</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">void MoveLast( ); //</font><font SIZE="3">滚动到记录集中的最后一个记录</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">void SetAbsolutePosition( 
        long nRows ); <br>
        </font><font SIZE="3">该函数用于滚动到由参数</font><font FACE="Times New Roman" SIZE="3">nRows</font><font SIZE="3">指定的绝对位置处．若</font><font FACE="Times New Roman" SIZE="3">nRows</font><font SIZE="3">为负数，则从后往前滚动．例如，当</font><font FACE="Times New Roman" SIZE="3">nRows</font><font SIZE="3">为</font><font FACE="Times New Roman" SIZE="3">-1</font><font SIZE="3">时，函数就滚动到记录集的末尾．注意，该函数不会跳过被删除的记录．</p>
        </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">virtual void Move( long 
        nRows, WORD wFetchType = SQL_FETCH_RELATIVE );<br>
        </font><font SIZE="3">该函数功能强大．通过将</font><font FACE="Times New Roman" SIZE="3">wFetchType</font><font SIZE="3">参数指定为</font><font FACE="Times New Roman" SIZE="3">SQL_FETCH_NEXT</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">SQL_FETCH_PRIOR</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">SQL_FETCH_FIRST</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">SQL_FETCH_LAST</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">SQL_FETCH_ABSOLUTE</font><font SIZE="3">，可以完成上面五个函数的功能．若</font><font FACE="Times New Roman" SIZE="3">wFetchType</font><font SIZE="3">为</font><font FACE="Times New Roman" SIZE="3">SQL_FETCH_RELATIVE</font><font SIZE="3">，那么将相对当前记录移动，若</font><font FACE="Times New Roman" SIZE="3">nRows</font><font SIZE="3">为正数，则向前移动，若</font><font FACE="Times New Roman" SIZE="3">nRows</font><font SIZE="3">为负数，则向后移动．</font></p>
      </blockquote>
    </blockquote>
    <p><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>