I need to store information for staff. Each database instance is per parent company with multiple outlets underneath. Some of the staff that work at an outlet can potentially also work at other outlets, however as each outlet is for the most part autonomous, each outlet will not want other outlets to see their staff list.
I wanted to create unique staff instances and just relate them to the outlets that employ them, keeping their details uniform across the database. However my colleague wishes to allow each outlet to create their own staff members. The consequence of this approach is that John Smith might be a staff member at outlet A, Jonathan smith at outlet B and J smith at outlet C (as each outlet could enter pretty much whatever they want). Also each staff member has a set of skills and services associated with them, which also will not be uniform between outlets.
Will this approach cause problems down the line? At the outlet level it probably won't make any difference, but I am concerned that if the parent group ask for reports, the results may be misleading as perhaps 5 staff members might be returned, which in reality are the same person, however may have different details.
What you are describing, if I understand you correctly, is choosing between the prospect of denormalizing a person record per outlet (giving each outlet a completely independent copy of John Smith) vs. defining a single John Smith and then defining related tables that could belong to the outlet level of access.
If you have a choice (the freedom to design the system either way) the normalized way with only 1 John Smith + auxiliary tables with outlet-specific details when necessary is the correct way. I hesitate to say 'correct', but in the absence of very large numbers of users I would say denormalization here would only lead to avoidable integrity errors.
If you choose to denormalize now and not relate John Smith at outlet A to John Smith at outlet B even though they are in reality the same person you are both opening the door to illogical data (updates to one John and not both) and losing the ability to do simple things like count the distinct number of people in the database.
Failing to identify unique people now will prevent you from being able to properly relate other entities to a person in the future. This will complicate your queries at the very least and give rise to logically incorrect but technically correct information in many cases.
In addition to the helpful answers (that actually answer the question), here is some useful info:
If these people are staff members (i.e. they have jobs), then why not use their social security number/national insurance number as the unique identifier? That's guaranteed to be unique and they are each guaranteed to have one.
EDIT: US Social Security numbers are guaranteed to be unique and are not reused. See Q20 here: http://www.ssa.gov/history/hfaq.html
EDIT: No they're not! D'oh! Thanks to Mitch Wheat for this link: http://www.dailyfinance.com/2010/08/12/your-social-security-number-may-not-be-unique-to-you/ I guess the task is an impossible one then as there is no real way of solving the problem...
If outlets can't know the stafflist of other outlets, and outlets can add staff, there doesn't seem to be any way of preventing 2 different outlets from adding the same person, but their being recorded as different people in your system. So unless there is some central clearinghouse, or a way for an outlet to see if a person is already listed as a staffmember (w/o getting any details about the outlet(s) she is associated with), I think you're stuck.
Related
Quick question about an ER/EER Diagram.
I have made this Entity Relationship Diagram, but I have been told, that there is something wrong with it by a friend. Is there something wrong with it?
The ER diagram is a design of a Library Management System, where a member can borrow 5 books at a time. The rest of the functionality of the system is how a normal library functions.
Library Management System EER
i don't understand the utility of the relationship between the librarian and the card and i don't understand why the books are splitted in two entities.
I would do 3 entities:
-member
-card
-book
every member has one card, every card is of one member;
every member can take many books, every book can be taken by many members,
the relation between member and book create another table in the logic schema: loans. before inserting a new loan you can check if the member has alredy 5 active loans (by checking the attribute active in the loans table).
Your given context is incomplete for me. I do not see the whole description of your problem/situation, so I will answer based on assumptions, and the experience I had during my life. So let's see...
The tino user questioned the existence of two entities, title and volume, which is something important. Let me explain this for a moment, which will eliminate this as an error. Previously (a time ago) we had video rental stores (I don't know if this the right name where you live, english is not my native language). Remember? We used to go there to rent VHS tapes to watch at home.
What we rented were not films, but more copies/midia of them. A film will always have the same actor, director, title, etc., but a copy could have different attributes/properties, like the year that the media was manufactured, the available languages, the expiration year, among other things. So we had distinctly two different things.
But despite this, we have to consider whether there is a need to create two entities for persistence. We have to remember if we need to persist this information. If a copy/midia has no attributes, then it's entity should not exist, and what a user would rent really would be the movies titles.
In your case, the relationship between volume and title, I belive, is really expressing this discrepancy.
Let's talk about the relationship between librarian and title. What a librarian manages? Does It manages the titles that never change and are abstract things, or the physical objects present in the library? :)
Finally, let's talk about the borrows relationship. When we break down 1-N (or N-1) relationships, we always pass the primary key from the 1 side to the N side, solving the relationship to the formation of the Physical Model in a Entity-Relationship Diagram.
