GridView循环显示有关问题

GridView循环显示问题？
一个GridView，一个DropDownList，一个Button ，
DropDownList为（0，最近7天），（1，最近15天），（2，最近30天），（3，查询所有）
当button点击时，根据DropDownList的SelectedIndex值查询。
该怎么循环查询好些呢。

C# code

    protected void btn_cha_Click(object sender, EventArgs e)    {        if (ddl7.SelectedIndex == 0)        {            GridView1.DataSource = null;            string sql = "select * from [jinhuo] where datediff(day,j_date,'" + DateTime.Now.ToShortDateString() + "')<7 and user_name='" + Session["userName"] + "'";            SqlConnection con = new SqlConnection(conString);            con.Open();            SqlCommand cmd = new SqlCommand(sql, con);            SqlDataReader sdr = cmd.ExecuteReader();            GridView1.DataSource = sdr;            GridView1.DataBind();            con.Close();            sdr.Close();        }        if (ddl7.SelectedIndex == 1)        {            GridView1.DataSource = null;            string sql = "select * from [jinhuo] where datediff(day,j_date,'" + DateTime.Now.ToShortDateString() + "')<15 and user_name='" + Session["userName"] + "'";            SqlConnection con = new SqlConnection(conString);            con.Open();            SqlCommand cmd = new SqlCommand(sql, con);            SqlDataReader sdr = cmd.ExecuteReader();            GridView1.DataSource = sdr;            GridView1.DataBind();            con.Close();            sdr.Close();        }        if (ddl7.SelectedIndex == 2)        {            GridView1.DataSource = null;            string sql = "select * from [jinhuo] where datediff(day,j_date,'" + DateTime.Now.ToShortDateString() + "')<30 and user_name='" + Session["userName"] + "'";            SqlConnection con = new SqlConnection(conString);            con.Open();            SqlCommand cmd = new SqlCommand(sql, con);            SqlDataReader sdr = cmd.ExecuteReader();            GridView1.DataSource = sdr;            GridView1.DataBind();            con.Close();            sdr.Close();        }        if (ddl7.SelectedIndex == 3)        {            GridView1.DataSource = null;            string sql = "select * from [jinhuo] where user_name='" + Session["userName"] + "'";            SqlConnection con = new SqlConnection(conString);            con.Open();            SqlCommand cmd = new SqlCommand(sql, con);            SqlDataReader sdr = cmd.ExecuteReader();            GridView1.DataSource = sdr;            GridView1.DataBind();            con.Close();            sdr.Close();        }    }

C# code

private void ExecSql(string sql){            GridView1.DataSource = null;            SqlConnection con = new SqlConnection(conString);            con.Open();            SqlCommand cmd = new SqlCommand(sql, con);            SqlDataReader sdr = cmd.ExecuteReader();            GridView1.DataSource = sdr;            GridView1.DataBind();            con.Close();            sdr.Close();}    protected void btn_cha_Click(object sender, EventArgs e)    {            string sql = "";        if (ddl7.SelectedIndex == 0)        {            sql = "select * from [jinhuo] where datediff(day,j_date,'" + DateTime.Now.ToShortDateString() + "')<7 and user_name='" + Session["userName"] + "'";        }        if (ddl7.SelectedIndex == 1)        {           sql = "select * from [jinhuo] where datediff(day,j_date,'" + DateTime.Now.ToShortDateString() + "')<15 and user_name='" + Session["userName"] + "'";        }        if (ddl7.SelectedIndex == 2)        {            sql = "select * from [jinhuo] where datediff(day,j_date,'" + DateTime.Now.ToShortDateString() + "')<30 and user_name='" + Session["userName"] + "'";        }        if (ddl7.SelectedIndex == 3)        {            sql = "select * from [jinhuo] where user_name='" + Session["userName"] + "'";        }        ExecSql(sql);    }