Have you ever wondered why you get an error message telling you that your ideal email address is already taken? Or how e-commerce stores can keep track of so many different orders, customers and products? The answer to these questions lies within the creation of a database which in other words is simply a collection of information that is organized in such a way that data can be easily stored, managed, updated, and retrieved.
Now there is a lot of moving information in a database, and understanding how the many elements of a database interact with each other can be difficult to grasp. Engineers need a visual way to understand how these separate elements are related to each other and how they work together. To visualize this, they build UML diagrams.
In both software and non-software systems across multiple business verticals, the use of UML diagrams has played a critical part in facilitating the design, development, and implementation of some successful logical and physical design models.
This UML Database Modeling guide has been prepared for beginners to help teach them the fundamentals of UML Database Modeling. By the end of this guide, you will be in a position to understand at a moderate level all the basics of database modeling, and more importantly, the concepts that you’ll take away will allow you to design UML database models from scratch.
Table of Contents
What is Database Modeling?
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Database Modeling is all about how we represent an application’s data within a database. Often as developers are designing applications that have persistence, they use higher-level data models that are specific for database design. These models are not implemented by the database system, rather they are translated into the model of the database system.
Object-relational database database models are one of the most common solutions for providing reliable and efficient object persistence for applications. A typical application is normally made up of an application layer and the database layer. A data model is used to represent the structure of the database layer and the relationships within it. The application model uses object-oriented class models to describe how the application layer maps to the database model.
For example, you have a Relational Database Management System (RDBMS). If you create a database in the relational system, the database is going to consist of relations. However, instead of designing the relations directly, the database designer will use a high-level design model. That model will then go through a translator that will translate it into the relationships that are implemented by the database.
There are two popular higher-level database design models. These are:
- Entity-Relationship Model (E/R).
- Unified Modeling Language (UML).
Both are graphical models and can be translated into relations. For decades, the E/R model was very popular but recently the UML model has gained more popularity. For the purposes of this guide, you will focus more on the data modeling subset of UML.
What is UML?
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The Unified Modeling Language (UML), is an industry-standard graphical language for specifying, visualizing, constructing, and documenting the artifacts of software systems. In 1997, the OMG (Object Management Group) established a set of agreed upon shapes, symbols, and notations that would be used to graphically depict each component that makes up a software system, its attributes, and how it relates to other components within the system.
It has proven to be effective in modeling large and complex systems and is a very important part of Object-Oriented design. UML diagrams are very easy to read and understand yet they are still able to simplify the complex process of software and database design. Using more comprehensible and easier to interpret graphical notation, UML diagrams communicate more clearly than natural language which can be imprecise or code which can be too detailed.
While this language was initially intended for software development, it is not dependent on any language or technology. UML can be used to map out processes even for non-technical processes such as business modeling and communication.
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What is UML Database Modeling?
UML diagrams can be used for UML Database Modeling and design. They have constructs that can be used to accurately model tables and relationships. In the data modeling subset of UML, there are 2 basic concepts:
UML Database Modeling: Classes
UML Database Modeling Class diagrams describe the type of objects in the system and the different kinds of static relationships that exist between them. The Class is essentially a template for creating objects as well as representing an object’s initial state, attributes, and behaviors/methods
The Class concept in UML is not specific to data modeling, it’s also used for designing programs. It is the central modeling technique that is used in almost all Object-Oriented methods. Just like in Programming, a Class consists of Name, Attributes, and Methods. However, for data modeling, we also add a private key “PK” and a foreign key “FK”, and we drop the methods that are associated with the Class since our focus is on data modeling and not the operations on the data.
| Class Name |
+ Attribute: string
# Attribute: string
+ Attribute: float
– Attribute: boolean
– Attribute: int |
When representing your Class, you start by writing its name in the upper most partition. This is followed by its attributes, and their data types which follow after the colon.
The public, protected and private attributes of a class are denoted using the preceding +, #, or – sign respectively.
UML Database Modeling: Relationships
The are 6 distinct relationships between UML Classes in UML Database Modeling:
These relationships can be represented using different styles of arrows as shown in the diagram below:
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Association
Associations capture relationships between objects of two different Classes in UML Database Modeling. If we assume that there are two Classes that interact with each other, a continuous connecting line should be drawn between them indicating the association in the diagram. Often, we can also use a verb above the line conveying its meaning. For example, consider the following Class diagram:
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If we want to have a relationship where customers can make orders, we can use a line between the Customer and Order Classes. This says that we have objects in the Customer Class and objects in the Order Class that are associated with each other.
In addition, the diagram also specifies the multiplicity/cardinality i.e., the number of objects that can take part in that relationship. Multiplicity captures how many objects of one Class can be related to an object of another Class in UML Database Modeling. Multiplicity is specified as a comma-separated list of intervals where each interval is represented as a minimum maximum range.
The different possible multiplicities have been summarized in the table below:
| Symbol | Meaning |
| 1 | Exactly 1 |
| 4,7,9 | Exactly 4, exactly 7, or exactly 9 |
| 0..1 | Zero or one |
| 2..9 | A value in the range from 2 to 9 |
| * | Zero or more (an unlimited number) |
| 0..* | Zero or more |
| 1..* | One or more |
Inheritance
Sometimes it’s also referred to as generalization. As the name implies, it’s a schematic representation of the relationship between a parent class and its descendants in UML Database Modeling. The hollow arrow is always pointed to the parent class. For example, in the same diagram, you can see that Payment is the parent class of Cash, Check, and Credit.
Realization
This refers to the relationships between an interface and objects that implement that interface in UML Database Modeling. Therefore, one Class can have some specifications that are implemented in another Class. A classic example is whereby several customers can realize the behavior of a single supplier.
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Dependency
When an object of one Class uses an object of another Class in its method and this object is not stored in any field, then this kind of relationship is modeled as a dependency. Changes in one Class will lead to changes in another in UML Database Modeling.
Aggregation
This is a special type of relationship between Classes that occurs when one Class is part of another in UML Database Modeling. It is shown as a continuous line with an unfilled diamond connected to the Class which represents the aggregate. For example, the Order class has a collection of OrderDetails.
Composition
This is a type of aggregation but in this case, the Classes that form the aggregate Class are destroyed when the aggregate Class is deleted in UML Database Modeling.
Conclusion
UML Database Modeling is not so much about a pretty picture, it is actually a powerful tool in the programmers tool kit. If you can understand and use it properly by getting into the habit of drawing your Class diagram before implementing your database, the time invested in drawing a succinct and clear UML diagram will eventually pay off and can save your project from design flaws and other errors you might encounter without a proper plan in front of you. You can draw them with a pen and paper but it’s a lot easier if you make use of a diagramming tool.
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