SPQuery qryReportLibrary = new SPQuery();
// Check the value is null ??
if (!string.IsNullOrEmpty(ddlMisson.SelectedValue))
qryReportLibrary.Query = string.Format(@"<Where>
<Eq>
<FieldRef Name='Misson' />
<Value Type='LookupMulti'>{0}</Value>
</Eq>
</Where>", ddlMisson.SelectedValue);
else
{
qryReportLibrary.Query = string.Format(@"<Where>
<IsNull>
<FieldRef Name='Mission'/>
</IsNull>
</Where>");
}
qryReportLibrary.Query = strQuery;