System models play an important role in systems development. As a systems analyst or user, you will constantly deal with unstructured problems. One way to structure such problems is to draw models.
A model is a representation of reality.
Models can be built for existing systems as a way to better understand those systems or for proposed systems as away to document business requirements or technical designs. An important concept, in both this chapter and the next, is the distinction between logical and physical models.
Logical models show what a system is or dose. They are implementation independent; that is, they depict the system independent of any technical implementation. As such, logical models illustrate the essence of the system. Popular synonyms include essential model, conceptual model, and business model.
Physical models show not only what a system is or does, but also bow the system is physically and technically implemented. They are implementation dependent because they reflect technology choices. Synonyms include implementation model and technical model.
Let’s say we need a system that can store students’ GRADE POINT AVERAGES. For any given student, the GRADE POINT AVERAGE must be between 0.00 and 4.00 where 0.00 reflects an average grade of F and 4.00 reflects an average grade of A. We further know that a new student does not have a grade point average until he receives his first grade report. Every statement in this paragraph reflects a business fact are constant－they are logical. Thus, GRADE POINT AVERAGE is a logical attribute whit a logical domain of 0.00-4.00 and a logical default value of null (meaning nothing).
今儿过来一看，貌似spaces有些许不一样，于是遂去the space carft看了下，果然更新了
A、可以给照片添加说明，可以添加评论，而且可以一次上传500张照片
B、US, UK, Canada, Germany, France or Japan这些地方的spaces用户可以通过spaces赚钱了，暂时没中国...
C、主题增加了
D、…………
Change is good!
We have some great changes in MSN Spaces.
Have a picture that has a great story around it…you can now add a description to your photos.Your viewers can now add comment to photos as well as blogs. So make sure to go to someone’s photo album and leave a comment on your favorite photo as well. We’ve also gotten rid of that pesky storage limit.You can now upload up to 500 PHOTOS A MONTH!
Great news for those of you living in the US, UK, Canada, Germany, France or Japan. You can now make money with MSN Spaces. You heard me. Make Money! With Amazon.com if you sign up for an associates ID, whenever anyone buys a book from Amazon.com by linking through from your space, you get a portion of that.So make sure to put up all of your favorite and maybe not so favorite books online.
In the same vain if you live in the US or Canada, you can earn money by allowing Kanoodle ads to run in your space.
In love with your brand new Xbox 360! Of course you are. Well now you can show that love by adding your Xbox Recent Games Played and your Xbox Gamercard to your MSN Space. There are even five new themes all about Xbox.
Speaking of themes there are now over 100 to choose from.
We haven’t forgotten blogging either. You can now post entries from three different email addresses instead of just one. For those of you who like to keep your space clean, we have also included multiple check delete. No more going click, delete, click, delete, click, delete. Select all the ones you want to get rid of and click delete…..EASY! We’ve included better blog navigation (i.e. View more entries links and Previous/Next within posts) and you can now choose how you want your blog entries to appear (by date or entry title)
And if all that is still not enough. We’ve beefed up permissions, Search , Integrated Member Directory and included lots of new fun stuff in Profile (General, Social, Contact Info), updated profile module with actionable photo, and included the Live Contacts Beta , which is launching as a feature in MSN Spaces.
We are launching in five new markets (Latin America, South Africa, New Zealand, Malaysia, and Singapore)
We’ve even cleaned up the URL’s. No more /members/
And for you Powertoy users….We have increased the character limit on the HTML Powertoy, AND included new extensions that play in the Windows Media Powertoy.
There is much more that we haven’t mentioned here and still more on the horizon. So keep your eyes peeled, and make sure to give us your feedback!
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