Data Access 数据访问

ODBC stands for Open Database Connectivity The Microsoft Open Database Connectivity (ODBC) interface is a C programming language interface that allows applications to access data from a variety of Database Management Systems (DBMS). a set

ODBC stands for Open Database Connectivity
The Microsoft Open Database Connectivity (ODBC) interface is a C programming language interface that allows applications to access data from a variety of Database Management Systems (DBMS).
a set of functions
a means of accessing several databases in different formats
 supports both 16-bit and 32-bit APIs,
several performance-boosting techniques- connection pooling
The set of API functions is complex, relatively few programmers write applications that directly call ODBC functions

DSN
A DSN is a set of values that an application needs to correctly connect to a database.

ODBC driver
machine the application runs on connects to a DLL called the ODBC Driver Manager, which in turn sends commands to (and retrieves data from) an ODBC driver specific to the particular database you want to use.

DAO
Data control.
This control lets you bind one or more controls on a form to a data source and offers buttons for navigating through the records of the database table you've connected to.

3-tier
3-tier (or n-tier) architectures, where one or more intermediate layers between the application and the database provide services such as data validation, business rules, workload balance, and security. Alas, if you want to embrace the n-tier philosophy, you should forget about the Data control.
 you can use one or more intermediate COM components, which consistently implement a more flexible 3-tier design.

RDO
combine the simplicity of DAO with the power of direct ODBC API programming.
bypasses the Jet Engine and the DAO DLL and works directly
resource-hungry Jet engine.

OLE DB
eventually replace ODBC as the primary means for connecting to databases.
counterpart to ODBC drivers are the OLE DB providers
OLE DB is based on COM, an architecture that has proven robust enough to move large quantities of data across the network.
any type of data source, not just relational and ISAM (indexed sequential access mode) databases
OLE DB is a comprehensive set of COM interfaces for accessing a diverse range of data in a variety of data stores.
OLE DB providers exist for accessing data in databases, file systems, message stores, directory services, workflow, and document stores.

ADO
ADO is the high-level interface to OLE DB. It fills more or less the same role that RDO does for the ODBC APIs.
Like ODBC APIs, OLE DB is a low-level interface that can't be easily (or at all) accessed from high-level languages such as Visual Basic.

ADO is probably its extensibility.
New features can be added to ADO in the form of special OLE DB providers, such as the MSDataShape provider
One ADO subsystem, named Remote Data Services, even lets you send a bunch of records to a client browser or activate COM components remotely over the Internet.

ADO Data Binding
binding is a technology that lets you place controls—such as TextBox, CheckBox, ListBox, and ComboBox controls—on a form and bind any or all of them to another control, called the Data control, which in turn is connected to a database.

ADO Data controls &  data consumers
 instead of Data controls, you should talk about one or more data consumers that are bound to a data source.
In Visual Basic 6, you can use many types of data consumers, such as an intrinsic or external control, a class, a COM component, a homemade ActiveX control (or UserControl), or the DataReport designer.
 You also have many data sources to choose from: the ADO Data control, a class, a COM component, a UserControl, or the DataEnvironment designer.

 In the Visual Basic 6 package, you'll also find some external ActiveX controls that support data binding, such as the ImageCombo, MonthView, DateTimePicker, MaskEdBox, RichTextBox, DataGrid, DataList, DataCombo, and Hierarchical FlexGrid controls.

data-aware ActiveX controls

DataEnvironment designer : form designer = ADO objects : forms and controls
When you use a form designer, you're actually defining at design time the forms and controls Visual Basic will create at run time. You make your choices in a visual manner, without worrying about what Visual Basic actually does when the program runs. Similarly, you can use the DataEnvironment designer to define the behavior of ADO Connections, Commands, and Recordset objects. You can set their properties at design time by pressing the F4 key to bring up the Properties window or by using their custom property pages, exactly as you would do with forms and controls.
In short, it's a design-time representation of the ADO objects that you would otherwise create at run time.

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