VB NET + Substring + Mysql + Check Rows - mysql

Question

I have a simple form with a button and 2 textbox.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
MysqlConn = New MySqlConnection(ConfigurationManager.ConnectionStrings("db.My.MySettings.dbConnectionString").ToString)
Try
'MysqlConn.Dispose()
'MysqlConn.Close()
MysqlConn.Open()
Dim str As String
str = "SELECT substring_index(substring(path,1,locate(substring_index(path,'\\',-1),path)-2),'\\',-1)as PATH FROM foto where id_product = '" & TextBox2.Text & "'"
Dim dbCommand As New MySqlCommand(str, MysqlConn)
Dim dbReader = dbCommand.ExecuteReader
While dbReader.Read()
If IsDBNull(dbReader(0)) OrElse String.IsNullOrEmpty(dbReader.GetString(0)) Then
TextBox1.Text = "0"
Else
TextBox1.Text = dbReader.Item("PATH")
End If
End While
Catch ex As Exception
MessageBox.Show(ex.Message)
Finally
MysqlConn.Dispose()
End Try
End Sub
when i put in textbox2 "1" i have my desired value.
But when i put "2", this id_product dont exist, i dont have "0".
It's blank and Textbox1 has my previous value.
What i do wrong ??

Answer 1

If the query doesn't return any record, your code doesn't enter the while loop (dbReader.Read return false) and thus you don't set the textbox to "0" but you should also start to use parameterized queries. It is very important to avoid possible parsing errors and mainly to avoid Sql Injection attacks
So, you could test if your query has produced any record verifying the property HasRows of the DataReader
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Using MysqlConn = New MySqlConnection(ConfigurationManager.ConnectionStrings("db.My.MySettings.dbConnectionString").ToString)
Try
MysqlConn.Open()
Dim str As String
str = "SELECT substring_index
(substring(path,1,
locate(substring_index(path,'\\',-1),path)-2),
'\\',-1) as PATH
FROM foto where id_product = #pid"
Dim dbCommand As New MySqlCommand(str, MysqlConn)
dbCommand.Parameters.Add("#pid", MySqlDbType.VarChar).Value = textBox2.Text
Using dbReader = dbCommand.ExecuteReader
if dbReader.HasRows Then
While dbReader.Read()
If IsDBNull(dbReader(0)) OrElse String.IsNullOrEmpty(dbReader.GetString(0)) Then
TextBox1.Text = "0"
Else
TextBox1.Text = dbReader.Item("PATH")
End If
End While
else
TextBox1.Text = "0"
End If
End Using
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Using
End Sub