Developer’s Guide Borland Delphi 7 for Windows PHẦN 7

• Các tài sản giá trị xác định một giá trị cho tham số được lựa chọn. Bạn không bao giờ có thể thiết lập giá trị cho thông số đầu ra và kết quả. Những loại của các thông số có giá trị được thiết lập bởi việc thực hiện các thủ tục lưu trữ. | 27-22 Developer s Guide Chapter 28 Using unidirectional datasets dbExpress is a set of lightweight database drivers that provide fast access to SQL database servers. For each supported database dbExpress provides a driver that adapts the server-specific software to a set of uniform dbExpress interfaces. When you deploy a database application that uses dbExpress you need only include a dll the server-specific driver with the application files you build. dbExpress lets you access databases using unidirectional datasets. Unidirectional datasets are designed for quick lightweight access to database information with minimal overhead. Like other datasets they can send an SQL command to the database server and if the command returns a set of records obtain a cursor for accessing those records. However unidirectional datasets can only retrieve a unidirectional cursor. They do not buffer data in memory which makes them faster and less resource-intensive than other types of dataset. However because there are no buffered records unidirectional datasets are also less flexible than other datasets. Many of the capabilities introduced by TDataSet are either unimplemented in unidirectional datasets or cause them to raise exceptions. For example The only supported navigation methods are the First and Next methods. Most others raise exceptions. Some such as the methods involved in bookmark support simply do nothing. There is no built-in support for editing because editing requires a buffer to hold the edits. The CanModify property is always False so attempts to put the dataset into edit mode always fail. You can however use unidirectional datasets to update data using an SQL UPDATE command or provide conventional editing support by using a dbExpress-enabled client dataset or connecting the dataset to a client dataset see Connecting to another dataset on page 19-10 . Using unidirectional datasets 28-1 Types of unidirectional datasets There is no support for filters because filters .