Despite this relationship here is a 0-5, to decompose it, we will not have exactly a 0-5 relationship. We would have in anyway to pass the primary key from the two sides to the table formed by this relationship. Therefore, here we have initially a N-N relationship between member and volume.
N-N relationships allow optional relations between entities. This means we can have the zero side cardinality here. To limit the number of books that can be rented, you need to implement a restriction/constraint with SQL, or with any procedural language in your database. In this case, you can implement a before insert trigger. This trigger has a duty to verify this restriction to allow or denny the completion of the operation as a whole.
Let it be clear that I'm not saying you should remove this notation. Your Conceptual Model should express it. But when you are decomposing, you have to remember that. I think you should just correct it.
Remember one important rule: Relations that have attributes/properties (the attributes/properties) can only exist in N-N relationships. If you have to put attributes/properties in a 1-N (or a N-1) relation, they (the attributes/properties) will always be on the N side. In summary, there are no N-1 (or 1-N) relationships with attributes in the relation. Only N-N relations can have attributes/properties. So be careful with this.
Any questions or clarification, please comment and I will answer.
I see no reason to distinguish member and card. Volume and Librarian don't have primary keys. Are they supposed to be weak entities? That doesn't make sense for Librarian and Volume needs an identifier to distinguish different copies.
I have to design a database for an advert site.
Until now this is the design i came up with:
Administrator(Id,Name,Password)
Advert(Advert_Id,Title,Description,Category,User,Date_created,Picture,Type)
User(User_Id, Name ,Phone,email,Address)
Category(Cat_id,Cat_Name)
Type(Id,Type_Name)
Picture(Picture_Id,Name)
The administrator refers to to the person that will manage the site.
The type refers to the type of the advert: selling, buying etc.
I must have minimum 5 tables at least 2 one to many relationships and at least one many to many relationship. The problem is that I can't find a many to many relationship that would make sense.
If an advertisement can have more than one category, then advertisements and category would be many to many.
If an ad can have more than one picture and a picture can be used in more than one ad, ads and pictures would be many to many.
However, you have to know the business rules. None of us will.
Like Dan mentioned on the answer, you have to know the Business Rules. It's a project for a class, so the business rules might be flexible, I remember back in college, for example, they would tell us that if a client wanted to have advertisement be of at least one category, but could have up to 3 or 4 different categories, that changes your DB structure a lot.
An advertisement can also have many pictures, not just one. It can even have some videos, and many of them as well. Make sure you clarify well the Business Rules and that you understand them fully, as in a real life situation, that can represent either the success of your project or the failure of it.
I'm new to database design and have some uncertainties about how best to model this particular case. I'd appreciate any suggestions for this fairly simple scenario.
When a production task begins, two people are involved at all times. One is in charge of the production, and a second is tasked with quality assurance. For any task in the database, it must be possible to identify these two people. They'll both exist in a Person table and have IDs, so I just want the best way to relate them to the production task. The following rules exist:
Either person may be swapped out for a different person at any time.
Each task always involves both people (Neither of these are null).
There are never any other people involved in the task that we want to record.
Each person may be involved in multiple tasks, or none at all.
If we had a whole host of relationships between the task and the people, I'd create some sort of convoluted relationship structure describing their relationship (As producer, quality assurance person, overseer, etc.), but here I feel as though it's sensible to just stick the IDs of the two people in the Task table, in separate columns for Production Person and Quality Assurance Person. Is this bad for some reason that I can't see?
What has really prompted my question is that I'm trying to design exactly that in DBDesigner 4, which I'm new to, and it just doesn't like it - When I try to set up a second non-identifying relationship between Task and Person, it won't give me a second field. It also won't seem to let me rename the fields in Task that refer to the persons, so it'd be impossible to differentiate between the two anyway. Since no-one else seems to share this problem, I've began to wonder whether it's a good idea at all. Is it standard to introduce additional tables as soon as there are two or more links between two entities? What would that look like if I wanted to enforce the above rules? I can't see how I'd ensure that an n:m table always has entries for both people working on the task.
If you are confident your requirements will stay this rigid forever, then just create two NOT NULL FKs:
This declaratively enforces that exactly two people are associated to the task at all times, which would not be readily achievable with just the junction table (as you already noted).
OTOH, if you anticipate your requirements might change at some point in the future, then the added flexibility of junction table might be more important than the completely declarative enforcement of your business rules.
