數據庫管理系統簡介(英文與譯文)
Introduction to DBMS
A database management system (DBMS) is an important type of programming system, used today on the biggest and the smallest computers.As for other major forms of system software, such as compilers and operating systems, a well-understood set of principles for database management systems has developed over the years, and these concepts are useful both for understanding how to use these systems effectively and for designing and implementing DBMS's. DBMS is a collection of programs that enables you to store, modify, and extract information from a database. There are many different types of DBMS's, ranging from small systems that run on personal computers to huge systems that run on mainframes. The following are the location of database between application programs and end-users.
There are two qualities that distinguish database management systems from other sorts of programming systems.
1) The ability to manage persistent data, and
2) The ability to access large amounts of data efficiently.
Point 1) merely states that there is a database which exists permanently; the contents of this database is the data that a DBMS accesses and manages. Point 2) distinguishes a DBMS from a file system, which also manages persistent data. A DBMS's capabilities are needed most when the amount of data is very large, because for small amounts of data, simple access techniques, such as linear scans of the data, are usually adequate.
While we regard the above two properties of a DBMS as fundamental, there are a number of other capabilities that are almost universally found in commercial DBMS's. These are:
Support for at least one data model, or mathematical abstraction through which the user can view the data.
Support for certain high-level languages that allow the user to define the structure of data, access data, and manipulate data.
Transaction management, the capability to provide correct, concurrent access to the database by many users at once.
Access control, the ability to limit access to data by unauthorized users, and the ability to check the validity of data.
Resiliency, the ability to recover from system failures without losing data.
Data Models Each DBMS provides at least one abstract model of data that allows the user to see information not as raw bits, but in more understandable terms. In fact, it is usually possible to see data at several levels of abstraction. At a relatively low level, a DBMS commonly allows us to visualize data as composed of files.
Efficient File Access The ability to store a file is not remarkable: the file system associated with any operating system does that. The capability of a DBMS is seen when we access the data of a file. For example, suppose we wish to find the manager of employee "Clark Kent". If the company has thousands of employees, It is very expensive to search the entire file to find the one with NAME="Clark Kent". A DBMS helps us to set up "index files," or "indices," that allow us to access the record for "Clark Kent" in essentially one stroke no matter how large the file is. Likewise, insertion of new records or deletion of old ones can be accomplished in time that is small and essentially constant, independent of the file’s length. Another thing a DBMS helps us do is navigate among files, that is, to combine values in two or more files to obtain the information we want.#p#
Query Languages To make access to files easier, a DBMS provides a query language, or data manipulation language, to express operations on files. Query languages differ in the level of detail they require of the user, with systems based on the relational data model generally requiring less detail than languages based on other models.
Transaction Management Another important capability of a DBMS is the ability to manage simultaneously large numbers of transactions, which are procedures operating on the database. Some databases are so large that they can only be useful if they are operated upon simultaneously by many computers: often these computers are dispersed around the country or the world. The database systems used by banks, accessed almost instantaneously by hundreds or thousands of automated teller machines (ATM), as well as by an equal or greater number of employees in the bank branches, is typical of this sort of database. An airline reservation system is another good example.
Sometimes, two accesses do not interfere with each other. For example, any number of transactions can be reading your bank balance at the same time, without any inconsistency. But if you are in the bank depositing your salary check at the exact instant your spouse is extracting money from an automatic teller, the result of the two transactions occurring simultaneously and without coordination is unpredictable. Thus, transactions that modify a data item must “lock out” other transactions trying to read or write that item at the same time. A DBMS must therefore provide some form of concurrency control to prevent uncoordinated access to the same data item by more than one transaction.
Even more complex problems occur when the database is distributed over many different computer systems, perhaps with duplication of data to allow both faster local access and to protect against the destruction of data if one computer crashes.
Security of Data A DBMS must not only protect against loss of data when crashes occur, as we just mentioned, but it must prevent unauthorized access. For example, only users with a certain clearance should have access to the salary field of an employee file, and the DBMS must be able associate with the various users their privileges to see files, fields within files, or other subsets of the data in the database. Thus a DBMS must maintain a table telling for each user known to it, what access privileges the user has for each object. For example, one user may be allowed to read a file, but not to insert or delete data; another may not be allowed to see the file at all, while a third may be allowed to read or modify the file at will.
