erd

问题 How to show composite keys in Chen E-R Diagram? I have not used this notation and I really don't like it. Any references would suffice, as I have excellent understanding of data modeling concepts (what a brag!). Note: I am aware of the symbol for associative entity, in such a symbol, one may have an auto-increment key as the PK, but then again, one may use the 2 FKs. In the later case, the PK would be composite. I don't know how to show it since Chen attribute symbol is an oval for each

问题 Let's say I have a 2 tables like below : Department : DeptId Name Employee : EmpId Name DeptId Here I have a 0-to-many relationship between Department and Employee table. Now when I read the relationship from Department to Employee , it says 1 department can have 0 or more employees. Does this mean while creating Employee table, I will set DepartmentId (foreign key) as null due to zero or many relationship? And how do we represent 1 or many relationship like for eg If relationship in ERD says

问题 I'm supposed to create a sort of second brand website. When someone wants to sell a product, he has the option to sell it for: a set amount free to bid can anyone give me an assist on how to continue? This is what i have so far, but i'm stuck and don't know how to continue. (sorry for the use of native language, the entities are : member, seller (top one) buyer (bottom one) and products, in that order. any form of help is appreciated! 回答1: I really don’t know where your problem is but I would

问题 Im getting back into database design and i realize that I have huge gaps in my knowledge. I have a table that contains categories. Each category can have many subcategories and each subcategory can belong to many super-categories. I want to create a folder with a category name which will contain all the subcategories folders. (visual object like windows folders) So i need to preform quick searches of the subcategories. I wonder what are the benefits of using 1:M or M:N relationship in this

问题 Im getting back into database design and i realize that I have huge gaps in my knowledge. I have a table that contains categories. Each category can have many subcategories and each subcategory can belong to many super-categories. I want to create a folder with a category name which will contain all the subcategories folders. (visual object like windows folders) So i need to preform quick searches of the subcategories. I wonder what are the benefits of using 1:M or M:N relationship in this

问题 Im getting back into database design and i realize that I have huge gaps in my knowledge. I have a table that contains categories. Each category can have many subcategories and each subcategory can belong to many super-categories. I want to create a folder with a category name which will contain all the subcategories folders. (visual object like windows folders) So i need to preform quick searches of the subcategories. I wonder what are the benefits of using 1:M or M:N relationship in this

问题 Can I improve my design on these 2 diagrams below? If so, how? I am confused by the use of association classes in my diagrams. Should I use them? Figure 1 Figure 2 回答1: For association classes, they make sense only if they represent associations with some behaviour and state. Look at Article'sSubject. It has nothing more than source, target and identity. You don't need a class for such a association, just use plain association, which has all those properties. There are more such unnecessary

问题 I have implemented RBAC (Role Based Access Control) tables. I have a requirement to add extra attribute on some roles, like 2nd_pwd attribute in Admin role, address attribute in Customer role. I have done this design, but it violates data integrity. When you removed (let's say) Admin role, you also need to remove admin data from Admin table. So I decided to add trigger for this job. Question: Is there a design for this requirement without trigger but still doesn't violate data integrity? What

问题 I'm looking for a free or paid software application that I can use for ER modelling. Since I mostly develop applications using Symfony2/Doctrine, does anyone know about software that has Doctrine support? 回答1: I think the following comparison is what you're searching: http://www.orm-designer.com/article/orm-designer-and-mysql-workbench-comparison Conclusion: both ORM Designer and MySQL workbench are able to work with Doctrine. Since MySQL workbench is free and under the wings of MySQL itself

问题 Here is the situation:- I have a table called Users. This contains user data for students and tutors as most of the data required is the same. Having completed the system I am now told that the client would like to be able to assign students to tutors. Is there a legitimate/ clean way I can create a one to many relationship within a single table, perhaps via a link table? I've tried to think this through but whatever solution I come up with seems messy. I would be grateful for any input.