The Entity Relationship Model - Learning MySQL [Book]
Need Criticism on my E-R Diagram for Video Rental Chain be in the system anymore because I will have removed the video from inventory. The Entity Relationship Model At a basic level, databases store information Similarly, an inventory and sales database might store information about In the ER diagram, an entity set is represented by a rectangle containing the entity name. .. Get unlimited access to videos, live online training, learning paths, books. Media Category: Entity Relationship Diagram. Visual Paradigm Community ERD – Inventory System · ERD – Inventory System · ERD – Video Rental System .
Are there some folks who work weekends at store 44 and weekday evenings at store 77? You want that video ID are never removed from the database.
Add some columns that deal with status: Of course you want more info too: That way, there's no transaction that has data missing. Bear in mind that modern databases handle multi-millions of records with ease.
Calculate how long you need to retain records, how many videos and customers are feasible in that time period, and stop worrying Things like how many are overdue ordered by days over; which members are often late, with statistics; employee transaction records who's the best salesperson, who's the best rental-pusher, is there an employee who's sales or rental records are associated with good or bad members more than other employees?
I don't know what your doc file holds: I tried opening it in my LibreOffice suite and it hung. I was thinking of that extra attribute in VIDEO as a possible solution too, maybe including a status on the video. That way it would be really easy to determine which videos I sold, which I have currently rented out, and which are in stock.
I don't think I have to go quite that in depth as far as the employees and their reports are concerned, but I do very much appreciate the ideas and your input. Those are some good ideas if I'm feeling like implementing some extra credit XD 0 griswolf 7 Years Ago I'm having a hard time wrapping my mind around individual UID for each item in stock.
That makes sense for one-off things like original artwork, or service calls; but for the things in a video store, not so much in my opinion. Just don't remove them. Use a status to indicate what happened. That way, you don't have to think about what happens in tables that have a FK referencing that item's entry when the item goes away.
If necessary its notyou can partition tables by date to avoid overfilling them. For example, each customer can buy any number of products, and each product can be bought by any number of customers.
Need Criticism on my E-R Diagram for Video | DaniWeb
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.
Entity Relationship Diagram Example: Movie Rental - Visual Paradigm Community Circle
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.
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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?
Need Criticism on my E-R Diagram for Video Rental Chain
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.
Media Category: Entity Relationship Diagram
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. This points us to the existence of a hidden intermediate entity, the booking, between the flight and the passenger entities. The requirement could be better worded as: The intermediate booking entity between the passenger and flight entities Each passenger can be involved in multiple bookings, but each booking belongs to a single passenger, so the cardinality of this relationship is 1: