Considering that this community has questions related to modeling databases, I am here seeking help.
I'm developing an auction system based on another one seen in a book I'm reading, and I'm having trouble. The problem context is the following:
In the auction system, a user makes product announcements (he/she defines a product). It defines
the product name, and the initial offer (called initial bid). The initial bid expresses the minimum amount to be offering. A
product only exists in the database when it is announced by a user. A
user defines a number of products, but a product belongs to a single
user.
Every product has an expiration date. When a certain date arrives, if
there are no offers for the product, it is not sold. If there are
offers for the product, the highest bidder wins the given product.
The offers have a creation date, and the amount offered. An offer is
made to a product from a user. A user can make different offers for
different products. A product can be offered by different users. The
same user can not do more than two offers for the same product.
This kind of context for me is easy to do. The problem is that I need to store a purchase. (I'm sorry, but I do not know if the word is that in English). I need a way to know which offer was successful, and actually "bought" a product. Relative to what was said, part of my Conceptual Model (Entity Relationship Diagram) is as follows:
I've tried to aggregate USERS with PRODUCTS, and make a connection/relationship between the aggregation and PRODUCTS, which would give me the PURCHASES event. When this was broken down (decomposed) I would have a table showing which offer bought what product.
Nevertheless, I would have a cardinality problem. This table would have two foreign keys. One for BIDS, and the other for PRODUCTS. This would allow an N-N relationship of these two, meaning that I could save more than one bid as the buyer of a product, or that the same bid could "buy" many products (so I say in the resulting PURCHASES table).
Am I not seeing something here? Can you guys help me with this? Thank you for reading, for your time, and patience. And if you need some more detail, please do not hesitate to ask.
EDIT
The property "Initial Bid" on the PRODUCTS entity is not a relationship.
This property represents a monetary value, a minimum amount that an offer must have to be given to a particular product.
You are approaching things backwards. First we determine a relevant application relationship, then we determine its properties. Given an application relationship and the possible application situations that can arise, only certain relationship/association sets/tables can arise. A purchase can have only one bid per product and one product per bid. So it is just a fact that the bid:product cardinality of PURCHASES is 1:1. Say so. This is so regardless of what other application relationships you wish to record. If it were a different application relationship between those entities then the cardinality might be different. Wait--USERS_PRODUCTS and BIDS form exactly such a pair of appplication relationships, with user:product being 1:0..N for the former and 0..N:0..N for the latter. Cardinality for each participant reflects (is a property of) the particular application relationship that a diamond represents.
Similarly: Foreign keys are properties of pairs of relationship/association sets/tables. Declaring them tells the DBMS to enforce that participant id values appear in entity sets/tables. Ie {bid} referencing BIDS and {product} referencing PRODUCTS. And candidate keys (of which one can be a primary key) are a property of a relationship set/table. It is the candidate keys of PURCHASES that lead to declaration of a constraint that enforces its cardinality. It isn't many:many on bid:product because "bid BID purchased product PRODUCT at price $AMOUNT on date DATE" isn't many:many on bid:product. Since either its bid or its product uniquely identify a purchase both {bid} and {product} are candidate keys.
Well... I tried to follow the given advices, and also tried to follow what I had previously spoken to do, using aggregation. It seems to me that the result is correct. Please observe:
Of course, a user makes offers for products. A user record can relate to multiple records in PRODUCTS. Likewise, a product can be offered by multiple users, and thus, a record in PRODUCTS can relate to multiple records on USERS. If I'm wrong on this, please correct me.
Looking at purchase, we see that a product is only properly purchased by a single bid. There is no way to say that a user buys a product, or a product purchase itself. It is through the relationship between USERS and PRODUCTS that an bid is created, and it is an bid that is able to buy a product. Thus, it is necessary to aggregate such a relationship, then set the purchase event.
We must remember that only one product can be purchased for a single bid. So here we have a cardinality of 1 to 1. Decomposition here will ask discretion of the data modeler.
By decomposing this Conceptual Model, we will have the following Logic Model:
Notice how relationships are respected with appropriate attributes. We can know who announced a product (the seller), and we can know what offers were made to it.
