Every time I program I recognize this relationship between classes and tables, or am I imagining it.
You can have a class per database table or a table per class i.e. :
tables: customer, products, order.
classes: customer, products, order, may have methods such as addRecord, deleteRecord, updateRecord.
what is this called? Object-Relational? I am not a DBA.
It all depends on the type of database you're using. If you're using an object oriented database (OODB), then there is no relationship, as the objects and the persisted data are the same thing. For example, if you have a Customer class, and you save it in an OODB, then that instance of the customer is what is stored in the DB.
If you are using a relational database, then the class instances, and the persisted representation of them in the DB, can be the same thing, but many times they aren't. This is because most folks use normalization to represent their data in an efficient way (in a relational DB). This means, instead of having a table per class, you can have a class represented by more than one table. In the Customer example, the tables might now be Customer (with Name, date of birth, and other properties), and Order (with order pointing to products in yet another table). The reason for this has to do with cardinality, and the ability for Customers to have more than one order. When your business logic needs this information from the DB, the data access layer's job is to map the data (called ORM) from the DB into your classes.
If you are using yet another type of DB, then there will be a different relationship between the classes (domain model) and what's persisted in the DB.
But, as far as having a name for this relationship? No, there is no name.
In additon to Bob's answer, the following.
In object modeling, the relationship between classes and subclasses is taken care of by inheritance, and object modelers know how to use inheritance to good advantage. The relational data model and by extension the SQL databases do not implement inheritance for you. You have to design tables to give you some of the same results.
In ER (Entity-Relationship) modeling, the corresponding concept is called generalization/specialization. This tells you how to model a class/subclass relationship, but it doesn't tell you how to design the tables when you go to build your database.
There are three techniques that are pretty well understood that can be really helpful when dealing with classes and subclasses. Here are their tags: single-table-inheritance class-table-inheritance shared-primary-key. Unfortunately, many tutorials on database design never cover these techniques. They can be enormously useful to people who know object modeling and want to come up to speed on relational modeling.
Related
We have a situation in a database design in our company. We are trying to figure out the best way to design the database to store transactional data. I need expert’s advice on the best relational design to achieve it. Problem: We have different kind of “Entities” in our system, for example; Customers, Services, Dealers etc. These Entities are doing transfer of funds between each other. We need to store the history of the transfers in database.
Solutions:
One table of transfers and another table to keep “Accounts” information. There are three tables “Customers”, “Services”, “Dealers”. There is another table “Accounts”. An account can be related to any of the “Entities” mentioned above; it means (and that’s the requirement) that logically there should be a one-to-one relationship to/from Entities and Accounts. However, we can only store the Account_ID in the Entities table, but we cannot store the foreign key of Entities in Accounts table. Here the problem happens in terms of database design. Because if there is a customer’s account, it is not restricted by the database design to not be stored in Services table etc. Now we can keep all transfers in one table only since Accounts are unified among all the entities.
Keep the balance information in the table primary Entities table and separate tables for all transfers. Here for all kind of transfers between the entities, we are keeping separate tables. For example, a transfer between a Customer and Service provider will be stored in a table called “Spending”. Another table will have transfer data for transfer between Service and Dealers called “Commission” etc. In this case, we are not storing all the transfers of the funds in a single table, but the foreign keys are properly defined since the tables “Spending” and “Commission” are only between two specific entities.
According to the best practices, which one of the above given solutions is correct, and why?
If you are simply looking for schemas that claim to deal with cases like yours, there is a website with hundreds of published schemas. Some of these pertain to storing transaction data concerning customers and suppliers. You can take one of these and adapt it.
http://www.databaseanswers.org/data_models/
If your question is about how to relate accounts to business contacts, read on.
Customers, Services, and Dealers are all sub classes of some super class that I'll call Contacts. There are two well known design patterns for modeling sub classes in database tables. And there is a technique called Shared primary Key that can be used with one of them to good advantage.
Take a look at the info and the questions grouped under these three tags:
single-table-inheritance class-table-inheritance shared-primary-key
If you use class table inheritance and shared primary key, you will end up with four tables pertaining to contacts: Contacts, Customers, Dealers, and Services. Every entry in Contacts will have a corresponding entry in one of the three subclass tables.
An FK in the accounts table, let's call it Accounts.ContactID will not only reference a row in Contacts, but also a row in whichever of Customers, Dealers, Services pertains to the case at hand.
This may work outwell for you. Alternatively, single table table inheritance works out well in some of the simpler cases. It depends on details about your data and your intended use of it.
You can make table Accounts with three fields with FK to Customers,Dealers and Services and it's will close problem. But also you can make three table for each type of entity with accounting data. You have the deal with multi-system case in system design. Each system solve the task. But for deсision you need make pros and con analyses about algorithm complexity, performance and other system requirements. For example one table will be more simple to code, but three table give more performance of sql database.
I'm new to data modelling and have started following tutorials to learn more.
I am trying to create a model for a hypothetical scenario and am struggling to validate what I have created to see if it is what would be considered a correct data model.
Essentially all im trying to do is correctly store data in a normalised form. In my scenario there are 3 types of people and each share some attributes and have one set of contact details each.
