ADO: Difference between revisions

Revision as of 11:45, 12 January 2014

This page is still a work in progress.

Most of the following text was taken from Wikipedia, connectionstrings.com:

Microsoft's ActiveX Data Objects ( ADO) is a set of Component Object Model (COM) objects for accessing data sources. As part of MDAC, it provides a middleware layer between programming languages and OLE DB (a means of accessing data stores, whether they be databases or otherwise, in a uniform manner). ADO allows a software developer to write programs that access data without knowing how the database is implemented; developers must be aware of the database for connection only. No knowledge of SQL is required to access a database when using ADO, although one can use ADO to execute SQL commands directly. The disadvantage of the latter is that it introduces a dependency upon the type of database used.

ADO is positioned as a successor to Microsoft's earlier object layers for accessing data sources, including Remote Data Objects (RDO) and Data Access Objects (DAO). ADO was introduced by Microsoft in October 1996.

NOTE
This tutorial only decribes a fraction of what can be done with ADO.
There are many ways to retrieve, add and change data using ADO. This tutorial only describes:

How to read data
How to use the OLEDB provider, ODBC and other providers are not part of this tutorial
The command to retrieve data is specified with the Open method for the RecordSet object. The Command object is not part of this tutorial

Internals

ADO is made up of four collections and twelve objects.

ADO collections

Fields: This collection contains a set of Field objects. The Collection can be used in either a Recordset object or in a Record object. In a Recordset object, each of the Field objects that make up the Fields collection corresponds to a column in that Record set object. In a Record object, a Field can be an absolute or relative URL that points into a tree-structured namespace (used for semi-structured data providers like the Microsoft OLE DB Provider for Internet Publishing) or as a reference to the default Stream object associated with that Record object.

Properties: An object can have more than one Property object, which are contained in the object's Properties collection.

Parameters: A Command object can have several Parameter commands to change its predefined behaviour, and each of the Parameter objects are contained in the Command object's Parameters collection

Errors: All provider-created errors are passed to a collection of Error objects, while the Errors collection itself is contained in a Connection object. When an ADO operation creates an error, the collection is cleared and a new group of Error objects is created in the collection.

ADO objects

Connection

The connection object is ADO's connection to a data store via OLE DB. The connection object stores information about the session and provides methods of connecting to the data store. As some data stores have different methods of establishing a connection, some methods may not be supported in the connection object for particular OLE DB providers. A connection object connects to the data store using its 'Open' method with a connection string which specifies the connection as a list of key value pairs (for example: "Provider='SQLOLEDB';Data Source='TheSqlServer'; Initial Catalog='Northwind';Integrated Security='SSPI';"). The start of which must identify the type of data store connection that the connection object requires:

an OLE DB provider (for example SQLOLEDB), using the syntax "provider=";
a file name, using the syntax "file name=";
a remote provider and server (see RDS), using the syntax "Remote provider=" and "Remote server="; or
an absolute URL, using the syntax "URL="

Command: After the connection object establishes a session to the data source, instructions are sent to the data provider via the command object. The command object can send SQL queries directly to the provider through the use of the CommandText property, send a parameterised query or stored procedure through the use of a Parameter object or Parameters collection or run a query and return the results to a dataset object via the Execute method. There are several other methods that can be used in the Command object relating to other objects, such as the Stream, RecordSet or Connection objects.

Recordset: A recordset is a group of records, and can either come from a base table or as the result of a query to the table. The RecordSet object contains a Fields collection and a Properties collection. The Fields collection is a set of Field objects, which are the corresponding columns in the table. The Properties collection is a set of Property objects, which defines a particular functionality of an OLE DB provider. The RecordSet has numerous methods and properties for examining the data that exists within it. Records can be updated in the recordset by changing the values in the record and then calling on the Update or UpdateBatch method.

Immediate: The recordset is locked using the adLockOptimistic or adLockPessimistic lock. The data are updated at the data source after the record is changed and the Update method is called.

Batch: The recordset is locked using adLockBatchOptimistic and each time Update is called the data are updated in a temporary buffer. Finally, when UpdateBatch is called the data are completely updated back at the data source. This has the advantage of it all being done in memory, and if a problem occurs then UpdateCancel is called and the updates are not sent to the data source

Transaction: If the OLE DB provider allows it, transactions can be used. To start the transaction, the programmer invokes the BeginTrans method and does the required updates. When they are all done, the programmer invokes the CommitTrans method. RollbackTrans can be invoked to cancel any changes made inside the transaction and rollback the database to the state before the transaction began

Record: This object represents one record in the database and contains a fields collection. A RecordSet consists of a collection of Record objects.

Stream: A stream, mainly used in a RecordSet object, is a means of reading and writing a stream of bytes. It is mostly used to save a recordset in an XML format, to send commands to an OLE DB provider as an alternative to the CommandText object and to contain the contents of a binary or text file.

Parameter: A parameter is a means of altering the behaviour of a common piece of functionality, for instance a stored procedure might have different parameters passed to it depending on what needs to be done; these are called parameterised commands.

Field: Each Record object contains many fields, and a RecordSet object has a corresponding Field object also. The RecordSet object's Field object corresponds to a column in the database table that it references.

Property: This object is specific to the OLE DB provider and defines an ability that the provider has implemented. A property object can be either a built-in property — it is a well-defined property implemented by ADO already and thus cannot be altered — or can be a dynamic property — defined by the underlying data provider and can be changed

Error: When an OLE DB provider error occurs during the use of ADO, an Error object will be created in the Errors collection. Other errors do not go into an Error object, however. For instance, any errors that occur when manipulating data in a RecordSet or Field object are stored in a Status property.

Usage

Some basic steps are required in order to be able to access and manipulate data using ADO:

Create a connection object to connect to the database
Create a recordset object in order to receive data in
Open the connection
Populate the recordset
Do all the desired searching/processing on the fetched data
Commit the changes you made to the data (if any) by using Update or UpdateBatch methods
Close the recordset
Close the connection

Create a connection object

In AutoIt this is simply done by:

Global $oConnection = ObjCreate("ADODB.Connection")

Create a recordset object

In AutoIt this is simply done by:

Global $oRecordSet = ObjCreate("ADODB.Recordset")

Open the connection

To open the connection you have to build a connection string or directly set the properties of the connection.

When your application connects to a database or a data file you let ADO utilize a provider to do the job for you. The connection string contains the information that the provider need to know to be able to establish a connection to the database or the data file. Because there are different providers and each providers have multiple ways to make a connection there are many different ways to write a connection string. It's like the address when sending a regular mail. Depending on the origin and destination and who is going to make the transport you need to write down the address in different ways.

Connection string

The connection string is really just a string inside the application you pass on to the connection object. The property is named ConnectionString or is passed through the Open function of the connection object.

As a string, there are no built in checks or constraints on how to format the connection string. A few rules to follow taken from www.connectionstrings.com

All blank characters, except those placed within a value or within quotation marks, are ignored
Blank characters will though affect connection pooling mechanism, pooled connections must have the EXACT same connection string
If a semicolon (;) is part of a value it must be delimited by quotation marks (")
Use a single-quote (') if the value begins with a double-quote (")
Conversely, use the double quote (") if the value begins with a single quote (')
No escape sequences are supported
The value type is NOT relevant
Names are case iNsEnSiTiVe
If a KEYWORD=VALUE pair occurs more than once in the connection string, the value associated with the LAST occurrence is used. But!... if the PROVIDER keyword occurs multiple times in the string, the FIRST occurrence is used.
If a keyword contains an equal sign (=), it must be preceded by an additional equal sign to indicate that it is part of the keyword.

A connection string consists of a list of argument/value pairs (that is, parameters), separated by semi-colons. For example: "arg1=val1; arg2=val2; ... argN=valN;" All the parameters must be recognized by either ADO or the specified provider. ADO recognizes the following five arguments in a connection string.

Argument	Description
Provider	Specifies the name of a provider to use for the connection.
File Name	Specifies the name of a provider-specific file (for example, a persisted data source object) containing preset connection information.
URL	Specifies the connection string as an absolute URL identifying a resource, such as a file or directory.
Remote Provider	Specifies the name of a provider to use when opening a client-side connection (Remote Data Service only).
Remote Server	Specifies the path name of the server to use when opening a client-side connection (Remote Data Service only).

