DAO: Microsoft Data Access Objects Database Interface

Index: Requirements Problems Companies Platforms Languages Software Research

Data Access Objects, database interface of Win32.
Microsoft Foundation Classes wrappers around DAO database access API objects.
Integrates DAO into COM and OLE. Covers most of DAO but not all. Some differences and enhancements, but pretty much one for one mapping, not a new model like the MFC ODBC stuff.
Preferred over ODBC (That is, Microsoft documentation encourages you to use DAO instead of ODBC.)
Uses Microsoft jet database engine, sql server, other databases that can be accessed with ODBC driver.
Installable ISAM database formats including fox, dbase, excel, btrieve, paradox, and delimited text.
Custom DFX routines for MFC DAO classes.
- DAO record field exchange mechanism.
- Extend DAO to support your own data types.
- Like ODBC's Custom RFX Routines.
- MSDEV Class Wizard will automatically write these RFX and DFX functions for you.
- Simplifies retrieving and updating data when using the CDaoRecordSet class.
- Static binding mechanism, maps database fields to C++ object data members. Retrieves every bound field each time the current record is changed. Example code:
```
myset.AddNew(); myset.m_strCustID = _T("MSFT"); myset.m_strCustName
= _T("Microsloth"); myset.Update(); 
```
- Dynamic binding via CDaoRecordSet::GetFieldValue and CDaoRecordset::PutFieldValue is more efficient, and the hybrid use of static and dynamic binding is supported. Example code:
```
COleVariant varFieldValue; rs.AddNew(); varFieldValue = _T("MSFT");
rs.SetFieldValue(_T("Customer_ID"), varFieldValue); varFieldValue
= _T("Microsloth"); rs.SetFieldValue(_T("Customer_Name"),
varFieldValue); rs.Update(); 
```
- DFX is similar to ODBC's RFX. The principles are the same but there are numerous internal differences. The design of DFX functions was such that virtually all the code is shared by the individual DFX routines.
- Types supported:
  - DFX_Bool, DFX_Byte, DFX_Short, DFX_Long, DFX_Currency, DFX_Single, DFX_Double, DFX_Date, DFX_Text, DFX_Binary, DFX_LongBinary
- At the highest level, DFX only does a few things:
  - Constructs the sql SELECT clause and sql PARAMETERS clause if necessary.
  - Constructs the binding structure used by DAO's GetRows function.
  - Manages the data buffer used to detect dirty fields (if double-buffering is being used).
  - Manages the NULL and DIRTY status arrays and sets values if necessary on updates.
- The heart of the mechanism is CDaoRecordSet::DoFieldExchange, which dispatches the following operations to DFX functions:
  - AddToParameterList, AddToSelectList, BindField, BindParam, Fixup, AllocCache, StoreField, LoadField, FreeCache, SetFieldNull, MarkForAddNew, MarkForEdit, SetDirtyField
- ICDaoRecordset::GetRows can work in several ways:
  - Fetch multiple records and multiple fields of data at one time. Faster data access. Lots of complicated arguments you have to use. MFC does not take advantage of multiple record fetching mechanism.
  - Allow programmers to specify binding addresses for the retrieved data of each field for one record of data.
  - Call back into the caller for variable length columns in order to allow the caller to allocate memory. Minimizes the number of copies of data as well as allowing direct storage of data into members of a class (the CDaoRecordset derived class). This is the method MFC uses to bind to data members in CDaoRecordset derived classes.
- CDaoRecordset::DoFieldExchange -- you override this.
- DAO and Microsoft jet database engine are responsible for converting fields in database into representations of the data member of the C++ object. MFC is not responsible for that.
- MFC uses direct address binding (DAOBINDING_DIRECT). For variable length columns bindings (like DFX_Text) callback binding is used so that MFC can control the memory allocation and specify an address of the correct length. MFC can always tell DAO where to put the data, allowing binding directly to member variables. The BindField handler fills binding structure in with address of memory allocation callback function and the type of column binding.
- Optional double buffered data cache used to detect when fields have changed and are dirty. Map used to match up dynamically allocated copies of the data with the address of the bound field.
- Facility for making direct DAO calls to do things that MFC classes do not cover (i.e. efficiency, getting around bulk fetching, security features (users and groups), refreshing collections, recordset cloning, database replication, as well as a few late additions to DAO.)
DDL -- Data Definition Language: for adding tables via the classes, instead of calling DAO.
DDE -- Dialog Data Exchange for CDaoRecordView
- A description of bindings between user interface dialog elements and C++ object member variables.
- Knows how to exchange data in both directions between UI dialog components and C++ instance variables.
- CRecordView and CDaoRecordView classes support form-based data access. Form view object whose controls are mapped directly to the field data members of a recordset object, and indirectly to the corresponding columns in a query result or table on the data source. Based on a dialog template resource.
- Works with RFX for ODBC, as well as with DFX for DAO.
- CMyDialog::DoDataExchange(CDataExchange *pDX)
  - Calls the base class version.
  - Handles the data map:
    - DDX_Check(pDX, IDC_MY_CHECKBOX, m_bVar); // map checkbox to boolean var
    - DDX_Text(pDX, IDC_MY_TEXTBOX, m_strName); // map text field to string var
    - DDV_MaxChars(pDX, IDC_MY_TEXTBOX, 20); // data validation specification, limits string to 20 characters
  - The MSDEV ClassWizard will write all of the DDX and DDE (?) calls in the data map for you.
  - You can also define your own custom DDX and DDV routines. See technical note 26, MFC technical notes in books online. Write new data types, exchange procedures, and validation procedures, as well as passing arbitrary expressions to the validation procedures. Can't be edited by the ClassWizard and therefor should be moved outside of the specially formatted comments that the class wizard inserts around its automatically generated code.
  - You can even customize the class wizard to know about your own DDX and DDV routines, for custom data types that you are going to use a lot.
- External Data Sources
  - The ODBC interface is an external data source.
  - 32 bit ODBC drivers: sql, Microsoft access, foxpro, excel, dbase, paradox, text files.
  - ODBC driver pack available.