Implement the application
The Entity Relationship Model At a basic level, databases store information about distinct objects, or entities, and the associations, or relationships, between these entities. Attributes describe the entity they belong to. .. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and. Relationship A relationship represents an association among objects. n ( unlimited) Supply relationships, and each Project object may participate in at least 0 All OSL events are refinements of the system-defined OSL Event construct. The Unified Modeling Language (UML) is a de-facto standard for modeling object-oriented systems. In UML there are five different types of relationships: association, aggregation, composition, dependency, and inheritance. Association is a semantically weak relationship (a semantic.
In a sales database, we could store the name, email address, postal address, and telephone number for each customer.
Attributes describe the entity they belong to. An attribute may be formed from smaller parts; for example, a postal address is composed of a street number, city, ZIP code, and country. Some attributes can have multiple values for a given entity. For example, a customer could provide several telephone numbers, so the telephone number attribute is multivalued. Attributes help distinguish one entity from other entities of the same type. We could use the name attribute to distinguish between customers, but this could be an inadequate solution because several customers could have identical names.
To be able to tell them apart, we need an attribute or a minimal combination of attributes guaranteed to be unique to each individual customer. The identifying attribute or attributes form a key. In our example, we can assume that no two customers have the same email address, so the email address can be the key.
However, we need to think carefully about the implications of our choices. For example, if we decide to identify customers by their email address, it would be hard to allow a customer to have multiple email addresses. Any applications we build to use this database might treat each email address as a separate person, and it might be hard to adapt everything to allow people to have multiple email addresses. Clearly, there may be several possible keys that could be used to identify an entity; we choose one of the alternative, or candidate, keys to be our main, or primary, key.
You usually make this choice based on how confident you are that the attribute will be non-empty and unique for each individual entity, and on how small the key is shorter keys are faster to maintain and use.
- Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
- UML Multiplicity and Collections
Attributes comprising the primary key are shown underlined. The parts of any composite attributes are drawn connected to the oval of the composite attribute, and multivalued attributes are shown as double-lined ovals. Similarly, a product price could be a positive rational number. Attributes can be empty; for example, some customers may not provide their telephone numbers. You should think carefully when classifying an attribute as multivalued: The sales database requirements may specify that a product has a name and a price.
To distinguish between products, we can assign a unique product ID number to each item we stock; this would be the primary key. Each product entity would have name, price, and product ID attributes.
The ER diagram representation of the product entity Representing Relationships Entities can participate in relationships with other entities. For example, a customer can buy a product, a student can take a course, an artist can record an album, and so on.
The Entity Relationship Model - Learning MySQL [Book]
Like entities, relationships can have attributes: Our database could then record each sale and tell us, for example, that at 3: For example, each customer can buy any number of products, and each product can be bought by any number of customers.
This is known as a many-to-many relationship. We can also have one-to-many relationships. For example, one person can have several credit cards, but each credit card belongs to just one person.
Looking at it the other way, a one-to-many relationship becomes a many-to-one relationship; for example, many credit cards belong to a single person. Finally, the serial number on a car engine is an example of a one-to-one relationship; each engine has just one serial number, and each serial number belongs to just one engine. We often use the shorthand terms 1: N for one-to-one, one-to-many, and many-to-many relationships, respectively. The number of entities on either side of a relationship the cardinality of the relationship define the key constraints of the relationship.
There are many relationships that may at first seem to be one-to-one, but turn out to be more complex. For example, people sometimes change their names; in some applications, such as police databases, this is of particular interest, and so it may be necessary to model a many-to-many relationship between a person entity and a name entity. Redesigning a database can be time-consuming if you assume a relationship is simpler than it really is.
In an ER diagram, we represent a relationship set with a named diamond. The cardinality of the relationship is often indicated alongside the relationship diamond; this is the style we use in this book.
The ER diagram representation of the customer and product entities, and the sale relationship between them. Partial and Total Participation Relationships between entities can be optional or compulsory. In our example, we could decide that a person is considered to be a customer only if they have bought a product. On the other hand, we could say that a customer is a person whom we know about and whom we hope might buy something—that is, we can have people listed as customers in our database who never buy a product.
These are referred to as the participation constraints of the relationship. In an ER diagram, we indicate total participation with a double line between the entity box and the relationship diamond. Understand object behaviour and their relationship from practical perspective How to Draw an Object Diagram? We have already discussed that an object diagram is an instance of a class diagram.
Association (object-oriented programming)
It implies that an object diagram consists of instances of things used in a class diagram. So both diagrams are made of same basic elements but in different form. In class diagram elements are in abstract form to represent the blue print and in object diagram the elements are in concrete form to represent the real world object. To capture a particular system, numbers of class diagrams are limited. However, if we consider object diagrams then we can have unlimited number of instances, which are unique in nature.
Only those instances are considered, which have an impact on the system.
UML - Object Diagrams
From the above discussion, it is clear that a single object diagram cannot capture all the necessary instances or rather cannot specify all the objects of a system. Second, consider only those instances, which will cover the functionality. Third, make some optimization as the number of instances are unlimited. The link in object diagram is used to connect objects.
Objects and links are the two elements used to construct an object diagram. The most important elements are to be identified.
The association among objects should be clarified. Values of different elements need to be captured to include in the object diagram.
Association (object-oriented programming) - Wikipedia
Add proper notes at points where more clarity is required. The following diagram is an example of an object diagram. It represents the Order management system which we have discussed in the chapter Class Diagram.
The following diagram is an instance of the system at a particular time of purchase. It has the following objects.