I'm not familiar with DBDesigner, and therefore with your particular problem, but in ER modeling in general, multiple relationships with the same entity are distinguished by their "rolenames" which determine the names of migrated attributes (see the section on "Rolenames" in the chapter 3 of the ERwin Methods Guide). Try locating something along those lines in the UI of your tool.
If you want to know the current state and not who held the role previously #Branko Dimitrijevic's solution will work.
But if the statement 'Either person may be swapped out for a different person at any time' implies you need to know who previously held that role consider a 3 table design
Task; TaskID, <other details>
Assignee; TaskID, PeopleID, role, start_date, end_date
People; PeopleID, <other details>
Then in the assignee table you need constraints to ensure that for each TaskID, Role combination the dates are reasonable e.g. dates don't overlap or have gaps. That you have only 1 of each role active for each task at a time. To manage this would probably require code either in triggers or the application.
I have a table named people
Each person can be a client, a manager, an accountant, or any combination of the three.
Also each of them have special table columns, besides the ones in the people table.
What I'm doing now is using a person_id in each of the tables... but I think it would be much simpler to just used the same table, and a different model for each one, so I can manage them separately.
Should I do that?
You don't have a nice inheritance hierarchy so I don't think STI applies. For example, how would you represent a person that was both a manager and an accountant in terms of (single) inheritance?
If a person could only have one of the three roles then maybe STI would make sense; but even then implementing roles using inheritance should be setting off warning bells in your head, you should know that one person will end up needing multiple roles sooner or later (and it will probably become a critical necessity immediately after delivery).
Update Edited to reflect clarifications requested by Chris Holmes below. Originally I was referring to a Terminal as a Site but changed it to better reflect my actual domain.
At heart, I think this is a question about modeling a many to many relationship between two root entities where the relationship itself carries some semantic meaning.
In my domain
You can think of a Terminal as a branch location of our company
A Terminal can have a relationship with any number of customers
A customer can have a relationship with any number of terminals (standard many to many)
A customer\terminal relationship means that a customer can potentially store products at the Terminal
This relationship can be enabled\disabled. To be disabled merely means you are temporarily not allowed to store product, so a disabled relationship is different from no relationship at all.
A customer can have many Offices
A Terminal that has a relationship with a customer (enabled or not) must have a default office for that customer that they communicate with
There are some default settings that are applied to all transactions between a Customer and a Terminal, these are set up on a Terminal-Customer Relationship level
I think my objects here are pretty clear, Terminal, Customer, Office, and TerminalCustomerRelationship (since there is information being stored specifically about the relationship such as whether it is enabled, default office, ad default settings). Through multiple refactorings I have determined that both Terminal and Customer should probably be aggregate roots. This leaves me with the question of how to design my TerminalCustomerRelationship object to relate the two.
I suppose I could make the traversal from Terminal to TerminalCustomerRelationship unidirectional toward the relationship but I don't know how to break the relation from the relationship to the customer, especially since it needs to contain a reference to an Office which has a relationship to a Customer.
I'm new to this stuff and while most of DDD makes perfect sense I'm getting confused and need a fresh outlook. Can someone give me their opinion on how to deal with this situation?
Please note that I say Relationship not relation. In my current view it deserves to be an object in the same way that a Marriage would be an object in an application for a wedding chapel. Its most visible purpose is that it relates two objects, but it has other properties that rightfully belong to it as well.
By your description, you definitely need a "TerminalCustomerRelationship" entity to track the associated information. I would also convert the 'IsEnabled' flag into a first class 'Event' entity with a timestamp - this gives you the ability to save a history of the state changes (a more realistic view of what's happening in the domain.)
Here's a sample application (in VS2008) that refects your problem. You can tweak/test the code until the relationships make sense. Run "bin/debug/TerminalSampleApp.exe" and right-click "Terminal->Create Example" to get started.
Let me know if you find it useful.
Names can often clarify an object's responsibilities and bring a domain model into focus.
I am unclear what a Site is and that makes the entire model confusing, which makes it difficult for me to offer better advice. If a Site were a Vendor, for instance, then it would be easy to rename SiteCustomerRelationship as a Contract. In that context it makes perfect sense for Contract to be its own entity, and have the the model look like Vendor-Contract-Customer-Office.
There are other ways to look at this as well. Udi has a decent post on this sort of many-to-many relationship here.
You should not have a object Like SiteCustomerRelationship, its DB specific.
If its truly DDD you should have a Relation like:
Aggregate<Site> Customer.Site
IEnumerable<Aggregate<Office>> Customer.Offices
and perhaps
Aggregate<Office> Customer.DefaultOffice