Now, as I said before, there is the PURCHASES relationship. As we have a relationship of 1 to 1 here, the rule tells us that we must choose which side the relationship will be interpreted (which entity shall keep such a relationship).
It is usually recommended to keep the relationship on the side that is likely in the future to become a "many". There is no need to do so in this case because it is clear that this relationship can not be preserved in a BIDS record. Therefore, we maintain such a relationship portrayed by "Winner Bid" on the PRODUCTS entity. As you can see, I set a unique identifier for BIDS. By defining the Physical Model, we have a surrogate key.
Well, I finish my answer here, but I think I can not consider it right yet. I would like someone to comment anything if possible. Thank you.
EDIT
I'd like to apologize. It seems that there was a mistake on my part. Well, I currently live in Brazil, and here we learn the ER-Diagram in a way that seems to me to be different from what many of you are used to. Until yesterday I've been answering some questions related to the subject here in the community, and I found odd certain different notations used. Only now I'm noticing that we are not speaking the same language. I believe it was embarrassing for me in a way. I'm sorry. Really, I'm sorry.
Oh, and one more thing (it may be interesting for someone):
The cardinality between BIDS and PRODUCTS is really wrong. It should
be 0 to 1 in both, as was said in the comments.
There are no relationships relating with relationships here. We have
what is called here (in my country, or in my past course)
"Aggregation" (Represented in the first drawing by the rectangle with clipped lines). The internal relationship (BIDS) when decomposed will
become an entity, and the relationship between BIDS and PRODUCTS is
made later. An aggregation says that a relationship needs to be resolved first so that it can relate with another entity. This creates a certain restriction, which may be necessary in certain business rules. It says that the joining of two entities define records that relate to records of another entity.
Certain relationships do not necessarily need to be named. Once you
break down certain types of relationships, there is no need (it's
optional) to name them (between relations N to N), especially if new
relationship does not arise from these. Of course, if I were to name the white relationships presented, we then would have USER'S BIDS (between USERS and BIDS), and PRODUCT'S BIDS (between BIDS and PRODUCTS).
With respect to my study material, it's all in portuguese, and I believe you will not understand, I do not know. But here's one of the books I read during my past classes:
ISBN: 978-85-365-0252-6
Title: PROJETO de BANCO de DADOS Uma Visão Prática
Authors: Felipe Machado, Mauricio Abreu
Publishing company: EDITORA ÉRICA
COVER:
LINK:
http://www.editorasaraiva.com.br/produtos/show/isbn:9788536502526/titulo:projeto-de-banco-de-dados-uma-visao-pratica-edicao-revisada-e-atualizada/
Well... What do I do now? Haha... I do not know what to do. I'll leave my question here. If someone with the same method that I can help me, I'll be grateful. Thank you.
To record the winning bid requires a functional dependency Product ID -> Bid ID. This could be implemented as one or more nullable columns (since not all products have been purchased yet) in the Products table, or better in a separate table (Purchases) using Product ID as primary key.
Related
This is an problem about drawing ERD in one of my course:
A local startup is contemplating launching Jungle, a new one stop
online eCommerce site.
As they have very little experience designing and implementing
databases, they have asked you to help them design a database for
tracking their operations.
Jungle will sell a range of products, and they will need to track
information such as the name and price for each. In order to sell as
many products as possible, Jungle would like to display short reviews
alongside item listings. To conserve space, Jungle will only keep
track of the three most recent reviews for each product. Of course, if
an item is new (or just unpopular), it may have less than three
reviews stored.
Each time a customer buys something on Jungle, their details will be
stored for future access. Details collected by Jungle include
customer’s names, addresses, and phone numbers. Should a customer buy
multiple items on Jungle, their details can then be reused in future
transactions.
For maximum convenience, Jungle would also like to record credit card
information for its users. Details stored include the account and BSB
numbers. When a customer buys something on Jungle, the credit card
used is then linked to the transaction. Each customer may be linked to
one or more credit cards. However, as some users do not wish to have
their credit card details recorded, a customer may also be linked to
no credit cards. For such transactions, only the customer and product
will be recorded.
And this is the solution:
The problem is the Buys action connect with 3 others entities: Product, Customer, and Card. I find this very hard to read and understand.
