PRB: Update or Delete Record Gives Error: Query is too complex

ID: Q125651

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.5, 1.51, 1.52
- Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 4.0, 4.1, 5.0, 6.0

SYMPTOMS

When trying to update or delete a record of a CRecordset object, the following message occurs:

With 16-bit MFC database classes and 16-bit ODBC desktop drivers:


   DBMS: ACCESS, Version: 2.0
   Query is too complex.
   State:S1000[Microsoft][ODBC Microsoft Access 2.0 Driver]

With 32-bit MFC database classes and 32-bit ODBC desktop drivers:


   DBMS: ACCESS
   , Version: 2.0
   Query is too complex.
   State:S1000,Native:-3071,Origin:[Microsoft]
      [ODBC Microsoft Access 2.0 Driver]

The examples above show "Microsoft Access 2.0" but could be any of the Microsoft desktop drivers.

CAUSE

The error occurs when the cursor library is loaded and the recordset retrieved by the CRecordset is opened as a snapshot object that contains more than 40 bound columns.

By default, the cursor library is loaded when a CRecordset is opened. MFC snapshots perform positioned updates and deletes by using the ODBC SQL WHERE CURRENT OF syntax. The cursor library changes the WHERE CURRENT OF clause to a full WHERE clause using all the columns in the recordset.

For example, the MFC Database classes might create the following SQL statement when updating a record:


   Update table1
   SET column1=?
   WHERE CURRENT OF XXXXX

The cursor library knows which record the application is currently positioned at and converts the WHERE CURRENT OF clause into a WHERE clause that will update the current record only. For example, assume the recordset has three columns. The cursor library changes the SQL statement to:


   Update table1
   SET column1=?
   WHERE column1=<current value> AND column2=<current value> AND
     column3 =<current value>

Here <current value> represents the value of that column before the update is performed.

You can see that more than 40 columns in a CRecordset will cause more than 40 AND predicates in the WHERE clause. The helpfile for the ODBC desktop drivers (ODBCJET.HLP) states in the SQL limitations section that no more than 40 AND predicates are supported. Thus, the SQL statement is too complex.

NOTE: Long binary columns (those using RFX_LongBinary) are not bound and do not count against the 40-column limit. However, an MFC ODBC application that references the RFX_LongBinary function may encounter this error when the recordset is opened. This is because the cursor library precedes calls to SQLGetData with the execution of a SELECT statement with a WHERE clause as described above.

RESOLUTION

To work around this behavior, do one of the following:

Reduce the number of columns in your recordset. If you are using the CRecordset object that AppWizard/ClassWizard created for you, you can reduce the number of columns returned in the recordset by removing record field exchange(RFX) functions from the CRecordset's DoFieldExchange() function.

Use dynasets. See the online books for more information about the differences between snapshots and dynasets. To use dynasets with the 16-bit MFC Database classes, please see the following article in the Microsoft Knowledge Base:
Q124915 SAMPLE: Using Dynasets with the 16-Bit MFC Database Classes
Special code must be acquired to use dynasets with the 16-bit MFC Database classes. CRecordsets which use a dynaset use the SQLSetPos() ODBC 2.0 functionality to perform positioned updates and deletes. This means the MFC database classes do not need to construct an SQL statement to send to the ODBC driver. A call to the SQLSetPos() function of the driver is all that is necessary to perform the update or delete.

STATUS

This behavior is by design.

REFERENCES

Appendix G of the ODBC 2.0 Programmer's Reference contains information about the cursor library. The ODBC Programmer's Reference is available in the onine books for Visual C++ version 2.0 or greater.

MFC TechNote #45 contains additional information about MFC support for long binary data and the use of RFX_LongBinary.

Additional query words:

Keywords : kbDatabase kbMFC kbODBC kbVC kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC410 kbVC500 kbVC600
Version : 1.50 1.51 1.52 | 2.00 2.10 4.00
Platform : NT WINDOWS
Issue type :