Data Access Objects
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Data Access Objects is a general programming interface for database access on Microsoft Windows systems and should not be confused with the Data Access Object design pattern used in object-oriented software design.
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[edit] History
DAO were originally called VT Objects. DAO 1.0 came up in November 1992. In version 3.5 it was able to bypass the Jet engine altogether and directly access ODBC data sources, including Microsoft SQL Server and other enterprise database systems. DAO 3.6 was the final version developed by Microsoft. Microsoft says that DAO will not be available in its future 64-bit operating systems.[1]
[edit] Design
DAO works by creating a "Workspace" object in which all database operations are performed. The workspace object exists as a session object that exists within a larger database engine object. There are two types of database engines: a Jet database engine object, and an ODBCDirect database engine.
[edit] Jet
The Jet database engine object consists of several objects:
- a workspace object containing
- a groups and users object
- a database object containing
- container objects which consist of
- containers of objects
- query definition (QueryDef) objects
- Recordset objects which are defined by a set of field objects
- relation objects which show the relationship between different fields in the database
- table definition (TableDef) objects which consists of fields and indexes of selected fields.
- container objects which consist of
- a series of error objects
[edit] ODBCDirect
The ODBCDirect database engine consists of a workspace object and an errors object. The main differences between this database engine and the Jet database engine are:
- the workspace object contains only a series of ODBC connection objects
- the database object consists of a series of recordset objects
The ODBC connection objects consist of QueryDef objects and recordset objects.
