Download objConn.Open()

Coding ADO.NET Objects:
Connection, Command, DataReader
A Simplified View of ADO.Net
Objects
Ado.Net
Data Provider
Data Consumer
Adapter
Dataset
WinForm
Connection
Command
WebForm
Reader
Connection with a Connection
Object
• A connection object represents a unique session
with a data source.
• Property:
– Connection string: string used to open a database.
• Data source, OLE DB provider, password, if any, security, etc.
• Methods:
– Use the Open/Close method to open/close a
connection.
Connection String
• Containing information about database,
OLE DB provider, password, if any,
security, etc.
• For Jet database:
– ="Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\salesDB.mdb"
Providers
• Microsoft.Jet.OLEDB.4.0
• MSDAORA – For Oracle
• MSDASQL
– Microsoft OLE DB Provider for ODBC
• SQLOLEDB – Microsoft SQL Server
Connection Object
• Example 1:
– dim strConn as string ="Provider=Microsoft.Jet.OLEDB.4.0;Data Source
= c:\salesDB.mdb"
– dim objConn as new OledbConnection(strConn)
– objConn.open()
• Example 2:
– dim strConn as string ="Provider=Microsoft.Jet.OLEDB.4.0;Data Source
= c:\salesDB.mdb"
– dim objConn as new OledbConnection()
– objConn.ConnectionString=strConn
– objConn.open()
• Object Browser: System.Data.OleDB/oledbConnection/New
• Basic Methods:
– Open, Close
– BeginTransaction
Command Object
• The command object allows us to execute a SQL statement.
• Properties:
–
–
–
–
CommandType: Text, stored procedure, tableDirect
CommandText: SQL statement, procedure name, table name
Connection
Other properties:
• Parameters
• Basic Methods:
– ExecuteReader: Creates a DataReader object that contains the results
of the query.
– ExecuteScalar: Executes the query, and returns the first column of the
first row in the result set returned by the query. Typically used to
execute a SQL aggregate function such as SUM, MAX, MIN, etc.
– ExecuteNonQuery: Change the data in a database without using a
DataSet by executing SQL’s UPDATE, INSERT, or DELETE
statements.
DataReader Object
• It is read-only and forward-only cursor.
• Basic Methods:
– Read: Reads the current record and advances the
pointer to the next record. It returns False if no more
record in the DataReader.
– Close: Closes the dataReader.
• Property:
– HasRows – Boolean, indicates whether this
System.Data.Common.DbDataReader contains one or
more rows.
ExecuteReader Example
dim strConn as string
="Provider=Microsoft.Jet.OLEDB.4.0;Data Source =
c:\salesDB.mdb"
dim objConn as new OledbConnection(strConn)
dim strSQL as string = "select * from customer;"
dim objComm as new OledbCommand(strSQL,objConn)
dim Results as string
objConn.open()
dim objDataReader as oledbDataReader
objDataReader=objComm.executeReader()
Note: No NEW in the Dim statement when define a
dataReader object.
Import NameSpace
• The Imports statement must appear before all
other declarations in a file and cannot appear
inside a class or module declaration.
– Imports System.Data.OleDb
– Public Class Form1
• Note: To import a nameSpace for all
forms/modules in a project:
– Use Project Property Page: Imports
Read Records in a DataReader
• dim Results as string
• do while objDataReader.Read()=true
Results+=objDataReader("cid") + “ “ + objDataReader("Cname") + vbCrLF
• loop
• Textbox1.text=Results
• Note: objDataReader.Item(0)
• Note: objDataReader.Item(“cid”)
• Note: The Read method will (1) read a record, (2) advance
pointer, (3) return true/false to indicate if more records exisit.
Add Items from a DataReader to a Listbox
Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source =
c:\salesDB.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select cname from customer;"
Dim objComm As New OleDbCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
LISTBOX1.Items.Add(objDataReader("cname"))
Loop
Note1: Listbox is not bound. 2. SelectedItem
Display Selected Customer Info in Textbox
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As
System.Object, ByVal e As System.EventArgs) Handles
ListBox1.SelectedIndexChanged
Dim strConn As String =
"Provider=Microsoft.Jet.OLEDB.4.0;Data Source =
c:\salesDB.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select * from customer where
cid='" & ListBox1.SelectedItem & "';"
Dim objComm As New OleDbCommand(strSQL,
objConn)
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
objDataReader.Read()
TextBox1.Text = objDataReader("cname")
TextBox2.Text = objDataReader("rating")
objDataReader.Close()
End Sub
Display Selected Customer Info in Textbox
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles ListBox1.SelectedIndexChanged
Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\salesDB.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select * from customer where cid = '" & ListBox1.SelectedItem & "'"
Dim objComm As New OleDbCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
objDataReader.Read()
TextBox1.Text = objDataReader("Cname")
TextBox2.Text = objDataReader("rating")
End Sub
Working with ODBC Data Source
Imports System.Data.Odbc
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load
Dim strConn As String = "DSN=MySalesDB"
Dim objConn As New OdbcConnection(strConn)
Dim strSQL As String = "select cname from customer;"
Dim objComm As New OdbcCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OdbcDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
