如何更新数据库中记录，求大神指点，纠正我的错误 - 编程论坛

小弟做了个“添加用户”窗体，实现了把用户名密码信息保存到Access数据库表中，并对用户名密码加密；

现要做“登录”窗体，请教如何对数据库表中的字段解密，以便解密后和输入TEXT文本中的值做比对。代码如下：

程序代码：

unit Frmlogin;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    Edit1: TEdit;
    Edit2: TEdit;
    Button1: TButton;
    Button2: TButton;
    Edit3: TEdit;
    Edit4: TEdit;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses DataT;

{$R *.dfm}
//解密函数
Function UncrypKey(Src: String; Key: String): string;
var
idx: integer;
KeyLen: integer;
KeyPos: integer;
offset: integer;
dest: string;
SrcPos: integer;
SrcAsc: integer;
TmpSrcAsc: integer;
Range: integer;
begin
KeyLen := Length(Key);
if KeyLen = 0 then
Key := 'Think Space';
KeyPos := 0;
SrcPos := 0;
SrcAsc := 0;
Range := 256;
offset := StrToInt('$' + copy(Src, 1, 2));
SrcPos := 3;
repeat
SrcAsc := StrToInt('$' + copy(Src, SrcPos, 2));
if KeyPos < KeyLen Then
KeyPos := KeyPos + 1
else
KeyPos := 1;
TmpSrcAsc := SrcAsc xor ord(Key[KeyPos]);
if TmpSrcAsc <= offset then
TmpSrcAsc := 255 + TmpSrcAsc - offset
else
TmpSrcAsc := TmpSrcAsc - offset;
dest := dest + chr(TmpSrcAsc);
offset := SrcAsc;
SrcPos := SrcPos + 2;
until SrcPos >= Length(Src);
Result := dest;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
    data.adoqlogin.close;
    data.adoqlogin.sql.clear;
    data.adoqlogin.sql.add('update users set username='张三',password='123456'');

       data.adoconnectBooks.connected:=true;
    data.adoqlogin.execsql;unit Frmlogin;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    Edit1: TEdit;
    Edit2: TEdit;
    Button1: TButton;
    Button2: TButton;
    Edit3: TEdit;
    Edit4: TEdit;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses DataT;

{$R *.dfm}
//解密函数
Function UncrypKey(Src: String; Key: String): string;
var
idx: integer;
KeyLen: integer;
KeyPos: integer;
offset: integer;
dest: string;
SrcPos: integer;
SrcAsc: integer;
TmpSrcAsc: integer;
Range: integer;
begin
KeyLen := Length(Key);
if KeyLen = 0 then
Key := 'Think Space';
KeyPos := 0;
SrcPos := 0;
SrcAsc := 0;
Range := 256;
offset := StrToInt('$' + copy(Src, 1, 2));
SrcPos := 3;
repeat
SrcAsc := StrToInt('$' + copy(Src, SrcPos, 2));
if KeyPos < KeyLen Then
KeyPos := KeyPos + 1
else
KeyPos := 1;
TmpSrcAsc := SrcAsc xor ord(Key[KeyPos]);
if TmpSrcAsc <= offset then
TmpSrcAsc := 255 + TmpSrcAsc - offset
else
TmpSrcAsc := TmpSrcAsc - offset;
dest := dest + chr(TmpSrcAsc);
offset := SrcAsc;
SrcPos := SrcPos + 2;
until SrcPos >= Length(Src);
Result := dest;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
    data.adoqlogin.close;
    data.adoqlogin.sql.clear;
    data.adoqlogin.sql.add('update users set username='张三',password='123456'');

       data.adoconnectBooks.connected:=true;
    data.adoqlogin.execsql;

编译提示UPDATE语句出错，求解？？？