commandtext问题

在三层架构中，客户端登陆代码：
procedure TForm2.Button1Click(Sender: TObject);
const
sSql :string = 'select Password from Popedom where';
var
sSelect,UserPass,UserName,Power : string;
i,j,t:integer;

begin
if (Form2.NameEdit.Text <> '') and (Form2.PassEdit.Text <> '') then
begin
if Form2.ComboBox1.Text = '一般用户' then
t := 1
else t:=0 ;
UserName := Form2.NameEdit.Text;
sSelect:=sSql+' UserName='''+UserName+'''';
CheckClient.Close;
CheckClient.CommandText := sSelect;
CheckClient.Open;

//如果有这个记录
if CheckClient.RecordCount=1 then
begin
UserPass := CheckClient.FieldByName('PassWord').asString;
Power := CheckClient.FieldByName('Mark').AsString;
end;
i:= CompareStr(UserPass,Form2.PassEdit.Text);
j:= CompareStr(inttostr(t),Power);
if (i=0) and (j=0) then
if t=0 then
begin
Form3.Show;
Form2.Hide;
end
else
begin
Form4.Show;
Form2.Hide;
end
else
MessageDlg('密码或权限错误！', mtWarning, [mbOK], 0)
end
else
MessageDlg('用户名或密码不能为空！',mtWarning,[mbOK],0)
end;

表定义：
create table Popedom(
UserName varchar2(15) primary key,
Password varchar2(15) not null,
mark char(1));
当登陆时报错，project client.exe raised exception class eoleexception
with message'commandtext changes are not allowed'.process stopped.
use step or run to continue.
谁能告诉我错在那儿了？