ListBox1.Items.Add(objDataReader("cname"))
Loop
End Sub
End Class
Use SQL to Retrieve Summaries
Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source =
c:\salesDB.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select count(cid) as custCount from customer;"
Dim objComm As New OleDbCommand(strSQL, objConn)
Dim Results As String
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
If objDataReader.Read = True Then
MessageBox.Show(objDataReader("custCount").ToString)
End If
Note: Alias ----- select
count(cid) as custCount
Using Command’s ExecuteScalar to
Run SQL Aggregates
• Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data
Source = c:\salesDB.mdb"
• Dim objConn As New OledbConnection(strConn)
• Dim strSQL As String = "select count(cid) from
customer';"
• Dim objComm As New OledbCommand(strSQL,
objConn)
• objConn.Open()
• TextBox1.Text = objComm.ExecuteScalar
ExecuteScalar Example
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As
System.Object, ByVal e As System.EventArgs) Handles
ListBox1.SelectedIndexChanged
Dim strConn As String =
"Provider=Microsoft.Jet.OLEDB.4.0;Data Source =
c:\EmpDataBase.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String
Select Case ListBox1.SelectedIndex
Case 0
strSQL = "select count(eid) from emp';"
Case 1
strSQL = "select Max(Salary) from emp';"
Case 2
strSQL = "select Min(Salary) from emp';"
End Select
Dim objComm As New OledbCommand(strSQL, objConn)
objConn.Open()
TextBox1.Text = objComm.ExecuteScalar
End Sub
Testing for Null
IsDBNull Function
If objDataReader.Read = True Then
If IsDBNull(objDataReader("custCount")) Then
TextBox1.Text = 0
Else
TextBox1.Text = objDataReader("custCount").ToString
End If
End If
Note: The keyword “Nothing” tests if an object is null.
Null Value Example
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles ListBox1.SelectedIndexChanged
Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\salesDB.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select * from customer where cid = '" & ListBox1.SelectedItem & "'"
Dim objComm As New OleDbCommand(strSQL, objConn)
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
objDataReader.Read()
TextBox1.Text = objDataReader("Cname")
TextBox2.Text = objDataReader("rating")
End Sub
‘ Statement won’t work if Rating is null in database
Setting a Field to Null with a Update
Statement
Dim strSQLUpd As String = "Update Customer Set Rating = null
Where cid = '" & ListBox1.SelectedItem & "'"
Error Handling
Try
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
Results += objDataReader("cid") + " " + objDataReader("Cname") + vbCrLf
Loop
TextBox1.Text = Results
objConn.Close()
Catch except As System.Exception
MessageBox.Show(except.Message)
End Try
Possible Database Errors
•
•
•
•
SQL syntax errors
Database connection not open
Null
Violate database constraints
•
•
•
•
Referential integrity
Field data type and length
Primary key constraint
Invalid data – database may have validation rules.
Use Command Object’s ExecuteNonQuery
to Insert A New Record
1. Create unbound text boxes to enter new record.
2. Add an Insert button with the following handler
Dim strSQLInsert As String
strSQLInsert = "Insert into Customer values ('"
strSQLInsert = strSQLInsert & TextBox1.Text & "','" & TextBox2.Text
& "','"
strSQLInsert = strSQLInsert & TextBox3.Text & "','" & TextBox4.Text
& "')"
Dim objCommInsert As New OleDbCommand(strSQLInsert, objConn)
objCommInsert.ExecuteNonQuery()
ExecuteNonQuery Example
dim strSQLUpd as string="Update
customer set rating = ‘A’”
strSQLUpd=strSQLUpd & " where
cname='" &
CnameList.selectedItem.text & "'“
Dim objCommUpd As New OleDbCommand(strSQLUpd, objConn)
objCommUpd.ExecuteNonQuery()
Note: How many records affected by this command?
ExecuteNonQuery method returns an integer indicating the number of
record affected by the update command.
Messagebox.Show(objCommUpd.ExecuteNonQuery())
Demo
• Create a project that do the following tasks:
– Use a DataReader to retrieve customer IDs and
populate a listbox.
– Select a new rating from radio buttons for the
selected customer.
– Update customer’s rating using the
ExecuteNonQuery method of a Command
object.
Declare OleDB objects and create listbox
Imports System.Data.OleDb
Public Class Form3
Inherits System.Windows.Forms.Form
Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source =
c:\salesDB.mdb"
Dim objConn As New OleDbConnection(strConn)
Dim strSQL As String = "select cid from customer;"
Dim objComm As New OleDbCommand(strSQL, objConn)
Private Sub Form3_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles MyBase.Load
objConn.Open()
Dim objDataReader As OleDbDataReader
objDataReader = objComm.ExecuteReader()
Do While objDataReader.Read() = True
ListBox1.Items.Add(objDataReader("cid"))
Loop
objConn.Close()
End Sub
Update customer rating
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles Button1.Click
objConn.Open()
Dim newRating As String
If RadioButton1.Checked = True Then
newRating = "A"
ElseIf RadioButton2.Checked Then
newRating = "B"
Else
newRating = "C"
End If
Dim strSQLUpd As String = "Update customer set rating = '" & newRating & "'"
strSQLUpd = strSQLUpd & " where cid='" & ListBox1.SelectedItem & "'"
Dim objCommUpd As New OleDbCommand(strSQLUpd, objConn)
objCommUpd.ExecuteNonQuery()
objConn.Close()
End Sub