Date 根据不同日期计算总和

我是Stackoverflow的新手，这个问题需要你的帮助。 我有一个表（sql 2014和使用vb.net），如下所示：

ID  Codfisc  Dal         Al
1   xxxxxxx  2017/08/08  2017/08/14

我开发了以下代码：

Dim a As Date = Date.Parse(TextBox2.Text)
    Dim b As Date = Date.Parse(TextBox3.Text)
    Dim source2 As String = WebConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
    Dim sql2 As String = "select SUM(DATEDIFF(day, dal, al)+1) AS Total FROM Tab1 WHERE codfisc = @codfisc and Dal >=@Dal and Al <=@al"
    Dim conn2 As New SqlConnection(source2)
    conn2.Open()
    Dim cmd2 As New SqlCommand(sql2, conn2)
    cmd2.Parameters.AddWithValue("@Codfisc", Me.GridView1.SelectedRow.Cells(8).Text)
    cmd2.Parameters.AddWithValue("@Dal", a)
    cmd2.Parameters.AddWithValue("@Al", b)
    Dim dr2 As SqlDataReader
    dr2 = cmd2.ExecuteReader()
    While dr2.Read()

        TextBox4.Text = dr2("Total").ToString()

        If IsDBNull(dr2("Total")) Then
            TextBox4.Text = "0"
        End If


    End While

    conn2.Close()

Dim a As Date=Date.Parse（TextBox2.Text）
Dim b As Date=Date.Parse（TextBox3.Text）
Dim source2 As String=WebConfiguration Manager.ConnectionString（“ConnectionString”）.ConnectionString
Dim sql2 As String=“从表1中选择SUM（DATEDIFF（day，dal，al）+1）作为总计，其中codfisc=@codfisc和dal>=@dal和al