database - Delphi + SQLite + DB Express select 参数错误

Question

我使用Delphi XE3并使用SQLite数据库和DB Express。

这是我的用户验证代码:

function validateUser(UserName, Password: string): Boolean;
var
  AParams: TParams;
  SQLTxt: string;
  MD5 : TIdHashMessageDigest5;
  RecCount: integer; 
begin
  AParams := TParams.Create(nil);
  MD5 := TIdHashMessageDigest5.Create;
  try
    Result := False;
    AParams.CreateParam(ftString, 'username', ptInput).Value := UserName;
    AParams.CreateParam(ftString, 'password', ptInput).Value :=
      MD5.HashBytesAsHex(MD5.HashString(Password));
    SQLTxt := 'SELECT login_id FROM login WHERE '+
    'login_username = :username AND login_password = :password ;';
    with Form1.sqlqry1 do
    begin
      SQL.Clear;
      SQL.Text :=  SQLTxt;
      Params := AParams;
      //Params.Items[0].Value := AParams.Items[0].Value;
      //Params.Items[1].Value := AParams.Items[1].Value;
      Prepared := true;
      Open;
    end;
    RecCount := Form1.sqlqry1.RecordCount;// I have error here
    if RecCount = 0 then
      Result := False
    else
      Result := True;
    // end
  finally
    AParams.Free;
    MD5.Free;
  end;
end;

应用程序向我显示[0x0005]:不支持操作错误。

有什么问题吗？为什么？

Answer 1

您也不需要使用 RecordCount(这是您指示发生错误的地方)。使用TDataSet.IsEmpty相反。

此外，来自documentation (强调我的):

Params is a collection of TParam objects that represent the parameters of a query or stored procedure specified by the SQL dataset. When you specify a query by setting the CommandText property (or the SQL property in TSQLQuery), the SQL dataset automatically parse the query and fills Params with a TParam object for every parameter in the query.

您不需要手动创建参数；当您分配 SQL 时，数据集会自动为您执行此操作。

function validateUser(UserName, Password: string): Boolean;
var
  SQLTxt: string;
  MD5 : TIdHashMessageDigest5;
  RecCount: integer; 
begin
  MD5 := TIdHashMessageDigest5.Create;
  try
    Result := False;
    SQLTxt := 'SELECT login_id FROM login WHERE '+
    'login_username = :username AND login_password = :password ;';
    with Form1 do
    begin
      // Clear not needed when setting SQL.Text directly.
      sqlqry1.SQL.Text :=  SQLTxt; 
      sqlqry1.Params.ParamByName('username').AsString := UserName;
      sqlqry1.Params.ParamByName('password').AsString :=
               MD5.HashBytesAsHex(MD5.HashString(Password));

      sqlqry1.Open;
      Result := not sqlqry1.IsEmpty;
    end;
  finally
    MD5.Free;
  end;
end;