Is an action involving more than 2 entities common in production? If it is, how should I understand and use it? Or if it's not, what is the better way of design for this problem?
While the bulk of relationships in practice are binary relationships, ternary and higher relationships are normal elements of the entity-relationship model. Some examples are supplies (supplier_id, product_id, region_id) or enrolled (student_id, course_id, semester_id). However, they often get converted into entity sets via the introduction of a surrogate identifier, due to dislike of composite keys or confusion with network data models in which only directed binary relationships are supported.
Reading cardinality indicators on non-binary relationships are a common source of confusion. See my answer to designing relationship between vehicle,customer and workshop in erd diagram for more info on how I handle this.
Your solution has some problems. First, Buys is indicated as an associative entity, but is used like a ternary relationship with an optional role. Neither is correct in my opinion. See my answer to When to use Associative entities? for an explanation of associative entities in the ER model.
Modeling a purchase transaction as a relationship is usually a mistake, since relationships are identified by the (keys of the) entities they relate. If (CustomerID, ProductID) is identifying, then a customer can buy a product only once, and only one product per transaction. Adding a date/time into the relationship's key is better, but still problematic. Adding a surrogate identifier and turning it into a regular entity set is almost certainly the best course of action.
Second, the Crow's foot cardinality indicators are unclear. It looks like customers and products are optional in the Buys relationship, or even as if multiple customers could be involved in the same transaction. There are three different concepts involved here - optionality, participation and cardinality - which should preferably be indicated in different ways. See my answer to is optionality (mandatory, optional) and participation (total, partial) are same? for more on the topic.
A card is optional for a purchase transaction. From the description, it sounds as if cards may participate totally, meaning we won't store information about a card unless it's used in a transaction. Furthermore, only a single card can be related to each transaction.
A customer is required for a purchase transaction, and it sounds like customers may participate totally, meaning we won't store information about customers unless they purchase something. Only a single customer can be related to each transaction.
Products are required for a purchase transaction, and since we'll offer products before they're bought, products will participate partially in transactions. However, multiple products can be related to each transaction.
I would represent transactions for this problem with something like the following structure:
I'm not saying converting a ternary or higher relationship into an entity set is always the right thing to do, but in this case it is.
Physically, that would require two tables to represent (not counting Customer, Product, Card or ProductReview) since we can denormalize TransactionCustomer and TransactionCard into Transaction, but TransactionProduct is a many-to-many relationship and requires its own table (as do ternary and higher relationships).
Transaction (TransactionID PK, TransactionDateTime, CustomerID, CardID nullable)
TransactionProduct (TransactionID PK, ProductID PK, Quantity, Price)
I have been studying datawarehouse in the last couple days, particularly, i have been reading The Data Wharehouse Toolkit - The Definitive Guide to Dimensional Modeling by Kimball and Ross.
Uppon that reading, i came to the 1st exapmle where there is a sales fact and it related to a product dimension, as you can see in the bellow image:
I think i can grasp the gist of how this relationship allows us to rotate the "cube" slicing and dicing data, however this is where i get lost:
In this example and many others on the web product is a one-to-one relationship with sales, which is fine i guess for most cases. But this generates a sales registtry for at least each kind of product that was in one sale.
So supposing i bought 1 banana, 2 apples and 1 orange, this would yield at least 3 sales registry. Again, which is fine i guess as it is storing the sale's ticket ID in the sales fact, we still can relate all itens in a given sale.
However if this was an use case: relate products on sales say i want to get every sale that had a banana and get stuff like: how many items each of these sales had, their price cost, their profit, stuff like that...
Wouldn't be better if the fact-product relation were Fact-one_to_many-Product relationship? Where fact would hold the sale's ticket ID and products would have its foreign key referencing where they are from or something?
I reckon these metrics should be in the fact table, and not in the product table as i think i would want. So, is this me not fighting my urge to normalize it or does it make sense in the way i would want to do that kind of filtering -> [given all sales with X product, get data from other products in the same sale].
If i were to follow the guidelines, product dimension would have one registry for every exclusive kind of product the store would have correct? And all the measurements i want i would store it on the fact itself, like price cost, sales price, profit, etc...
On the other hand, if i were to one-to-many product dimension would have many copies of each product. Which is bad, i think. However, i think it would give me better queries in that regard.
As you can see, i'm a beginer and really in the early stages of this path, so if you would endulge me in a Explain Like I'm Five kind of answer I would appreciate.
EDITED:
Sorry #Nick.McDermaid, you are right. I meant from the perspective of the sales fact where for every sale fact i will have only one product, but are correct that for one product it can have N sales related. And so, we have one record of product in the database for every different product on our store. This is the right way to do it, how to rightfully model it. Also, the many indicator is the "sales quantity" i'm guessing.
Anyhow, while this allows for slicing and dicing when/if we have sales as the point of view, but what if i want to for example:
Get all sales that had a banana in it, with all the other items in those sales. We can still do it with this structure but its harder than if the products were repeated and we had the sale id as a foreign key in the product table.
Cuz ultimetly i want to get all the sales(and products within that sale) that had a banana. And then take metrics out of them.
What you are somewhat hinting at would be a degenerate dimension, consisting of the sales id/invoice #/purchase order # of the transaction that took place. The whole purpose of a degenerate dimension is to group items that are related by a meaningless piece of data. For example, a PO # of A1234 is meaningless on its own, it doesn't tell you anything about the purchase. However, it can be used to identify other meaningful data, such as the date of purchase of the products for the customer. In that context, the PO # is defined by the collection of the entities it brings together to describe an event.
Another critical concept in data-warehousing is the abstraction of the schema in the database from the model in the cube. You don't join and group data in a cube model. You slice and filter. There are no foreign keys in a cube model. Those are used in the underlying data schema, but all of that work is handled behind the scenes of the cube model.
I was a little miffed about the one-to-one relationship explanation on the 'I Think You Mean A Many To One' article.
In this instance for example, a product has one price because the business in question is small, niche, localized and supports only a single currency. Multiple prices per product make no sense in this case? I'm doubtful I'm grasping the concept correctly though, because everywhere I read says it will probably be a many-to-one even if you think it isn't?
Can somebody enlighten me please? :)
In an attempt to gain more reputation so that I can help in comments instead of an "answer" The one-to-many vs one-to-one is this
View a one-to-one as an extension of the table you are looking at.
Table B extends Table A. Meaning the information wasn't necessarily relevant enough to include in the table directly, but has a bidirectional relationship with each other. Basically meaning that As Table A, I am not dependent on the information in Table B, but Table B's information is very dependent on me. For the price example it means that Table A has a row related to a row in table B. So if you entering unique information in your Price table around every item to match in Table A, then this would be useful. As in say you had a description column about the item in your price table. Otherwise the price table in this case may just be irrelevant to have in the schema.
in a one-to-many relationship Table B usually has no reference back to Table A. So in the case of price, the items you are looking at do have a price, but prices aren't exclusive to items. So to better define, A number of things may have the price 9.99, but 9.99 only needs to exist in your pricing table once.
I am not familiar with the article you refer to. However, price is a classic example of a slowly changing dimension. Price may be constant at any point in time, but over time, the price changes.
Such dimensions are typically implemented by having effective and end dates for the period in question.
Now, at a given point in time, a product probably does have only one price. Things that affect the price -- coupons, discounts for the purchaser, volume discounts, for example -- are not properties of the product. These are properties of the transaction.
That said, there may be circumstances where a fixed volume discount does not make sense. So, the "price" for a product might include volume, as well as time.
In any case, I would agree with you that price is not a good example of a 1-1 relationship. There are other factors such as time and volume that affect it.
Using the snowflake schema image from wikipedia:
http://en.wikipedia.org/wiki/File:Snowflake-schema-example.png
Would it ever make sense to have a "Brand_Id" foreign key in Fact_Sales as you do in Dim_Product? There is a many-to-one relationship of sales/brands just like sales/products or products/brands, so is there any logical reason not to? You may want to join directly to the Dim_Brand table.
I'm probably not seeing something obvious.
The type of relationship you're looking at is a has-a relationship.
A product has a brand. A sale has a product; it's the thing that was sold. But a sale does not have a brand. Or, a better way of saying this, you cannot sell a brand. (don't read too far into that one...)