Does the below data model look feasible?
The relationship between person and one of defendant, magistrate, or staff-member is a case of the class/subclass pattern. There are two common ways of modeling this pattern in relational tables.
One way is called "Class Table Inheritance". You can find out more by visiting this tag: class-table-inheritance or by searching the web for Martin Fowler's treatment of the same subject. Your design resembles this design.
Another way is called "Single Table Inheritance", which you can also research the same way. single-table-inheritance. It's simpler, and works ok in some cases. You deal with fewer joins, but you deal with more NULLS.
Many people who go for class table inheritance also apply a technique called "Shared Primary Key". shared-primary-key. Using this technique, Defendant, Magistrate, and Staff_Member would each use a copy of person_id as the primary key. This primary key also functions as a foreign key. Shared primary key enforces the one-to-one nature of the IS-A relationships that exist in this case.
If you want to go further in data modeling, you might want to learn ER modeling as a distinct data model from the relational model. What you've done here is essentially to use ER diagramming to diagram a relational model. There's nothing wrong with that, but it obscures a whole new field of study, generally called conceptual data modeling.
If you generate an ER model at the conceptual level, you don't attempt to implement it in terms of tables. There is a diagramming convention in ER that goes under the name "generalization/specialization" that allows you to depict a class/subclass situation, while remaining silent on how it's going to be implemented.
Conceptual data models have an area of usefulness, in addition to relational data modeling. What makes conceptual data models useful is precisely the fact that they present the information requirements without stating how those requirements are going to be met.
Once you are proficient at creating conceptual data models, it's not hard to convert one of them to a relational model.
This may be more than you bargained for, but since you are taking on learning modeling, I thought I'd survey some of the field for you.
This is probably a simple problem for an experienced database developer, but I'm struggling... I have trouble translating a certain ER diagram to a DB model, any help is appreciated.
I have a setup similar to slide 17 of this presentation:
http://www.cbe.wwu.edu/misclasses/mis421s04/presentations/supersubtype.ppt
Slide 17 shows an ER diagram with an Employee supertype having an Employee Type attribute and as subtypes the Employee Types themselves (Hourly, Salaried and Consultant), which is very similar to my design situation.
In my case, suppose Salaried Employees are the only ones that can be bosses of other employees and I wanted to somehow indicate if a certain Salaried employee is the boss of the Hourly and/or Salaried Employee and/or Consultant (either, none or both), how could that be designed in a database model, also considering these are one-to-many relationships?
I can put a PK-FK relationship between them, which would result in all tables having two FKeys and (like Consultant having FK_Employee and FK_SalariedEmployee) and SalariedEmployee referencing itself, but I keep thinking that might not be the wisest solution....although I'm not sure why (integrity issues?).
Is this or an acceptable solution or is there a better one?
Thanks in advance for any help!
Your case looks like an instance of the design pattern known as “Generalization Specialization” (Gen-Spec for short). The gen-spec pattern is familiar to object oriented programmers. It’s covered in tutorials when teaching about inheritance and subclasses.
The design of SQL tables that implement the gen-spec pattern can be a little tricky. Database design tutorials often gloss over this topic. But it comes up again and again in practice.
If you search the web on “generalization specialization relational modeling” you’ll find several useful articles that teach you how to do this. You’ll also be pointed to several times this topic has come up before in this forum.
The articles generally show you how to design a single table to capture all the generalized data and one specialized table for each subclass that will contain all the data specific to that subclass. The interesting part involves the primary key for the subclass tables. You won’t use the autonumber feature of the DBMS to populate the sub class primary key. Instead, you’ll program the application to propagate the primary key value obtained for the generalized table to the appropriate subclass table.
This creates a two way association between the generalized data and the specialized data. A simple view for each specialized subclass will collect generalized and specialized data together. It’s easy once you get the hang of it, and it performs fairly well.
In your specific case, declaring the "boss of" FK to reference the PK in the Salaried Employees table will be enough to do the trick. This will produce the two way association you want, and also prevent employees who are not salaried from being referenced as bosses.
CWM is data modeling
UML is object modeling.
Can someone explain the difference that a layman can understand?
Object Model: deals with object oriented "blue-print" of your system. This includes, class diagrams (classes you will be creating), relationship between these classes, methods in the classes, properties etc.
Data model: deals with entities at the database level. Like how the classes in the OM will get stored in the database, in which tables etc. So DM deals with Table schema, relationship between different tables (PKs, FKs) etc.
DM does not have complex OO features like polymorphism, inheritance, overloading etc which are usually listed in an OM.
As a rough example, two classes in the OM can get stored (mapped) to a single Table in the DM, like both Employee and Manager persons can be stored in a single DB table.
Data modeling deals with the design and creation of your database structure, ie. how the data is stored.
Object modeling deals with how the application interacts with the information received from an external source, e.g. an end-user, a database, a web service, etc.
Let's say, for example, you are tracking customer history for the sales department. The department needs the customer's name, address, phone, email, and purchase history.
In the data model, you define the tables and fields that will store each individual piece of data. In that definition, you may include information like the maximum length, data type or whether or not the data is required.
