Problem to Dropdownlist

I have the codes like
        sql = LibUtil.GetSqlCodeVG(accessRight, dept);
        dt = DBUtil.GetCodeTable(conn, sql);
        ddlStore.DataSource = dt;
        ddlStore.DataValueField = "code";
        ddlStore.DataTextField = "dscr";
    public static string GetSqlCodeVG(string accessRight, string dept)
        string sql = @"select store_no code, sfzone, concat(ifnull(dscr,''),',  ',ifnull(store_no,'')) as dscr
            from c_store_vg c
            where deleted = '0' and dscr is NOT NULL ";
        sql += "order by dscr ";
        return sql;

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to fill the Dropdownlist. But after I've selected one value to the DDL, I check that
is having empty value inside. why?
LVL 12
HuaMin ChenProblem resolverAsked:
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SelectedValue will work, but only if you keep the databinding of the list in !IsPostBack. Otherwise the selected value will reset to the default selected value.

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HuaMin ChenProblem resolverAuthor Commented:
Can you please show with example codes to your advice? thanks
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