So, no, you wouldn't want to add brand to sales.
If you are working in a dimensional model (the Star/Snowflake schema note in your question makes be think you are), then adding the BRAND_ID to the sales fact makes sense from a performance perspective, if the questions that the business is trying to answer are "what were the sales for brand X across all products in this time frame".
It also may be useful if the product dimension is a Type 1 SCD, and a product changes brands. You may want to preserve the prior sales as being of the "old" brand.
Keep in mind you are not doing entity - relationship modeling when you build a star/snowflake reporting schema. Questions of is-a or has-a aren't pertinent to a dimensional model.
I think that would be nice as a way to cache the data... but in all honesty, your probably better off just relying on the links as they are.
The reasoning is that you already have that definition of what that table does, store sales. To add in what brand those products are that the store sold is going to muddy the 'topic' or 'theme' of that table, recording sales of a store.
Now if by some way you had a product that can be sold under different brands (heck if I know how a package can have split personalities...) then yea, it would make sense to a degree, but a more reasonable solution is to give each product it's own SKU code then.
There are couples of questions around asking for difference / explanation on identifying and non-identifying relationship in relationship database.
My question is, can you think of a simpler term for these jargons? I understand that technical terms have to be specific and unambiguous though. But having an 'alternative name' might help students relate more easily to the concept behind.
We actually want to use a more layman term in our own database modeling tool, so that first-time users without much computer science background could learn faster.
cheers!
I often see child table or dependent table used as a lay term. You could use either of those terms for a table with an identifying relationship
Then say a referencing table is a table with a non-identifying relationship.
For example, PhoneNumbers is a child of Users, because a phone number has an identifying relationship with its user (i.e. the primary key of PhoneNumbers includes a foreign key to the primary key of Users).
Whereas the Users table has a state column that is a foreign key to the States table, making it a non-identifying relationship. So you could say Users references States, but is not a child of it per se.
I think belongs to would be a good name for the identifying relationship.
A "weak entity type" does not have its own key, just a "partial key", so each entity instance of this weak entity type has to belong to some other entity instance so it can be identified, and this is an "identifying relationship". For example, a landlord could have a database with apartments and rooms. A room can be called kitchen or bathroom, and while that name is unique within an apartment, there will be many rooms in the database with the name kitchen, so it is just a partial key. To uniquely identify a room in the database, you need to say that it is the kitchen in this particular apartment. In other words, the rooms belong to apartments.
I'm going to recommend the term "weak entity" from ER modeling.
Some modelers conceptualize the subject matter as being made up of entities and relationships among entities. This gives rise to Entity-Relationship Modeling (ER Modeling). An attribute can be tied to an entity or a relationship, and values stored in the database are instances of attributes.
If you do ER modeling, there is a kind of entity called a "weak entity". Part of the identity of a weak entity is the identity of a stronger entity, to which the weak one belongs.
An example might be an order in an order processing system. Orders are made up of line items, and each line item contains a product-id, a unit-price, and a quantity. But line items don't have an identifying number across all orders. Instead, a line item is identified by {item number, order number}. In other words, a line item can't exist unless it's part of exactly one order. Item number 1 is the first item in whatever order it belongs to, but you need both numbers to identify an item.
It's easy to turn an ER model into a relational model. It's also easy for people who are experts in the data but know nothing about databases to get used to an ER model of the data they understand.
There are other modelers who argue vehemently against the need for ER modeling. I'm not one of them.
Nothing, absolutely nothing in the kind of modeling where one encounters things such as "relationships" (ER, I presume) is "technical", "precise" or "unambiguous". Nor can it be.
A) ER modeling is always and by necessity informal, because it can never be sufficient to capture/express the entire definition of a database.
B) There are so many different ER dialects out there that it is just impossible for all of them to use exactly the same terms with exactly the same meaning. Recently, I even discovered that some UK university that teaches ER modeling, uses the term "entity subtype" for the very same thing that I always used to name "entity supertype", and vice-versa !
One could use connection.
You have Connection between two tables, where the IDs are the same.
That type of thing.
how about
Association
Link
Correlation