This page is for data modellers or application designers. It explains how to use an ERD to understand how your system's data is connected.
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Overview
An ERD is an entity relationship diagram. Use an ERD to represent the information that your application will use and how it is connected. PhixFlow uses the connections between data when you come to make views. When you select something to add to a view, PhixFlow can offer all the connected tables and attributes. This means you can easily create views that combine data from different tables.
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PhixFlow's ERDs are simplified Entity-Relationship Diagrams. The concepts are explained in this article: ER Diagram Tutorial in DBMS. |
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todo better image Doc simple 1
Entities are the data tables that your application has or will have.
First create your "entities"
table, attributes
Set up relationships
e.g. invoice is for a customer. Tell PhixFlow "customer name" is connected via customer ID
This creates arelation and marks it as a forign key - with glyph to show it.
Its important to understand that the connection is there for other stuff - for creating views that combing information from multiple streams so that you can
If you wanted a screen with cust info at top and a list of depts it would need to know how to do this.
The line means go and find it from the other table and can then find the matching attribute and preport the records (by following the line). (show it as an animation
Turn this into a simple explanation with a sequence
Start with tables and attribures
Explain primary keys
Explain relations and foriegn keys.
A table (was stream) represents a thing or entity.
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Choose a name that reflects the entity your table represents. |
Entities:
- Company
- Employee
- Department
- Product
The table in a relationship diagram has a list of attributes.
Employee attributes:
- EmployeeID
- Name
- Address
- Phone
- Department
PhixFlow automatically generates a unique identifier, UID.
(Integer, sequence)
attribute that has unique values. The unique attribute is the primary key.
Primary Keys
- Employee stream: EmployeeID
- Department stream: DepartmentID
Improve
When the data from one stream's primary key also appears in a different stream, it is a foreign key.
This gets set automatically when you connect up an ERD
todo update using UX terminology
Use an example to show that the things are linked by relationship and that the data is connected and may not have the same name (can be a later concept)
Tell PhixFlow there is a relationship
Employee attributes:
- EmployeeID (primary key)
- Name
- Address
- Phone
- Department = DepartmentID - foreign key
Department attributes
- DepartmentID - primary key
- Department Name
- Purpose
- Department Manager = foreign key
Tables or attributes can have various relationships to each other
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By convention, a relationship is expressed as an action (verb) that follows the left-right flow of the diagram. |
Attributes can have relationships. On the diagram the relationship is shown as an arrow.
Relationships can only be one-to-many, from primary key to foreign key.
When you are designing screens, you can create views to display attributes from a table AND from tables with a direct relationship.
Relationships
- one company → employs → many employee
- one employee → manages → several departments
- one department → makes → many products
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We recommend that a primary key attribute should be an integer, because PhixFlow can automatically generate a unique integer value for each new data record. |
More About Relationships
The name for the relationship should reflect its direction. For example:
- either company → employs → people
- or people→ work for → company
- but not company → work for → people
PhixFlow imposes no restrictions on the names for relationships, Between two tables with multiple relations, each connection must have a unique name.
Many-to-Many Relationships
You can show a many-to-many relationship by using an intermediate stream. This stream has attributes that are foreign keys from the two streams you want to connect, with a one-to-many relationship into the foreign keys. For example
- A customer buys many products
- And a product can be bought by many customers
- The intermediate stream is a Customer Purchase Record, which has both the CustomerID and ProductID as foreign keys.
Example
In a relationship diagram, a stream and its attributes are displayed as a box. You can expand or collapse the box using the icon in the top left. PhixFlow shows all the attributes when the stream is expanded, and only the primary and foreign keys when it is collapsed.
todo Update labels with UX terminology Departments needs to go and there is not foriegn key here...
The following diagram shows some relationships between streams that represent a company, its employees and departments, the products it makes and the customers who buy them.
All of the plural foreign keys are wrong
Limitations - single hop relationships. You cannot go Project → Opportunity → Statuses at the moment. You need to go from one table
- Project → Opportunity
- Project → statuses
The company "contains" many departments. An employee "works in" a department, and a department can have many team members. In this company, the department "makes" several products, which "ship to" many customers.
Relationship diagrams are useful because you can design relational views that use data from multiple streams. For example, you might want a view that shows the employee details by department. This will display data from the Employee stream and the Department stream. The relationship diagram shows these are connected by the DepartmentID attribute.
Relationship Diagrams in the Repository
When you create a relationship diagram, you start in the repository. Find theAs you draw the diagram, PhixFlow adds items to the repository:
- a stream for each table; see
- a stream attributes for each attribute. These are nested under the stream in the repository.
- a relation for each relationship.
todo Draft image needs improving
Related things
Relationship diagrams do not show changes over time, responsibilities or processes. To represent this type of information, use a Workflow Diagram.
Relationship Diagrams and Relational Views
Relationship diagrams underpin the ability to create views that combine data from different streams.
