Why combo box is not loaded from join sql query - vb.net

Question

Dim cmd As New SqlCommand("SELECT MainDepartmentsTable.ID, FeesTable.ID,FeesTable.Particular,FeesTable.Fee from MainDepartmentsTable inner join SubDepartmentsTable on MainDepartmentsTable.ID = SubDepartmentsTable.MainDeptID inner join FeesTable on SubDepartmentsTable.MainDeptID = FeesTable.SubDepartmentID WHERE FeesTable.ID = '" & TextDepartmentID.Text & "' ", con)
Dim DataAdopter = New SqlDataAdapter(cmd)
Dim dt As New DataTable
DataAdopter.Fill(dt)
Load_Deparments()

Answer 1

First you need to validate the user input.
Using blocks ensure that you connection and command are closed and disposed.
Parameters save your database from sql injection. I had to guess at the datatype.
Private ConStr As String = "Your connection string"
Private Sub OPCode()
Dim DeptID As Integer
If Not Integer.TryParse(TextDepartmentID.Text, DeptID) Then
MessageBox.Show("Please enter a valid department ID.")
Exit Sub
End If
Dim dt As New DataTable
Using con As New SqlConnection(ConStr),
cmd As New SqlCommand("SELECT MainDepartmentsTable.ID,
FeesTable.ID,
FeesTable.Particular,
FeesTable.Fee
from MainDepartmentsTable
inner join SubDepartmentsTable on
MainDepartmentsTable.ID = SubDepartmentsTable.MainDeptID
inner join FeesTable on
SubDepartmentsTable.MainDeptID = FeesTable.SubDepartmentID
WHERE FeesTable.ID = #DepartmentID;", con)
cmd.Parameters.Add("#DepartmentID", SqlDbType.Int).Value = DeptID
con.Open()
dt.Load(cmd.ExecuteReader)
End Using
Load_Departments(dt)
End Sub
Private Sub Load_Departments(dt As DataTable)
ComboBox1.DataSource = dt
ComboBox1.DisplayMember = "Particular"
ComboBox1.ValueMember = "Fee"
End Sub

Answer 2

Let's simplify a bit:
Dim sql = "SELECT FeesTable.ID, CoNCAT(FeesTable.Particular, '-', FeesTable.Fee) as DisplayText
FROM
SubDepartmentsTable
inner join FeesTable on SubDepartmentsTable.MainDeptID = FeesTable.SubDepartmentID
WHERE SubDepartmentsTable.MainDeptID = #id"
Dim da As New SqlDataAdapter(sql, con)
da.SelectCommand.Parameters.AddWithValue("#id", Convert.ToInt32(TextDepartmentID.Text)) 'less worried about AWV with ints but read https://blogs.msmvps.com/jcoehoorn/blog/2014/05/12/can-we-stop-using-addwithvalue-already/
Dim dt As New DataTable
da.Fill(dt)
'this part connects the data to the combo - it's the part you didn't show in your Q and what people are telling you is missing
yourCombo.DisplayMember = "DisplayText"
yourCombo.ValueMember = "ID"
yourCombo.DataSource = dt
This isn't intended to be a paste in fix for your problem; you've left out too much info from your question to know how to answer it exactly, so you're gonna have to modify this
Things you need to change:
The columns that are selected in the SQL - load one column you will show (I demonstrated how to combine two bits of info under one name of DisplayText) and one column for the backing value (ID)
The column that is filtered on in the where clause or the name of the text box TextDepartmentID - it seems odd to me that a Fee ID should be equal to a Department ID, so I've altered the SQL to retrieve the Fee details for a MainDepartmentID because I've assumed your textbox name is correct and that you intended to show the fees for a department via the subdepartments
Read the blog linked and perhaps choose adding a parameter in a different way
Change yourCombo to the name of your combobox
Change DisplayMember and ValueMember setting to match the names of the two columns your SQL query selects
You only need two columns if youre plumbing up a combo, but if you pass the datatable off somewhere you can add more