在线考试网站系统英文翻译

在线考试网站系统英文翻译内容摘要：

uts by using folders and subfolders. When you do this, you’ re the database manager. But what do you do when the problems you’ re trying to solve get too big? How can you easily collect information about all customers and their orders when the data might be stored in several document and spreadsheet files? How can you maintain links between the files when you enter new information? How do you ensure that data is being entered correctly? What if you need to share your information with many people but don’ t want two people to try updating the same data at the same time? How do you keep duplicate copies of data proliferating when people can’t share the same data at the same time? Faced with these challenges, you need a database management system (DBMS). Relational Databases Nearly all modern database management systems store and handle information using the relational database management model. In a relational database management system, sometimes called an RDBMS, the system manages all data in tables. Tables store information about a single subject (such as customers or products) and have columns (or fields) that contain the different kinds of information about the subject (for example, customers’ addresses or phone numbers) and rows (or records) that describe all the attributes of a single instance of the subject (for example, data on a specific customer or product). Even when you query the database (fetch information from one or more tables), the result is always something that looks like another table. The term relational stems from the fact that each table in the database contains information related to a single subject and only that subject. If you study the relational database management model, you’ ll find the term relation applied to a set of rows (a table) about a single , data about two classes of information (such as customers and orders) can be manipulated as a single entity based on related data values. For example, it would be redundant to store customer name and address information with every order that the customer places. In a relational database system, the information about orders contains a field that stores data, such as a customer number, which can be used to connect each order with the appropriate customer information. You can also join information on related values from multiple tables or queries. For example,you can join pany information with contact information to find out the contacts for a particular pany. You can join employee information with department information to find out the department in which an employee works. Database Capabilities An RDBMS gives you plete control over how you define your data, work with it, and share it with others. The system also provides sophisticated features that make it easy to catalog and manage large amounts of data in many tables. An RDBMS has three main types of capabilities: data definition, data manipulation, and data control. Data definition You can define what data is stored in your database, the type of data (for example, numbers or characters), and how the data is related. In some cases, you can also define how the data should be formatted and how it should be validated. Data manipulation You can work with the data in many ways. You can select which data fields you want, filter the data, and sort it. You can join data with related information and summarize the data. You can select a set of information and ask the RDBMS to update it, delete it, copy it to another table, or create a new table containing the data. Data control You can define who is allowed to read, update, or insert data. In many cases,you can also define how data can be shared and updated by multiple users. All this functionality is contained in the powerful features of Microsoft Access. Let’ s take a look at how Access implements these capabilities and pare them to what you can do with spreadsheet or word processing programs. Microsoft Access as an Application Development System Being able to define exactly what data you need, how it should be stored, and how you want to access it solves the data management part of the problem. However, you also need a simple way to automate all the mon tasks you want to perform. For example, each time you need to enter a new order, you don’ t want to have to run a query to search the Customers table, execute a mand to open the Orders table, and then create a new record before you can enter the data for the order. And after you’ ve entered the data for the new order, you don’ t want to have to worry about scanning the table that contains all your products to verify the order’s sizes, colors, and prices。