Relational Database Design
CS Home Data Modeling ERD Entities Relationships Attributes Example A single entity instance in one entity class (parent) is related to multiple entity. Entity-Relationship model is used in the conceptual design of a database Examples are a person, car, customer, product, gene, book etc. • Attributes: an . A many-one relationship type (and the counterpart one-many) is also often called a. Learn about entity relationship diagram symbols. Read the ER diagram tutorial. See ERD For example, an employee entity can have multiple skill values.
However, the relationships that can be created among the tables enable a relational database to efficiently store huge amount of data, and effectively retrieve selected data. Database Design Objective A well-designed database shall: This is because duplicate data not only waste storage spaces but also easily lead to inconsistencies. Ensure Data Integrity and Accuracy: Databases are usually customized to suit a particular application.
No two customized applications are alike, and hence, no two database are alike. Guidelines usually in terms of what not to do instead of what to do are provided in making these design decision, but the choices ultimately rest on the you - the designer.
Define the Purpose of the Database Requirement Analysis Gather the requirements and define the objective of your database, e.
Drafting out the sample input forms, queries and reports, often helps. Gather Data, Organize in tables and Specify the Primary Keys Once you have decided on the purpose of the database, gather the data that are needed to be stored in the database.
Divide the data into subject-based tables. Choose one column or a few columns as the so-called primary key, which uniquely identify the each of the rows. Primary Key In the relational model, a table cannot contain duplicate rows, because that would create ambiguities in retrieval. To ensure uniqueness, each table should have a column or a set of columnscalled primary key, that uniquely identifies every records of the table.
For example, an unique number customerID can be used as the primary key for the Customers table; productCode for Products table; isbn for Books table. A primary key is called a simple key if it is a single column; it is called a composite key if it is made up of several columns.
The primary key is also used to reference other tables to be elaborated later. You have to decide which column s is to be used for primary key. The decision may not be straight forward but the primary key shall have these properties: The values of primary key shall be unique i.
For example, customerName may not be appropriate to be used as the primary key for the Customers table, as there could be two customers with the same name. The primary key shall always have a value.
Many-to-many (data model)
In other words, it shall not contain NULL. Consider the followings in choose the primary key: The primary key shall be simple and familiar, e. The value of the primary key should not change.
- A Quick-Start Tutorial on Relational Database Design
Primary key is used to reference other tables. If you change its value, you have to change all its references; otherwise, the references will be lost. For example, phoneNumber may not be appropriate to be used as primary key for table Customers, because it might change.
Primary key often uses integer or number type. But it could also be other types, such as texts. However, it is best to use numeric column as primary key for efficiency. Primary key could take an arbitrary number.
This arbitrary number is fact-less, as it contains no factual information. Unlike factual information such as phone number, fact-less number is ideal for primary key, as it does not change. Primary key is usually a single column e.
But it could also make up of several columns. You should use as few columns as possible. Let's illustrate with an example: Name may not be unique.
Phone number and address may change. Hence, it is better to create a fact-less auto-increment number, say customerID, as the primary key. Create Relationships among Tables A database consisting of independent and unrelated tables serves little purpose you may consider to use a spreadsheet instead.
The power of relational database lies in the relationship that can be defined between tables.
Many-to-many (data model) - Wikipedia
The most crucial aspect in designing a relational database is to identify the relationships among tables. The types of relationship include: In a "company" database, a manager manages zero or more employees, while an employee is managed by one and only one manager.Entity-Relationship Diagrams
In a "product sales" database, a customer may place many orders; while an order is placed by one particular customer.
This kind of relationship is known as one-to-many. One-to-many relationship cannot be represented in a single table. For example, in a "class roster" database, we may begin with a table called Teachers, which stores information about teachers such as name, office, phone and email.
To store the classes taught by each teacher, we could create columns class1, class2, class3, but faces a problem immediately on how many columns to create.
On the other hand, if we begin with a table called Classes, which stores information about a class courseCode, dayOfWeek, timeStart and timeEnd ; we could create additional columns to store information about the one teacher such as name, office, phone and email. However, since a teacher may teach many classes, its data would be duplicated in many rows in table Classes.
Design pattern: many-to-many (order entry)
To support a one-to-many relationship, we need to design two tables: We can then create the one-to-many relationship by storing the primary key of the table Teacher i. The column teacherID in the child table Classes is known as the foreign key.
A foreign key of a child table is a primary key of a parent table, used to reference the parent table. Take note that for every value in the parent table, there could be zero, one, or more rows in the child table. For every value in the child table, there is one and only one row in the parent table. Many-to-Many In a "product sales" database, a customer's order may contain one or more products; and a product can appear in many orders.
In a "bookstore" database, a book is written by one or more authors; while an author may write zero or more books. This kind of relationship is known as many-to-many. Let's illustrate with a "product sales" database. Class diagram Other views of this diagram: We need to know the subtotal for each order line that is, the quantity times the unit sale price and the total dollar value of each order the sum of the subtotals for each line in that order. So for every many-to-many, we will need a junction table in the database, and we need to show the scheme of this table in our diagram.
If there is an association class like OrderLinesits attributes will go into the junction table scheme. If there is no association class, the junction table sometimes also called a join table or linking table will contain only the FK attributes from each side of the association. Other views of this diagram: Large image - Data dictionary text The many-to-many association between Orders and Products has turned into a one-to-many relationship between Orders and Order Lines, plus a many-to-one relationship between Order Lines and Products.
You should also describe these in English, to be sure that you have the fk's in the right place: UPCs are defined for virtually all grocery and manufactured products by a commercial organization called the Uniform Code Council, Inc. To uniquely identify each order line, we need to know both which order this line is contained in, and which product is being ordered on this line.
The two fk's, from Orders and Products, together form the only candidate key of this relation and therefore the primary key. There is no need to look for a smaller pk, since OrderLines has no children.
Data representation The key to understanding how a many-to-many association is represented in the database is to realize that each line of the junction table in this case, OrderLines connects oneline from the left table Orders with one line from the right table Products.
Each pk of Orders can be copied many times to OrderLines; each pk of Products can also be copied many times to OrderLines. But the same pair of fk's in OrderLines can only occur once. In this graphic example, we show only the pk and fk columns for sake of space. Large image - Description text [ Previous: Keys ] [ More: