select case and error join multiple table - vb.net

Question

Imports System.Data.OleDb
Public Class Form1
Dim conn As OleDbConnection
Dim cmd As OleDbCommand
Dim da As OleDbDataAdapter
Dim strSQL As String
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles btnsearch.Click
Dim fieldselect As String = ""
Select Case ComboBox1.Text
Case "startYear"
fieldselect = "startYear"
Case "genres"
fieldselect = "genres"
Case "Rating"
fieldselect = "Rating"
End Select
conn = New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=Movies.accdb; Persist Security Info=False;")
strSQL = "SELECT startYear, genres, averageRating FROM (basic.tconst Inner JOIN Rating.tconst on basic.tconst=Rating.tconst)" & fieldselect & "'" & TextBox1.Text & "%'"
conn.Open()
da = New OleDbDataAdapter(strSQL, conn)
Dim ds As New DataSet("Movies")
da.Fill(ds, "Movies")
DataGridView1.DataSource = ds.Tables("Movies")
conn.Close()
End Sub

Answer 1

Keep you data objects local so you can control there closing and disposing. Using...End Using blocks do this for you even it there is an error.
Are all your fields in the Select Case the same datatype? If not you will have to adjust the code. If you need help, tell me the datatypes so I can fix the code.
Always use parameters to avoid sql injection. A text box can hold a Drop Table command. Parameters are treated as values not executable code.
If you need to update later in the code, you will need to have a primary key available in your Select.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Since your field name exactly matches the combo text the Case statement is not necessary
Dim fieldselect As String = ComboBox1.Text
Dim dt As New DataTable
Using conn = New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=Movies.accdb; Persist Security Info=False;")
Dim strSQL = "SELECT startYear, genres, averageRating FROM basic.tconst Inner JOIN Rating.tconst on basic.tconst=Rating.tconst Where " & fieldselect & " = #FieldValue;"
Using cmd As New OleDbCommand(strSQL, conn)
cmd.Parameters.Add("#FieldValue", OleDbType.VarChar).Value = TextBox1.Text
conn.Open()
dt.Load(cmd.ExecuteReader)
End Using
End Using
DataGridView1.DataSource = dt
End Sub

Answer 2

i wanted to use combo box as the filter for my program, i need to filter data from 2 different table. rating has it own table and basic is different table. both of the field mention above does not have any connection.

Answer 3

It's a bit had to help without context but I gave it a try :
I would replace
strSQL = "SELECT startYear, genres, averageRating FROM (basic.tconst Inner JOIN Rating.tconst on basic.tconst=Rating.tconst)" & fieldselect & "'" & TextBox1.Text & "%'"
With
"SELECT startYear, genres, averageRating " &
"FROM basic.tconst " &
"INNER JOIN Rating.tconst on basic.tconst=Rating.tconst " & fieldselect & "'" & TextBox1.Text & "%'"
To get better help, could you explain how tables Rating and basic works and what fieldselect is ?