In the object model, in addition to enforcing the rules you set up in the data model, you may also add additional behaviors, such as making sure that the email address is formatted correctly, or capitalizing the first letter of the customer's first and last name. These type of rules tend to be more complex and detailed than the rules set within the data model.
At any rate, the purpose of the object model is to ease the management of the data within the application itself and to perform higher-level validation on the data before it gets sent to the database.
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Closed 10 years ago.
Is it things like tables? Or would it also include things like constraints, stored procedures, packages, etc.?
I've looked around the internet, but finding elementary answers to elementary questions is sometimes a little difficult.
That's quite a general question!
Basically, all types that the database system itself offers, like NUMERIC, VARCHAR etc., or that the programming language of choice offers (int, string etc.) would be considered "atomic" data(base) types.
Anything that you - based on your program's or business' requirements - build from that, business objects and so forth, are entities.
Tables, constraints and so forth are database-internal objects needed to store and retrieve data, but those are general not considered "entities". The data stored in your tables, when retrieved and converted into an object, that then is an entity.
Marc
In the entity relationship world an entity is something that may exist independently and so there is often a one-to-one relationship between entities and database tables. However, this mapping is an implementation decision: For example, an ER diagram may contain three entities: Triangle, Square and Circle and these could potentially be modelled as a single table: Shape.
Also note that some database tables may represent relationships between entities.
This seems helpful: http://en.wikipedia.org/wiki/Entity-relationship_model
In a database an entity is a table. The table represents whatever real world concept you are trying to model (person, transaction, event).
Contraints can represents relationships between entities. These would be foreign keys. They also enforce rules like first_name can not be blank (null). A transaction must have 1 or more items. An event must have a date time.
Stored Procedures / Packages / Triggers could handle more complex relationships and/or they can handle business rules, just depends on what it's doing.
it kind of depends how you think about it and how you model your problem domain. most of the time when you hear about entities, they are database tables (one or many) mapped onto object classes. So it's not really an entity until it's been queried for and turned into a class instance.
but again, it depends on your modeling methodology, and there are multiple :-)
This thread is demomnstrating one reason why it is difficult to find "elementary answers to elementary questions". Certain words have been used by different programming paradigms to mean different things (try asking a bunch of OO programmers what is the difference between a Class and an Object sometime).
Here's my take on it.
I first came across Entity as a modelling term in SSADM (ask your dad). In that context an Entity is used to model a logical clump of datas during the requirements gathering / analysis phase. The relationships between entities were modelled using the Entity Relationship diagrams, and the profile of an Enity was modelled using Entity Life Histories. ELH diagrams were very useful in COBOL systems but utterly horrible in relational databases. ERDs on the other hand continue to be useful to this day.
During the design and implementation phases the Entities get resolved into database tables, objects or records in a COBOL input file. In the course of that process a logical entity may get split across multiple tables, or several entities may get squidged into a single table, or there may be a one-to-one mapping. Sometimes an entity is resolved away entirely or lingers on as a view or a stored procedure.
My answer is obviously a little late, but here it is as defined in a database certification text book:
Entity: A uniquely identifiable element about which data is stored in a database.
and to clear up entity and table confusion,
Entity is not a table. Tables can be called "tables" or "relations" the words are synonymous.
We'd need to know some context. One thing people sometimes do when analysing data in prepartion for designing a database is to create an Entity Realtionship Diagram, where you are considering what data items you are managing and their relationships.
I wonder if that's the context you mean?
If so perhaps a read of this article would get you started?
Entities are "things of significance" to the users/business/enterprise/problem domain.
Update:
See this article in my blog in which I try to cover the subject in more detail:
What is entity-relationship model?
An entity is a term from the entity-relationship model.
A relational model (your database schema) is one of the ways to implement the ER model.
Relational tables represent relations between simple types like integers and strings, which, in their turn, can represent everything: entities, attributes, relationships.
You cannot tell what is it only from the relational structure, you need to see the ER model.
For table persons,
id name surname
1 John Smith
id, name and surname are entities in the real world and may or may not represent entities in the underlying ER model.
The fact of a record exists in the table means that these entities are in the following relation: "person 1 has name John and has surname Smith".
In the example above, the entity is defined by id (from the model's point of view).
If a person changes his name from John to Jack, the person remains the same (again, from the model's point of view), but gets related to another name.
In example above name and surname can be treated as attribute (as opposed to entity), but again, you need to see the ER model which this schema implements to tell what is it.
In some ER-to-relational model mappings, an entity should be defined in a table referenceable with a FOREIGN KEY to be considered an entity (which should constrain its domain).
However, this constraint can exist but not be represented in a database (due to technological limitations or something else).
Like, we cannot keep a list of all possible names, but the name of ##$^# is most probably a non-name, hence, it does not belong to the domain of names.
Therefore, an attribute is an entity which can participate in a relationship but cannot be contained in a domain-defining table.
For instance, the table prices:
good_id price
defines relationships between the set of goods (which is defined by the table goods) and the set of real numbers (which cannot be contained in a table since it's not even countable).
Still each price (like $2.00) is a real-world entity just as well.