为什么日期查询无法显示结果？

Question

procedure TForm1.Button1Click(Sender: TObject);
var
t1,t2: string;
begin
   t1:=DateTimePicker1.Date;
   t2:=DateTimePicker2.Date;
   with adoquery1 do
   begin
     close;
     sql.Clear;
     sql.Add('select * from abc where date between '+t1+' and '+t2);
     
     open;
   end;
end;
双击没有反映，请问会是怎么回事？
后来改成下面样子也是一样的：
procedure TForm1.Button1Click(Sender: TObject);begin   with adoquery1 do
   begin
     close;
     sql.Clear;
     sql.Add('select * from abc where date>:a');
     Parameters.ParamByName('a').value := DateTimePicker1.Date;
     open;
   end;
end;

Answer 1

procedure TForm1.Button1Click(Sender: TObject);
var
t1,t2: string;
begin
   t1:=DateTimePicker1.Date;
   t2:=DateTimePicker2.Date;
   with adoquery1 do
   begin
     close;
     sql.Clear;
     sql.Add('select * from abc where date between #'+t1+'# and #'+t2+'#');  //查詢日期型字段是要加#號的.
     
     open;
   end;
end;

Answer 2

还是出错:#附近有语法错误 
还有t1:=DateTimePicker1.Date忘记加datetostr了

Answer 3

sql.Add('select * from abc where date between '+QuotedStr(t1)+' and '+QuotedStr(t2));

Answer 4

第二种用法，在open前执行Prepared := True;