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Mỗi bảng cần phải có một khóa chính - một hoặc nhiều lĩnh vực với một giá trị duy nhất cho mỗi bản ghi. (Nguyên tắc này được gọi là thực thể toàn vẹn trong thế giới quản lý cơ sở dữ liệu.) TblContacts, lĩnh vực ContactID là chìa khóa chính. Liên lạc với nhau có giá trị khác nhau ContactID để bạn có thể xác định một từ khác. | Creating Access Tables 34 Setting the Primary Key Every table should have a primary key one or more fields with a unique value for each record. This principle is called entity integrity in the world of database management. In tblContacts the ContactID field is the primary key Each contact has a different ContactID value so that you can identify one from the other. ContactID 17 refers to one and only one record in the Contacts table. If you don t specify a primary key unique value field Access can create one for you. Understanding unique values Without the ContactID field you d have to rely on another field or combination of fields for uniqueness. You couldn t use the LastName field because two customers could easily have the same last name. In fact you couldn t even use the FirstName and LastName fields together multi-field key for the same reason two people could be named James Smith. You need to come up with a field that makes every record unique. Looking at the table you may think that you could use a combination of the LastName FirstName and Company fields but theoretically it s possible that two people working at the same company have the same name. The easiest way to solve this problem is to add an AutoNumber field for the express purpose of using it as the table s primary key This is exactly the situation with the Contacts table. The primary key of this table is ContactID an AutoNumber field. If you don t designate a field as a primary key Access can add an AutoNumber field and designate it as the table s primary key. This field contains a unique number for each record in the table and Access maintains it automatically. Generally speaking you may want to create and maintain your own primary key even if you always use AutoNumber fields as primary keys A primary key is always an index. An index maintains a presorted order of one or more fields that greatly speeds up queries searches and sort requests. When you add new records to your table Access checks for .
