Room: Database Relationships - knowledge Transfer
Learn about different types of database relationships and how companies use them. Plus For example, if you own a record store, the database might have a table for Only one patron can have a copy of a library book checked out at a time. Database System of LIBRARY MANAGEMENT SYSTEM Semester Project upto 2Gb Design view The library has the following tables in its database; 1. Create a table for those fields and enter the sample data. Think about what the primary key for each table would be and about the relationship between the tables. Database design and a more indepth look at keys are also covered. Our app will be used to manage a library of SQL books and allow users to check out the For example what if for one of the 'My Second SQL Book' checkouts the title is.
This constraint is consistent with the enterprise being modeled. This frequently happens when we need to keep a history of events over time. In a library, customers can borrow many books and each book can be borrowed by many customers, so this seems to be a simple many-to-many association between customers and books.
But any one customer may borrow a book, return it, and then borrow the same book again at a later time. The library records each book loan separately. There is no invoice for each set of borrowed books and therefore no equivalent here of the Order in the order entry example.
You have already seen other parts of the library model in exercises. This relationship has Year and Semester attributes that identify its sequence position. Student and Course are related through the many-to-many Attempts relationships; a course can exist without a student, and a student can be enrolled without attempting any courses, so the participation is not total. When a student attempts a course, there are attributes to capture the Year and Semester, and the Mark and Grade.
For a real university, many more aspects would need to be captured by the database. The airline has one or more airplanes.
Database model - Wikipedia
An airplane has a model number, a unique registration number, and the capacity to take one or more passengers. An airplane flight has a unique flight number, a departure airport, a destination airport, a departure date and time, and an arrival date and time.
Each flight is carried out by a single airplane. A passenger has given names, a surname, and a unique email address.
Database Design - Many-to-many 2
A passenger can book a seat on a flight. The ER diagram of the flight database An Airplane is uniquely identified by its RegistrationNumber, so we use this as the primary key. A Flight is uniquely identified by its FlightNumber, so we use the flight number as the primary key. The departure and destination airports are captured in the From and To attributes, and we have separate attributes for the departure and arrival date and time. Because no two passengers will share an email address, we can use the EmailAddress as the primary key for the Passenger entity.
An airplane can be involved in any number of flights, while each flight uses exactly one airplane, so the Flies relationship between the Airplane and Flight relationships has cardinality 1: N; because a flight cannot exist without an airplane, the Flight entity participates totally in this relationship. A passenger can book any number of flights, while a flight can be booked by any number of passengers.
N Books relationship between the Passenger and Flight relationship, but considering the issue more carefully shows that there is a hidden entity here: We capture this by creating the intermediate entity Booking and 1: N relationships between it and the Passenger and Flight entities.
Identifying such entities allows us to get a better picture of the requirements.
There are no requirements to capture passenger details such as age, gender, or frequent-flier number. If, instead, we assumed that the capacity is determined by the model number, we would have created a new AirplaneModel entity with the attributes ModelNumber and Capacity.
The Airplane entity would then not have a Capacity attribute. Airlines typically use a flight number to identify a given flight path and schedule, and they specify the date of the flight independently of the flight number.Illustration on ER Model to Relational Tables- 1 - Database Management System
For example, there is one IR flight on April 1, another on April 2, and so on. Let's go ahead and add some data to our table. Referential Integrity We're going to take a slight detour here to discuss a topic that's extremely important when dealing with table relationships: This is a concept used when discussing relational data which states that table relationships must always be consistent.
Different RDBMSes might enforce referential integrity rules differently, but the concept is the same. The constraints we've defined for our addresses table enforce the one to one relationship we want between it and our users table, whereby a user can only have one address and an address must have one, and only one, user.
Design pattern: many-to-many with history (the library loan)
This is an example of referential integrity. Let's demonstrate how this works. What happens if we try to add another address for a user who already has one?
How about if we try to add an address for a user who doesn't exist? Here we get a different error.
If you're wondering why we can add a user without an address but can't add an address without a user, this is down to the modality of the relationship between the two entities. Don't worry about exactly what this means for now, just think of it as another aspect of entity relationships. Adding this clause, and setting it to CASCADE basically means that if the row being referenced is deleted, the row referencing it is also deleted. Determining what to do in situations where you delete a row that is referenced by another row is an important design decision, and is part of the concept of maintaining referential integrity.
Table of Contents
One-to-Many Okay, time to get back to our different table relationship types with a look at one-to-many. A one-to-many relationship exists between two entities if an entity instance in one of the tables can be associated with multiple records entity instances in the other table. The opposite relationship does not exist; that is, each entity instance in the second table can only be associated with one entity instance in the first table.