1. Tonga Institute of Higher Education
Data Access
Tonga Institute of Higher Education

2. Introduction Many programs work with large amounts of data
Databases are an good place to store data
Programs need a way to connect to, get data from and save data to databases
ActiveX Data Objects .Net (ADO .Net) provides way to access data from Object Linking and Embedding Database (OLE DB)-compliant data sources such as SQL Server and Oracle.

3. ADO .Net Components
.Net Data Providers – Objects designed for connecting to a database, executing commands and retrieving results
Connection
Command
DataReader
DataAdapter

4. Connection Object
Connection Object – Provides connectivity to a data source
SQLConnection object – Allows connection to a Microsoft SQL Server 7.0 or later database
Located in System.Data.SQLClient namespace
OLEDbConnection object – Allows connection to an OLE DB data source
Located in System.Data.OleDb namespace
Microsoft Access is an OLE DB compliant database

5. Connection Strings
The SQLConnection and OLEDbConnection objects both use connection strings to connect to a data source
Connection String – Provides information needed to connect to a database
You do not need to include everything in a connection string. Only include what is needed.
SQLConnection object
SQL Server
Initial Catalog=pubs;Data Source=(local);User ID=sa;password=;
OLEDbConnection object
Oracle
Provider=msdaora;Data Source=MyOracleDB;User ID=myUsername;Password=myPassword;
Microsoft Access
Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:\northwind.mdb
Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\\tihe1\student.mdb
You don’t always
need a User ID
and Password
Local DB
Network DB

6. Opening and Closing Connections
Before a Connection object can work with a database, the connection must be opened
Use the Open() method
After the Connection object is used, the connection must be closed
Use the Close() method

7. Command Object
Command Object – Enables the execution of queries and return of data from a data source
A Connection object must be specified to connect to a data source
A SQL statement must be specified for execution at the data source
There are different ways to execute SQL statements
ExecuteNonQuery() – Use this when data will not be returned
ExecuteReader() – Use this when data will be returned

8. ExecuteNonQuery Code
ExecuteNonQuery does
not return anything

9. Connections, Commands and ExecuteNonQuery
Demonstration
Connections, Commands and ExecuteNonQuery

10. DataReader Object
DataReader Object – Provides a stream of data from the data source
Useful members
FieldCount
Get<DataType>
GetByte
GetString
And more
HasItems
Item
Read

11. DataReader and ExecuteReader Code
Using a DataReader
to display data from
database
ExecuteReader returns
A DataReader
Keeps looping
until all rows
are read
Gets string from first index

12. Connections, Commands and DataReader.ExecuteReader Code
Demonstration
Connections, Commands and DataReader.ExecuteReader Code

13. DataSet Object DataSet –An in-memory database
Useful members (* covered later)
Clear
Tables*
DataSet

14. DataTable Object
A DataTable object is an in-memory database table. It contains:
Rows and columns of data
Primary key, foreign key, constraint and relation information about the data
Useful members (* covered later)
ChildRelations*
Clear
Columns*
Constraints*
DataSet
NewRow
ParentRelations*
PrimaryKey
TableName
DataTables

15. DataColumn Object A DataColumn object represents a table column
Useful members
AllowDBNull
AutoIncrement
AutoIncrementSeed
AutoIncrementStep
Caption
ColumnName
DataType
DefaultValue
MaxLength
Ordinal
Table
Unique
DataColumn

16. DataRow Object A DataRow Object represents a row of data in a table
Useful members
Item
Table
DataRow

17. DataSets, DataTables, DataColumns and DataRows
Demonstration
DataSets, DataTables, DataColumns and DataRows

18. DataTableCollection A DataTableCollection contains DataTables
Access the DataTableCollection of a DataSet by using the Tables property
Useful members
Add
Count
Clear
Item
Remove

19. Demonstration
DataTableCollection

20. DataColumnCollection
A DataColumnCollection contains DataColumns
Access the DataColumnCollection of a DataTable by using the Columns property
Useful members
Add
Count
Clear
Item
Remove

21. DataColumnCollection
Demonstration
DataColumnCollection

22. DataRowCollection A DataRowCollection contains DataRows
Access the DataRowCollection of a DataTable by using the Rows property
Useful members
Add
Count
Clear
Item
Remove

23. Demonstration
DataRowCollection

24. DataAdapter Object
DataAdapter – Used to retrieve data from a database and put it in a DataSet. Can also send changes from the DataSet to the database
Useful members
Fill
Update

25. DataAdapter Code
The Fill method opens the Connection if the connection is not already open. If Fill opened the connection, it will also close the connection.

26. Demonstration
DataAdapter Object

27. Programmatically Creating DataSet Objects
DataSets, DataTables, DataRows and DataColumns are all objects
We can create and customize all of these objects

28. Programmatically Creating DataSet Objects Code

29. Programmatically Creating DataSet Objects
Demonstration
Programmatically Creating DataSet Objects

30. DBNull, Nothing and “” There is a difference between having
No Data
And “”
In a database, use null when you have no data
In VB.Net
Use “” when you have an empty string
Use nothing when there is no data in a variable
Use dbnull when there is no data in a database field

31. Demonstration
DBNull, Nothing and “”

32. Storing Single Quotation Marks in a Database
We often store SQL queries in a string
Dim SQL as String = “Insert into Students (FirstName) values (‘Tevita’)”
SQL queries use single quotation marks
To store a single quote in a database, use the single quote twice
Dim SQL as String = “Insert into Students (FirstName) values (‘Ma’’afu’)”

33. Storing Single Quotation Marks in a Database
Demonstration
Storing Single Quotation Marks in a Database

34. Data Access Exceptions
Often, errors occur when databases are used.
The name of the file is changed
The location of a file is incorrect
The program does not have the proper username and password to access the database
Common Data Access Exceptions
DataException - Exception that is thrown when errors are generated using ADO.NET components.
ConstraintException - Exception that is thrown when attempting an action that violates a constraint.
NoNullAllowedException - Exception that is thrown when attempting to insert a null value into a column where AllowDBNull is set to false.
ReadOnlyException - Exception that is thrown when attempting to change the value of a read-only column.

35. Data Access Exceptions
Demonstration
Data Access Exceptions