麻烦请问为什么下面2个查询，只能用一个，若2个同时使用就查询不到想要的结果了

麻烦请问为什么下面2个查询，只能用一个，若2个同时使用就查询不到想要的结果了？谢谢

begin
ADOQuery1.ConnectionString :='Provider=Microsoft.jet.OLEDB.4.0; Data Source='
+ ExtractFilePath(Application.ExeName)+'LD.mdb;Persist Security Info=False';
With ADOQuery1 do
begin
  Close;
  Sql.Text:='Select distinct lifetime from data1 where lamptype= "E19.5H 225W"' ;
Open;
end;
if ADOQuery1.RecordCount>0 then
    begin
      while Not ADOQuery1.Eof do
      begin
        ComboBox4.Items.Add(ADOQuery1.Fields[0].AsString);
          ADOQuery1.Next;

With ADOQuery1 do
begin
  Close;
/SQL.Clear;
  //Sql.Add('Select distinct sn from data1') ;
//Open;
//end;
//if ADOQuery1.RecordCount>0 then
    //begin
     // while Not ADOQuery1.Eof do
     // begin
        //ComboBox3.Items.Add(ADOQuery1.Fields[0].AsString);
         // ADOQuery1.Next;