如何用topendialog打开db数据库

[iamsunqun]

各位大虾，我想用topendialog选择我需要的.db数据库，然后放到dbgrid中去，下面是我写的程序，运行出错，请指正，谢谢！
procedure Tmainform.N2Click(Sender: TObject);
var
f:file;
i:integer;
s:string;
opendialog:Topendialog;
begin
for i:=0 to 5 do //居中显示字段
begin
DBGrid1.Columns.Items[i].Width:=round((DBGrid1.Width-50)/6);
DBGrid1.Columns.Items[i].Title.Alignment:=taCenter;
DBGrid1.Columns.Items[i].Alignment:=taCenter;
end;
opendialog:=Topendialog.Create(application);
opendialog.Filter:='(*.db)|*.db';
opendialog.DefaultExt:='.db';
opendialog.FileName:='';
if opendialog.Execute then
s:=opendialog.FileName;
//s:='db_tiankong.db';
dbgrid1.Visible :=true;
mainform.query1.Close;
mainform.query1.SQL.Clear;

mainform.query1.SQL.Add('select 题号,题型,章节,难度,分数,成功 from '+s);

mainform.dbgrid1.Columns[0].FieldName:='题号';
mainform.dbgrid1.Columns[1].FieldName:='题型';
mainform.dbgrid1.Columns[2].FieldName:='章节';
mainform.dbgrid1.Columns[3].FieldName:='难度';
mainform.dbgrid1.Columns[4].FieldName:='分数';
mainform.dbgrid1.Columns[5].FieldName:='成功';
mainform.query1.Open;

end;

[iamsunqun]

难道这你没有大虾，还是我给的分数不够，尽然没人回复！！！！！！！！！