When you create a GUI screen for an application, a view can display the data records for selected attributes using a Stream View. This can be a grid (table), a graph or a chart. If a table has no relationships to other tables, PhixFlow can only show attributes and records from that stream.
When a table has a direct relationship to other tables in a relationship diagram, PhixFlow can display the data for the attributes from the related tables.
For example, with the following relationship diagram, you can create a view based on the "Departement" stream that shows
- Company name from table Company
- Department names from table Department
- Product category and Status from table Products
todo better diagram and example
Check its in PhixFlow the delete
ToolbarOrganise selected objects in a grid.
Align all selected objects left.
Align all selected objects right.
Align all selected objects to top.
Align all selected objects to bottom.
or click to open the list of streams in the repository and drag a stream in from the repository list.
Context Menu
PhixFlow ERDs define the logical structure of the data in your application.
The following example shows an ERD for a school. A school has entities, for example: departments, teachers, students, classrooms and so on. All these entities are represented as tables. The lines connecting the tables show the relationships between them.
The following illustration shows how an ERD represents a table.
The table has a list of attributes, which represent the different pieces of data you want to record.
Attributes are configured with a data type and associated properties; see Understanding Data Types. For example the Address attribute is a String which has an associated length. To change the attribute to reflect the nature of your data, click on the attribute's name to open and edit its properties.
It is important to set the correct data type and properties before loading records into a table, as you cannot change the data type once the table contains records. If you do have data and want to change the type you will need to add a new attribute or clear your data; see Clearing and Loading Data in an ERD.
You can create tables in an ERD manually, but if you already have data, PhixFlow can automatically create tables from it; see Adding Content to an ERD.
Primary Keys and UIDs
PhixFlow expects every table to have a special attribute called a primary key, which uniquely identifies each record. For example, teachers have a UID as the primary key. This is because they can have the same first or family name, or even both, so these attributes cannot be a primary key. When you create a table, PhixFlow adds an attribute called UID (short for Unique IDentifier) and configures it as follows:
- as the table's
primary key. - as an integer.
- to automatically create a unique number for every record in a table.
This means PhixFlow can ensure all the UIDs have unique values.
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Display Name
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Typically a primary key is not user-friendly and you do not want to display it. Instead, you want to display something that makes more sense to a person. For this reason, when you create a table, PhixFlow automatically adds an attribute called Name and ticks its Display Name property. The display name is shown in place of the primary key when displayed on a view or screen. Display names do not need to be unique and should be something user-friendly.
Let's look at the following example. For the SchoolDept table, PhixFlow has created:
- a UID, for example 1490.
- a Name, for example Mathematics.
When any view makes reference to the SchoolDept using it's primary key, PhixFlow displays the department name, not its number; see Using Relational Views. The configuration is shown below:
SchoolDept SchoolDept
Primary Key Display Name
Relationships and Foreign Keys
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Tables in PhixFlow are relational, which means information in one table can be related to information in another table. The key benefit is you can access data in many different tables without needing to duplicate it.
A foreign key is an attribute in one table, that refers to the primary key in another table.
In an ERD you can connect tables by drawing a line, called a relationship between them. For example, the Teacher table needs to have the name of a department. The ERD below shows this relationship using a line that joins SchoolDept.DeptID to Teacher.Department.
When you create screens that show data, PhixFlow uses the relationships defined in the ERD to provide options for displaying data from related attributes in the same grid, form, card, graph or chart; see Displaying Data (Views). This means you only need to store the School DeptID against a teacher to be able to access all of the SchoolDept details.
When you create a relationship from a primary key to an attribute, PhixFlow automatically sets the attribute to be a 
foreign key. Foreign keys must have the same data type and properties as the corresponding primary key. For example, in the illustration above, the Teacher.Department attribute contains the same data type as SchoolDept.DeptID; see Understanding Data Types.
The foreign key represents the many side and the primary key the one side. This means that SchoolDept.DeptID is unique, there is only one record containing this unique information. The Teacher.Department, can have one or more instances of a value of DeptID. For example, the Maths Department UID could occur 10 times, once for each of the 10 maths teachers.
One-to-many and Many-to-one
PhixFlow draws a relationship line between a primary key and a foreign key. This represents a one-to-many relationship. For example, one teacher runs many courses. A many-to-one relationship is implied when you read a relationship in the opposite direction. For example, a department has many teachers, and several teachers work for one department.
Many:many
To create a many-to-many relationship, you need an intermediate table that has foreign key attributes from each of the tables you want to have a relationship. For example, many students take many courses. A many-to-many relationship is shown below, using the intermediate table called CourseAttendee. Notice that CourseAttendee does not need a unique identifier.
- A course is attended by many students
- A student takes many courses.
Viewing ERD Data
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Using ERDs in Views
PhixFlow displays data to application users via views most commonly a grid view, which displays data in rows and columns.