"O" is overlapping, meaning there could be overlapping between each subclass entities. (A manager could also be a salesperson)
Besides, we can use "D" for disjoint subclasses.
成都创新互联是一家集网站建设,嘉荫企业网站建设,嘉荫品牌网站建设,网站定制,嘉荫网站建设报价,网络营销,网络优化,嘉荫网站推广为一体的创新建站企业,帮助传统企业提升企业形象加强企业竞争力。可充分满足这一群体相比中小企业更为丰富、高端、多元的互联网需求。同时我们时刻保持专业、时尚、前沿,时刻以成就客户成长自我,坚持不断学习、思考、沉淀、净化自己,让我们为更多的企业打造出实用型网站。
A single line between Employee and its subclasses implies an optinal participation (an employee may not belong to any subclass), while a double line implies a mandatory participation (an employee must be in one or more subclasses).
An U-like symbol identify subclasses by pointing to them. (the entity the bottom of "U" points to is a subclass)
Schemarepresents abstract (strong and weak) entities and associated attributes and necessary relationships between entities.
A schema looks like this:
STUDENT(
Stu_num, Stu_Lname, Stu_Fname, program_num@, age, gender)
(PK "Stu_num" should be underscored. I use italic instead)
Data dictionarycontains metadata of attributes, with which you could understand the what, where, how, how much, who, when questions about data.
A good logical model should follow normal formof database.
UNF
Contains repeating values.
1NF
Each cell in the table contains only one value (so that the table can be represented in a relational database).
2NF
Satisfies 1NF;
No partial dependencies.
Meaning: A non-primary-key attribute cannot be dependent on part of primary-key.
For example, in R(
A BC D), if
(A+B) -> C, D,
(B) -> D,
D is partially dependent on primary key (A+B), thus not satisfying 2NF.
3NF
Satisfies 2NF;
No transitive dependencies.
Meaning: A non-primary-key attribute cannot determine non-primary-key attributes. (But a non-primary-key attribute can determine a primary-key attribute)
BCNF
Satisfies 3NF;
No non-CK dependencies.
Another way to say this: For all functional dependencies exist in the table, all their determinants are candidate keys.
Meaning: the second and third normal forms normalize non-prime attributes, and BCNF normalizes relationships between prime attributes.
For example (a model in 3NF but not in BCNF)
In R(
A BC D), there are
(A+B) -> C, D;
(A+C) -> B, D;
(C) -> B.
With a non-primary-key attribute C determines primary-key attribute B, the model does not satisfy BCNF.
标题名称:RelationshipDatabaseDesign
转载来于:https://www.cdcxhl.com/article10/geiddo.html
成都网站建设公司_创新互联,为您提供企业建站、自适应网站、动态网站、静态网站、定制开发、
声明:本网站发布的内容(图片、视频和文字)以用户投稿、用户转载内容为主,如果涉及侵权请尽快告知,我们将会在第一时间删除。文章观点不代表本网站立场,如需处理请联系客服。电话:028-86922220;邮箱:631063699@qq.com。内容未经允许不得转载,或转载时需注明来源: 创新互联