VB+access图书管理系统代码问题，希望有高手帮忙详细解释下，急~！！！！十分感谢

Option Explicit
Dim panduan As String
Private Sub Command1_Click()
Dim rs_findbook As New ADODB.Recordset
Dim sql As String
If Check1.Value = vbChecked Then
sql = "书名='" & Trim(Text1.Text & " ") & "'"
End If
If Check2.Value = vbChecked Then
If Trim(sql) = "" Then
sql = "类别='" & Trim(Combo1.Text & " ") & "'"
Else
sql = sql & "and 书名='" & Trim(Combo1.Text & " ") & "'"
End If
End If
If Check3.Value = vbChecked Then
If Trim(sql) = "" Then
sql = "作者='" & Trim(Text2.Text & " ") & "'"
Else
sql = sql & "and 作者='" & Trim(Text2.Text & " ") & "'"
End If
End If
If Check4.Value = vbChecked Then
If Trim(sql) = "" Then