外文翻譯----數(shù)據(jù)庫和數(shù)據(jù)倉庫(文件)
【正文】 re arranged in a strict parent child relationship. Each parent record may have many children, but each child record has exactly one parent. Figure 31 shows a simple hierarchical database, indicating the relationship between a customer and the orders it has placed with a pany. Searching a hierarchical database is rapid and convenient as long as it is searched from the top down. IBM’s IMS (Information 畢業(yè)設(shè)計（論文）外文翻譯 2 Management System) is the most widely used hierarchical DBMS. Hierarchical DBMS are best suited for problems that require a limited number of structure answers that can be specified in advance. Once data relationships have been specified, they cannot easily be changed without a major programming effort. Thus, the hierarchical model cannot respond flexibly to changing requests for information. The Network Model The work database model is best at representing manytomany relationship among data. In other words, a ”child” can have more than one “parent”, as Figure32 illustrates. Computer Associates ’IDMS is a work DBMS for puter mainframes. Network DBMS are more flexible than hierarchical DBMS, but access paths must still be specified in advance. There are practical limitations to the number of links, or relationships ,that can be established among records. If they are too numerous, the software will not work efficiently. Neither work no hierarchical database management models can easily create new relationships among data elements or new patterns of access without major programming efforts. The Relational Model In the early 1970s the relational database approach was developed to provide a much more userfriendly organization. Instead of using plex structures of pointers the relational database stores information simply as tables called relations. These tables are sometimes called flat because the rows of the table really are very much the same as the records of a file. Each row in a relation is called a record. Each column corresponds to a particular field within the record (fields are also called domains).The fields are headed by attributes, which describe the entries in the column. A relational database consists of one or more relations. A relational DBMS performs three primary operations on relations to form new relations: (1) Two relations may be joined (bined). (2) A relational may be projected (some of the columns are extracted form the relations and used to form the columns of the new relation). (3) Records may be selected according to various user specified criteria’s(Structured Query Language) is the most important query language based on the relational model. For instance, a relation named Accounts, record bank accounts, their balance, and type might look like: 畢業(yè)設(shè)計（論文）外文翻譯 3 Accounts Account Balance Type 173921 Checking 251101 Savings … … … Heading the columns are the three attributes: Account No, Balance, and Type. Below the attributes are the rows, or records. The first row says that account number 173921 has a balance of one thousand and seven hundreds dollars, and it is checking account .Suppose we wanted to know the balance of account could ask this Query in SQL as follows: SELECT balance FORM Accounts WHERE accountNo=173921 IBM’s DB2 and Oracle from the Oracle Corporation are examples of mainframe relational database management systems. Microsoft Access is a PC relational database management system. (Oracle also has a PC version). ClientServer Architecture Many varieties of modern software use a clientserver architecture, in which request
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