Recordset: Filtering Records (ODBC)

项目
09/15/2006

This article applies to the MFC ODBC classes. For DAO recordsets, see the article DAO Recordset.

This article explains how to filter a recordset so that it selects only a particular subset of the available records. For example, you might want to select only the class sections for a particular course, such as MATH101. A filter is a search condition defined by the contents of an SQL WHERE clause. When the framework appends it to the recordset’s SQL statement, the WHERE clause constrains the selection.

You must establish a recordset object’s filter after you construct the object but before you call its Open member function (or before you call the Requery member function for an existing recordset object whose Open member function has been called previously).

To specify a filter for a recordset object

Construct a new recordset object (or prepare to call Requery for an existing object).
Set the value of the object’s data member.

The filter is a null-terminated string. It contains the contents of the SQL WHERE clause but not the keyword WHERE. For example, use
```
m_pSet->m_strFilter = "CourseID = 'MATH101'";
```
not
```
m_pSet->m_strFilter = "WHERE CourseID = 'MATH101'";
```
****Note ****The literal string “MATH101” is shown with single quotation marks above. In the ODBC SQL specification, single quotes are used to denote a character string literal. Check your ODBC driver documentation for the quoting requirements of your DBMS in this situation. This syntax is also discussed further near the end of this article.
Set any other options you need, such as sort order, locking mode, or parameters. Specifying a parameter is especially useful. For information on parameterizing your filter, see the article Recordset: Parameterizing a Recordset (ODBC).
Call Open for the new object (or Requery for a previously opened object).

Tip Using parameters in your filter is potentially the most efficient method for retrieving records.

Tip Recordset filters are useful for joining tables and for using parameters based on information obtained or calculated at run time.

The recordset selects only those records that meet the search condition you specified. For example, to specify the course filter described above (assuming a variable strCourseID currently set, for instance, to “MATH101”), do the following:

// Using the recordset pointed to by m_pSet

// Set the filter
m_pSet->m_strFilter = "CourseID = " + strCourseID;

// Run the query with the filter in place
if ( m_pSet->Open( CRecordset::snapshot, NULL, CRecordset::readOnly ) )

// Use the recordset

The recordset contains records for all class sections for MATH101.

Notice how the filter string was set in the example above, using a string variable. This is the typical usage. But suppose you wanted to specify the literal value 100 for the course ID. The following code shows how to set the filter string correctly with a literal value:

m_strFilter = "StudentID = '100'";   // correct

Note the use of single quote characters; if you set the filter string directly, the filter string is not:

m_strFilter = "StudentID = 100";   // incorrect for some drivers

The quoting shown above conforms to the ODBC specification, but some DBMSs may require other quote characters. For more information, see the article SQL: Customizing Your Recordset’s SQL Statement (ODBC).

****Note ****If you choose to override the recordset’s default SQL string by passing your own SQL string to Open, you should not set a filter if your custom string has a WHERE clause. For more information about overriding the default SQL, see the article SQL: Customizing Your Recordset’s SQL Statement (ODBC).

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Recordset: Filtering Records (ODBC)

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