How To Use Updatable Joins in an MFC ODBC Application (135109)

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Background

A join links records from multiple tables matching one or more key fields in each table to create a "view." When a join is updated, the fields in the view and the corresponding fields in the tables are modified.

Generally, a join is performed with a SQL statement of this nature:

SELECT Table1.colA, Table2.colB
FROM Table1, Table2
WHERE Table1.primary_key=Table2.foreign_key

You can specify such SQL in your recordset's SELECT statement, but the resulting join will not be updatable for reasons to be discussed.

When are joins not updatable?

The Microsoft desktop ODBC drivers do not support updatable joins of the form just described in the following two cases:

• Using snapshots.
• Executing a SQL SELECT statement with multiple tables

Using snapshots forces joins non-updatable. MFC snapshots by default loads the cursor library (see appendix G of the ODBC 2.0 Programmer's Reference for more information about the cursor library) which doesn't support updatable joins. The alternative to using snapshots is to use dynasets which require you to explicitly inhibit the loading of the cursor library.

If you attempt to use snapshots with joins, you will first encounter the fact that MFC will mark the recordset read only. Even if you set m_bUpdatable and m_bAppendable members of CRecordset TRUE yourself, you will receive the following error:

In VC++ 1.5x: In VC++ 2.x: The Microsoft Desktop Drivers are unable to process SQLSetPos updates on a recordset opened using a join in its select statement. This means that a dynaset won't allow updatable joins unless you use a predefined query as mentioned above. If the SELECT statement contains a join, the driver will reject attempts to perform updates. This is a limitation of the drivers.

If you specify a join in the SQL SELECT statement of your recordset Open() call or in the GetDefaultSQL() call and are using dynasets, you will receive the following error:

In VC++ 1.5x: In VC++ 2.x:

How to perform a join that will be updatable

To perform updatable joins based on a predefined query, you must meet the criteria laid out in the summary section of this article. Additional information about using predefined queries in MFC ODBC applications is provided below.

When using a predefined query in MFC employing dynasets you can utilize two methods of opening the recordset:

• Use the "{CALL predefined_query}" syntax in the recordset open to explicitly execute a predefined query and use its result set in the recordset.
  
  For example:

       m_pSet->Open(CRecordset::dynaset, "{CALL QueryName}");

  Note: If you use the "CALL" syntax, you must force m_bUpdatable and m_bAppendable member variables of your recordset class to TRUE since they will be FALSE by default since predefined queries are assumed read-only. Do this in a member function of your CRecordset-derived class since these are protected members.
• Open the view that a predefined query creates just as if it were a regular table in the database.
  
  For example:

       m_pSet->Open(CRecordset::dynaset);

  where the GetDefaultSQL() member of your recordset derived class is: {return("QueryName");}
  
  Note: When using predefined queries, regardless of how you open the recordset, you refer to the tables specified in the query, not to the query itself, in your RFX functions.
  
  For example, if Table1 is a table referenced in predefined query QueryName, a representative RFX function would be:

              RFX_Text(pFX, "Table1.primary_key", m_member);