DBMS Types
Designers developed three different types of database structures: hierarchical, network, and relational. Hierarchical and network were first developed but relational has become dominant. While the relational design is dominant, the older databases have not been dropped. Companies that installed a hierarchical system such as IMS in the 1970s will be using and maintaining these databases for years to come even though new development is being done on relational systems. These older systems are often referred to as legacy systems.#p#
譯文：
DBMS簡介
數據庫管理系統是編程系統中的重要的一種，現今可以用在最大的以及最小的電腦上。其他主要形式的系統軟件，比如說匯編以及操作系統，近些年來開發出一系列容易理解的數據庫管理系統原則，并且這些概念既有助于理解如何有效利用系統，又可以幫助設計和執行DBMS系統。DBMS是一個程序的集合，它使你能夠存儲、修改以及從數據庫中提取信息。有很多種不同類型的DBMS系統，從運行在個人電腦上的小型系統到運行在大型主機上的巨型系統。下圖是數據庫在應用程序和終端用戶之間的位置。
DBMS的功能
有兩種功能使數據庫管理系統區別于其他設計系統：
1）管理固有數據的能力，以及
2）高效訪問大量數據的能力。
第一點只是表明現有一個固定存在的數據庫；而這個數據庫的內容也就是DBMS所要訪問和管理的那些數據。第二點將DBMS和同樣能管理固有數據的文件系統區分開來。通常在數據量非常大的時候才需要用到DBMS系統的功能，因為對于小量數據而言，簡單的訪問技術（如對數據的線性掃描）就足夠了。
雖然我們將以上兩點作為DBMS的基本特性，但是其他一些功能也是在商業DBMS系統中常見的，它們是：
·支持至少一種用戶可以據之瀏覽數據的數據模式或數學提取方式。
·支持某種允許用戶用來定義數據的結構、訪問和操縱數據的高級語言。
·事務管理，即對多個用戶提供正確、同時訪問數據庫的能力。
·訪問控制，即限制未被授權用戶對數據的訪問能力，以及檢測數據有效性的能力。
·恢復功能，即能夠從系統錯誤中恢復過來而不丟失數據的能力。
數據模型 每個DBMS提供了至少一種允許用戶不是以原始比特位的方式，而是以更容易理解的術語來觀看信息的抽象數據模型。實際上，通常要觀察以幾個不同級別提取出來的數據是可能的。在相關的低級別中，DBMS一般允許我們將數據形象化為文件的組成部分。#p#
高效數據訪問 存儲一個文件的能力并不特別：操作系統中結合的文件系統都能夠如此。DBMS的能力在我們訪問文件的數據時才能顯露出來。比如，假設我們希望找到員工經理“克拉克·肯特”。如果這個公司有上萬員工，則要通過NAME=“克拉克·肯特”搜索整個文件來找到這個人是非常費時的。而DBMS幫助我們建立“索引文件”或“索引”，不管文件有多大，它都使我們能夠一舉訪問到“克拉克·肯特”的記錄。同樣的，新記錄的插入或者原有記錄的刪除都可以在較短并且本質上恒定的時間內完成，而不依賴于文件的長度。DBMS還可以幫助我們進行文件間的導航，即，通過結合兩個或更多文件的值來獲得我們所需的信息。
查詢語言 為了使訪問文件更容易，DBMS提供了查詢語言（或者說數據控制語言）來表達對文件的操作。查詢語言對用戶所提供的細節的詳細程度要求有所不同，基于關系數據模型的系統通常比基于其他模型的系統所需的細節要少。
事務管理 DBMS的另外一項重要功能就是同時管理大量事務的能力。事務即是數據庫中運行的進程。某些數據庫是如此之大，它們只有在被多臺計算機同時操作時才有用武之地：通常這些計算機分散在全國甚至世界各地。銀行中使用的數據庫系統就是這類數據庫的一個典型，它們兒乎同時被成千上萬的自動取款機所訪問，也同時被同樣多甚至更多的支行員工所訪問。機票預定系統是另一個好例子。
兩個訪問不會互相打擾。舉個例了說，任意多的事務可以同時讀取你銀行的結余而不引起任何沖突。但是如果你正在銀行里查詢工資，與此同時，你的愛人在一臺自動取款機上取款，兩個事務同時發生且沒有彼此協調，那你的查詢結果就很難說了。因此，會引起數據項改變的事務必須“上鎖”，將其他在同一時刻試圖讀寫該項數據的事務關在外面。因此，DBMS必須提供某種并發控制狀態以阻止多個事務對于同一數據項的非協調訪問。
更復雜的問題發生在數據庫分布在許多不同計算機系統上的時候，它們多半使用數據副本來允許高速的本地訪問以及避免由于某臺計算機崩潰而破壞數據。
數據安全 DBMS不只可以在計算機崩潰時保護數據不被丟失，正如上文提到的那樣，而且它還能夠阻止非法訪問。比如，只有擁有特定權限的用戶可以訪問職工文件的工資區域，DBMS還能結合不同用戶的權利來判定他們有權看到哪些文件，文件的哪些區域或者數據庫中數據的哪些子集。因此DBMS必須擁有一個表，表中給出每個用戶對于每個對象的訪問權限。比如，某個用戶可能被允許讀取文件，但不能夠插入或刪除數據；另一個用戶可能根本都不被允許看文件，而第三個用戶可能被允許讀取或隨意修改文件。
DBMS類型
設計人員開發了三種不同類型的數據庫結構：層次數據庫，網狀數據庫以及關系數據庫。層次數據庫和網狀數據庫是首先被開發出來的，但關系數據庫己經成為了主導數據模型。盡管關系數據庫的設計己經成為主導，但舊的數據庫也仍然沒有被拋棄。盡管關系數據庫不斷得到發展，但在20世紀70年代安裝了層次數據庫的一些公司，如IMS，在未來仍然將維持使用這些數據庫。這些舊的數據庫系統通常被稱作遺留系統。
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