Other arguments are passed to the provider named in the Provider argument, without any processing by ADO.

Depending on the data source you want to connect to the connection string varies.
The following pages document connection strings for OLEDB providers only.

IBM DB2
Microsoft Access
Microsoft Excel
Microsoft Project
Microsoft SQL Server
MySQL
Oracle Database Server
SQLite
Text files

Connection properties

To be added.

Populate the recordset

The ADO Recordset object is used to contain the set of data extracted from a database and is composed of records (rows) and fields (columns).

The Recordset object is the heart of ADO. Via this object, we can select desired data and change the data. Equally important is the ability to move around inside the database. However, the functionality of the provider may impose limitations.

RecordSet.Open(Source, Connection, CursorType, LockType, Options)

The Open method is called on a Recordset object to open a cursor which gives you access to the records contained in the base table, the results from a query (used in the examples of this tutorial), or a previously saved Recordset.
There are five optional parameters.

Source

This optional parameter is a variant that can be any one of the following data sources:

SQL query string (used in the examples of this tutorial)
table name
Others: More details can be found under External Links - ADO

Connection: This optional parameter is a valid Connection object (used in the examples of this tutorial) or a connection string that defines the connection.

CursorType: This optional parameter specifies the type of cursor to use when you open a Recordset object.; Possible values for the CursorTypeEnum are:

Constant	Value	Description
adOpenDynamic	2	A dynamic cursor with both forward and backward scrolling where additions, deletions, insertions, and updates made by other users are visible
adOpenForwardOnly	0	(Default) A forward scrolling only, static cursor where changes made by other users are not visible
adOpenKeyset	1	A keyset cursor allows you to see dynamic changes to a specific group of records but you cannot see new records added by other users
adOpenStatic	3	A static cursor allowing forward and backward scrolling of a fixed, unchangeable set of records
adOpenUnspecified	-1	Cursor type not specified

LockType: This optional parameter indicates the type of lock in effect on a Recordset.; Possible values for the LockTypeEnum are:

Constant	Value	Description
adLockBatchOptimistic	4	Multiple users can modify the data and the changes are cached until BatchUpdate is called
adLockOptimistic	3	Multiple users can modify the data which is not locked until Update is called
adLockPessimistic	2	The provider locks each record before and after you edit, and prevents other users from modifying the data
adLockReadOnly	1	(Default) Read-only data
adLockUnspecified	-1	Lock type unknown

Options: This optional parameter tells the provider how to evaluate the Source parameter when it contains something other than a Command object. The appropriate use of this option can speed up performance since ADO will not have to determine the type of the data source.; Possible values for the CommandTypeEnum are:

Constant	Value	Description
adCmdFile	256	Evaluate as a previously persisted file
adCmdStoredProc	4	Evaluate as a stored procedure
adCmdTable	2	Have the provider generate an SQL query and return all rows from the specified table
adCmdTableDirect	512	Return all rows from the specified table
adCmdText	1	Evaluates as a textual definition of a command or stored procedure call
adCmdUnknown	8	The type of the CommandText parameter is unknown. ADO will make several attempts to interpret the CommandText: adCmdText then adCmdStoredProc then adCmdTable
adCmdUnspecified	-1	(Default) does not specify how to evaluate

Search/process the fetched data

To be added.

Commit the changes

Not covered by this tutorial (at the moment).

Close the recordset

Close the recordset and assign a new value to the object to release it:

$oRecordSet.Close()
$oRecordSet = 0

Close the connection

Close the connection and assign a new value to the object to release it:

$oConnection.Close()
$oConnection = 0

Examples

IBM DB2
Microsoft Access
Microsoft Excel
Microsoft Project
Microsoft SQL Server
MySQL
Oracle Database Server
SQLite
Text files

Tools

Here are some (AutoIt) tools to make life easier.

External links

ADO

Connection Strings

@@ Line 231: / Line 231: @@
 === Commit the changes===
-To be added.
+Not covered by this tutorial (at the moment).
 === Close the recordset===