问题
I am in the process of designing this E-R diagram for a shop of which I have shown part of below (the rest is not relevant). See the link please: E-R diagram
The issue that I have is that the shop only sells two items, Socks and Shoes. Have I correctly detailed this in my diagram? I'm not sure if my cardinalities and/or my design is correct. A customer has to buy at least one of these items for the order to exist (but has the liberty to buy any number).
The Shoe and Sock entities would have their respective ID attribute, and I am planning to translate to a relational schema like this:
(I forgot to add to my diagram the ORDER_CONTAINS relationship to have an attribute called "Quantity". )
Table: Order_Contains
ORDER_ID | SHOEID | SOCKID | QTY
primary key | FK, could be null |FK, could be null | INT
This clearly won't work since the Qty would be meaningless. Is there a way I can reduce the products to just two products and make all this work?
回答1:
Having two one-to-many relationships combined into one with nullable fields is a poor design. How would you record an order containing both shoes and socks - a row per shoe with SOCKID set to NULL and vice-versa for socks, or would you combine rows? In the former case the meaning of QTY is clear though it depends on the contents of SHOEID/SOCKID fields, but what would the QTY mean in the latter case? How would you deal with rows where both SHOEID and SOCKID are NULL and the QTY is positive? Keep in mind Murphy's law of databases - if it can be recorded it will be. Worse, your primary key (ORDER_ID) will prevent you from recording more than one row, so a customer couldn't buy more than one (pair of) socks or shoes.
A better design would be to have two separate relations:
Order_Socks (ORDER_ID PK/FK, SOCKID PK/FK, QTY)
Order_Shoes (ORDER_ID PK/FK, SHOEID PK/FK, QTY)
With this, there's only one way to record the contents of an order and it's unambiguous.
回答2:
You have not explained very well the context here. I'll try to explain from what I understand, and give you some hints.
Do your shop only and always (forever) sell 2 products? Do the details of these products (color, model, weight, width, etc...) need to be persisted in the database? If yes, then we have two entities in the model, SOCKS and SHOES. Each entity has its own properties. A purchase or a order is usually seen as an event on the ERD. If your customers always buys (or order) socks with shoes, then there will always be a link between three entities:
CLIENTS --- SHOES --- SOCKS
This connection / association / relationship is an event, and this would be the purchase (or order).
If a customer can buy separate shoes and socks, then socks and shoes are subtypes of a super entity, called PRODUCTS, and a purchase is an event between CUSTOMERS and PRODUCTS. Here in this case we have a partitioning relationship.
If however, your customers buy separate products, and your store will not sell forever only 2 products, and details of the products are not always the same and will not be saved as columns in a table, then the case is another.
Shoes and socks are considered products, as well as other items that can be considered in future. Thus, we have records/rows in a PRODUCTS table.
When a customer places an order (or a purchase), he (she) is acquiring products. There is a strong link between customers and products here, again usually an event, which would be the purchase (or a order).
I do not know if you do it, but before thinking of start a diagram, type the problem context in a paper or a document. Show all details present in the situation.
The entities are seen when they have properties. If you need to save the name of a customer, the customer's eye color, the customer's e-mail, and so on, then you will have certainly a CUSTOMER entity.
If you see entities relate in some way, then you have a relationship, and you should ask yourself what kind of relationship these entities form. In your case of products and customers, we have a purchasing relationship there between. The established relationship is a purchase (or an order, you call it). One customer can buy various products, and one product (not on the same shelf, is the type, model) can be purchased for several customers, thus, we have a Many-To-Many relationship.
The relationship created changes according to the context. Whatever, we'll invent something crazy here as examples. Say we have customers and products. Say you want to persist a situation where customers lick Products (something really crazy, just for you to see how the context says the relationship).
There would be an intimate connection between customers and products entities (really close... I think...). In this case, the relationship represents a history of customers licking products. This would generate an EVENT. In this event you could put properties such as the date, the amount of times a customer licked a proper product, the weather, the time, the traffic light color on the street, etc., only what you need to persist according to your context, your needs.
Remember that for N-N relationships created, we need to see if new entities (out of relationship) will emerge. This usually happens when you are decomposing the conceptual model to the logical model. Probably, product orders will generate not one but two entities: The ORDER and the products of orders. It is within the products of orders that you place the list of products ordered from each customer, and the quantity.
I would like to present various materials to study ERD, but unfortunately they are all in Portuguese. I hope I have helped you in some way. If you want to be more specific about your problem, I think I can really help you best. Anything, please ask.
来源：https://stackoverflow.com/questions/35391046/e-r-diagram-confusion