Total and partial relationships
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. An entity relationship model, also called an entity-relationship (ER) diagram, is a work for a department, this sketched by double lines, and called “total or a department, this is sketched by a single line and called “partial or. What is total participation and partial participation in DBMS? 1, Views Can a weak entity be a part of ternary relationship in an ER-diagram? 74 Views.
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. 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.
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.
From time to time, we encounter cases where we wonder whether an item should be an attribute or an entity on its own. For example, an email address could be modeled as an entity in its own right. When in doubt, consider these rules of thumb: Is the item of direct interest to the database?
Objects of direct interest should be entities, and information that describes them should be stored in attributes. Our inventory and sales database is really interested in customers, and not their email addresses, so the email address would be best modeled as an attribute of the customer entity. Does the item have components of its own? If so, we must find a way of representing these components; a separate entity might be the best solution.
In the student grades example at the start of the chapter, we stored the course name, year, and semester for each course that a student takes. Can the object have multiple instances? If so, we must find a way to store data on each instance. Roles aren't often talked about in database tutorials, but they're an important concept. Consider a marriage - a relationship with two mandatory roles filled by the same entity set. In most relationships, the entity sets also define the roles, but when an entity set appears multiple times in a single relationship, we distinguish them in different roles.
In the example above, a Patient can Purchase Medicine with or without a Prescription. A Purchase can't exist without a Patient and Medicine, but a Prescription is optional overall, though it may be required in specific cases. A weak entity is an entity that can't be identified by its own attributes and therefore has another entity's key as part of its own.
An identifying relationship is the relationship between a weak entity and its parent entity. Both the identifying relationship and the weak entity are indicated with double borders.
Weak entity sets must necessarily participate totally in their identifying relationship. In this example, a Prescription contains LineItems which are identified by the Prescription's key and a line number. For examples of non-identifying relationships, see the previous examples. While Medicine participates totally in the Produce relationship, it has its own identity e.
Note that surrogate keys always imply regular entities.
DATA MODELING USING THE ENTITY-RELATIONSHIP MODEL
Total or partial participation indicate whether a certain relationship is required for an entity to exist. A Laboratory can exist without producing any medicine, but Medicine can't be Produced without a Laboratory. Medicine can't exist without being Produced, and a Laboratory can't Produce something unspecified.
A Prescription can exist without being Purchased, and a Purchase can exist without a Prescription. That leaves optional total participation, which I had to think about a bit to find an example: Partial participation in an identifying relationship isn't possible. Total participation doesn't imply an identifying relationship - Medicine can't exist without being Produced by a Laboratory but Medicine is identified by its own attributes.
Partial participation in a non-identifying relationship is very common. For example, Medicine can exist without being Purchased, and Medicine is identified by its own attributes.