# Strong and weak entity relationship design

### Difference Between Strong Entity and Weak Entity (with Comparison Chart) - Tech Differences

Design of an E-R Database Schema Relationship sets that involve two entity sets are binary (or . key of the strong entity set on which the weak entity set is. about ERD Shapes? Check out this guide for a comprehensive look at all ER diagram symbols and notation. Strong Entity Symbol. Strong entity Weak Relationships are connections between a weak entity and its owner. . Did you know you can create a free account and start diagramming with just an email address?. Weak entity sets aren't determined by an existence dependency. Regular entity sets can participate totally in a relationship and so have an existence.

A field can be a FK or a PK or both or neither but you aren't discussing those. Every entity table has a PK. It may or may not have a FK.

### Weak entity - Wikipedia

Thus a relationship may be weak breakable or strong unbreakable. A strong relationship just means that the dependent entity cannot exist apart from the relationship. Take Class, Course and Room, for example. Imagine the following conversation: What course will you teach?

Since a course must be specified before the class is created, the relationship is a strong one. Also, when considering which class to take, a class in "Bead Rendering" and a class in "Cybercrafting" are two completely different things even though they may both meet in the same room at the same time just on different days.

The identity of each class is inexorably bound to a course. So this relationship is also identifying.

What room will it be in? Yes, a room will have to be assigned before the semester starts, but at the moment we can still create the class, put "TBD" in the catalog and allow students to enroll. The class can exist without a room for a while, anyway so the relationship is weak. Also, two classes in "Discretionary Logic" are functionally equivalent, even though they are taught in different rooms.

The relationship with the room has nothing to do with the type of class. The relationship is non-identifying. So if students had signed up to take Bead Rendering in room 17 were notified that the room had changed to 12, they would not think, "This is a completely different class!

However, if they were told the class was now "Second-hand Carnival Staffing" then they would be right. This is now a completely different class. This is the difference between identifying and non-identifying relationships.

It's important to realize that all relationships consist of at least two entities.

• The Entity-Relationship Model
• Weak entity

In this way a database relationship is similar to a human relationship -- it takes two to Tango. The "unary" just means that both sides of the relationship are filled by the same logical entity. It is easy to see that a "meets in" relationship between a Class entity and a Room entity cannot be satisfied between, say, two Class entities or two Room 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.

## Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi

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.

### The Entity-Relationship Model

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?

Weak Entity - Database Management System

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.

## Difference Between Strong and Weak Entity

Can the object have multiple instances? If so, we must find a way to store data on each instance. The cleanest way to do this is to represent the object as a separate entity. In our sales example, we must ask whether customers are allowed to have more than one email address; if they are, we should model the email address as a separate entity. Is the object often nonexistent or unknown? If so, it is effectively an attribute of only some of the entities, and it would be better to model it as a separate entity rather than as an attribute that is often empty.

Consider a simple example: The ER diagram representation of student grades as a separate entity Entity or Relationship? An easy way to decide whether an object should be an entity or a relationship is to map nouns in the requirements to entities, and to map the verbs to relations.

All else being equal, try to keep the design simple, and avoid introducing trivial entities where possible; i. Intermediate Entities It is often possible to conceptually simplify many-to-many relationships by replacing the many-to-many relationship with a new intermediate entity sometimes called an associate entity and connecting the original entities through a many-to-one and a one-to-many relationship. A passenger participates in an M: Any given flight can have many passengers with a booking.

Any given passenger can have bookings on many flights. Hence, we can consider the many-to-many relationship to be in fact two one-to-many relationships, one each way.