香奈儿cf是什么意思:VB的问题

今天看一段程序，有个地方不是很明白，希望大哥大姐们帮我解决一下，谢谢了~！
Private Sub cmdInquire_Click()
Dim txtSQL As String
Dim MsgText As String
Dim dd(4) As Boolean
Dim mrc As ADODB.Recordset
txtSQL = "select * from student_info where"
If Check1.Value Then
If Trim(txtSID.Text) = "" Then
smeg = "学号不能为空"
MsgBox smeg, vbOKOnly + vbExclamation, "警告"
txtSID.SetFocus
Exit Sub
Else
If Not IsNumeric(Trim(txtSID.Text)) Then
MsgBox "请输入数字！", vbOKOnly + vbExclamation, "警告"
Exit Sub
txtSID.SetFocus
End If
dd(0) = True

txtSQL = txtSQL & "student_id='" & Trim(txtSID.Text) & "'"
End If
End If
If Check2.Value Then
If Trim(txtName.Text) = "" Then
smeg = "姓名不能为空"
MsgBox smeg, vbOKOnly + vbExclamation, "警告"
txtName.SetFocus
Exit Sub
Else

dd(1) = True
If dd(0) Then
txtSQL = txtSQL & "and student_name='" & txtName.Text & "'"
Else
txtSQL = txtSQL & "student_name='" & txtName.Text & "'"
End If
End If
If Check3.Value Then
If Trim(txtClassno.Text) = "" Then
smeg = "班号不能为空"
MsgBox smeg, vbOKOnly + vbExclamation, "警告"
txtClassno.SetFocus
Exit Sub
Else
dd(2) = True
If dd(0) Or dd(1) Then
txtSQL = txtSQL & " and class_no='" & txtClassno.Text & "'"
Else
txtSQL = txtSQL & " class_no='" & txtClassno.Text & "'"
End If
End If
End If
If Not (dd(0) Or dd(1) Or dd(2) Or dd(3)) Then
MsgBox "请设置查询方式！", vbOKOnly + vbExclamation, "警告"
Exit Sub
End If
txtSQL = txtSQL & "order by student_ID"

就这段~！
不明白的有二：1：txtSQL = txtSQL & "and student_Name = '" & txtName.Text & "'" 这句话是什么意思？这句里的and student_Name = '这一句是什么意思？加一个’又是什么意思？
2：DD（）表示什么？谢了！