英语翻译This model is more flexible than the hierarchical or network model.In a hierarchical or network database,record definitions are fixed in order to embed linked list data pointers in the records.Since the relational model uses keyed field t

英语翻译This model is more flexible than the hierarchical or network model.In a hierarchical or network database,record definitions are fixed in order to embed linked list data pointers in the records.Since the relational model uses keyed field t
英语翻译
This model is more flexible than the hierarchical or network model.In a hierarchical or network database,record definitions are fixed in order to embed linked list data pointers in the records.Since the relational model uses keyed field to relate records,record definitions may be easily changed and new records created.However,this flexibility comes with an integrity and performance price.To ensure integrity,user-defined integrity constraints must be established and checked before processing transactions.In the hierarchical or network model,the constraints are automatically established when the sets are implemented,while in the relational model,the constraints are user-defined in the application program.Since it is so easy to get the system up and running,the design of the user-integrity constraints may not be well thought out in advance and as the database grows in size and complexity,more tables are often added to the system,and therefore more joins are needed to access information in the database.(A join is one of the set operators of relational algebra used to concatenate data found in different tables.) The more joins needed to process a query,the slower the system responds.Another disadvantage for some information management systems based on the relational model is that complex data cannot be adequately described in simple rows and columns.In the LIMS environment,using the relational model to describe complex data,it is not uncommon to do 10 or more record joins to process a single information request.Despite its disadvantages,this model requires less initial planning and design to implement and is the quickest and easiest of the models to set up.It soon became the most recognized and widely accepted database model for microcomputers and minicomputers.The relational model became the panacea for every information storage problem.Whenever changes or additions to a database were needed,new tables were added or existing tables extended,often without determining the performance impact on the as information systems system.Consequently,changed over time,the performance often degraded dramatically.

英语翻译This model is more flexible than the hierarchical or network model.In a hierarchical or network database,record definitions are fixed in order to embed linked list data pointers in the records.Since the relational model uses keyed field t
这种模式是更加灵活的多层次或网络模式.在一个等级或网络数据库,记录定义是固定的,以嵌入链表指针数据的记录.自关联模型利用键控领域涉及记录,记录的定义,可以很容易改变,新的纪录产生.但是,这种灵活性是一个完整的性能和价格.为确保其完整性,用户定义的完整性约束,必须建立和检查后,处理交易.在等级或网络模型,以约束是自动成立时,集实施,而在关系模型中,约束是用户自定义的应用程序.因为它是那么容易,使整个系统建成运行,设计中的用户完整的制约未必是深思熟虑的超前性,为数据库增长 规模和复杂程度,更表往往添加到系统,因此,加入更多的是需要获取信息的数据库.(一加入,是一个集商关系代数用来串数据发现,在不同的表) .的 加入更需要一个过程的质疑,减慢系统响应.另一个不利的一些信息管理系统,基于关系模型,是复杂的数据不能充分描述 简单整齐和栏目.在实验室环境中,利用关联模型来描述复杂的数据,这是不寻常的做10个或更多的记录加入到进程的一个信息请求.尽管有其不利之处,这种模式需要较少的初步规划和设计实践,是最快捷,最简便的模式 成立.它很快就成了最普遍认可和广泛接受的数据库模式微机和微型.关联模型成了灵丹妙药,为每个信息存储问题.每当修改或增加了一个数据库,需要新表中添加或现有表延长 往往没有确定的表现,对信息资源系统.因此,随时间推移,其性能往往急剧退化.