DropDownList控制gridview的问题

Question

大家好，我有两个DropDownList控制gridview。两个DropDownList都有自己的items和一个叫做ALL的选项（值为""）。我想点ALL就没有filter,select所有的东西。问题是有时候一个选ALL，另一个由自己的值（不是ALL）。于是我有了以下的code，我给两个DropDownList各写了一份。基本思路是：If both DropDownList have "" value, then select everything
Elseif DropDownList1="" then select everything where value=DropDownList2
Elseif DropDownList2="" then select everything where value=DropDownList1
Endif
当我两个DropDownList都选ALL，顺利得到数据。不动第二个的选项(ALL)，把第一个改为其他值，结果什么都没有，没有任何数据，再乱点一下，再点回来就好了，就是很不稳定，时好时坏。是不是AccessDataSourceGridView.SelectParameters.Clear()
除了问题，大家帮帮我，谢谢 Protected Sub DropDownListSchool_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownListProgram.SelectedIndexChanged
        If DropDownListSchool.SelectedValue = "" And DropDownListProgram.SelectedValue = "" Then
            AccessDataSourceGridView.SelectParameters.Clear()
            AccessDataSourceGridView.SelectCommand = "SELECT Students.Username, Students.LastName, Students.FirstName, ProgramsStudents.CourseID, ProgramsStudents.NeedsVisa, Schools.SchoolName, Schools.SchoolID FROM ((Students INNER JOIN ProgramsStudents ON Students.Username = ProgramsStudents.UserName) INNER JOIN Schools ON Students.SchoolID = Schools.SchoolID)"
        ElseIf DropDownListSchool.SelectedValue = "" Then
            AccessDataSourceGridView.SelectParameters.Clear()
            AccessDataSourceGridView.SelectCommand = "SELECT Students.Username, Students.LastName, Students.FirstName, ProgramsStudents.CourseID, ProgramsStudents.NeedsVisa, Schools.SchoolName, Schools.SchoolID FROM ((Students INNER JOIN ProgramsStudents ON Students.Username = ProgramsStudents.UserName) INNER JOIN Schools ON Students.SchoolID = Schools.SchoolID) WHERE (ProgramsStudents.CourseID = '" & DropDownListProgram.SelectedItem.Text & "')"
        ElseIf DropDownListProgram.SelectedValue = "" Then
            AccessDataSourceGridView.SelectParameters.Clear()
            AccessDataSourceGridView.SelectCommand = "SELECT Students.Username, Students.LastName, Students.FirstName, ProgramsStudents.CourseID, ProgramsStudents.NeedsVisa, Schools.SchoolName, Schools.SchoolID FROM ((Students INNER JOIN ProgramsStudents ON Students.Username = ProgramsStudents.UserName) INNER JOIN Schools ON Students.SchoolID = Schools.SchoolID) WHERE (Students.SchoolID = '" & DropDownListSchool.SelectedItem.Text & "')"
        End If
    End Sub    Protected Sub DropDownListProgram_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownListProgram.SelectedIndexChanged
        If DropDownListSchool.SelectedValue = "" And DropDownListProgram.SelectedValue = "" Then
            AccessDataSourceGridView.SelectParameters.Clear()
            AccessDataSourceGridView.SelectCommand = "SELECT Students.Username, Students.LastName, Students.FirstName, ProgramsStudents.CourseID, ProgramsStudents.NeedsVisa, Schools.SchoolName, Schools.SchoolID FROM ((Students INNER JOIN ProgramsStudents ON Students.Username = ProgramsStudents.UserName) INNER JOIN Schools ON Students.SchoolID = Schools.SchoolID)"
        ElseIf DropDownListSchool.SelectedValue = "" Then
            AccessDataSourceGridView.SelectParameters.Clear()
            AccessDataSourceGridView.SelectCommand = "SELECT Students.Username, Students.LastName, Students.FirstName, ProgramsStudents.CourseID, ProgramsStudents.NeedsVisa, Schools.SchoolName, Schools.SchoolID FROM ((Students INNER JOIN ProgramsStudents ON Students.Username = ProgramsStudents.UserName) INNER JOIN Schools ON Students.SchoolID = Schools.SchoolID) WHERE (ProgramsStudents.CourseID = '" & DropDownListProgram.SelectedItem.Text & "')"
        ElseIf DropDownListProgram.SelectedValue = "" Then
            AccessDataSourceGridView.SelectParameters.Clear()
            AccessDataSourceGridView.SelectCommand = "SELECT Students.Username, Students.LastName, Students.FirstName, ProgramsStudents.CourseID, ProgramsStudents.NeedsVisa, Schools.SchoolName, Schools.SchoolID FROM ((Students INNER JOIN ProgramsStudents ON Students.Username = ProgramsStudents.UserName) INNER JOIN Schools ON Students.SchoolID = Schools.SchoolID) WHERE (Students.SchoolID = '" & DropDownListSchool.SelectedItem.Text & "')"
        End If
    End Sub