This page is for data modellers or application designers. It explains how to use an ERD to define your system's data structure.
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Overview
PhixFlow ERDs are simplified Entity-Relationship Diagrams. "Entities" are things in the world that you want to represent as data tables in your application.| Insert excerpt | ||||||||
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Overview
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.
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Tables and Attributes
The table name is displayed in the header.
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
You can see that the Address and Name attributes have the word "String" next to them. PhixFlow needs to know what type of data the attribute contains. To make it easy to create an attribute, PhixFlow sets the type to string by default. It is important that you change the attribute to reflect the nature of the data. For example, an attribute of Telephone Number would be integer.
Primary Key UID
Each table must have a special attribute that contains a unique value for every record. For example, several teachers 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
If your data already has a unique attribute you can use that as the UID. For example, our Teacher table could have a National Insurance attribute.
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If you:
PhixFlow cannot make sure all UIDs are unique. In these cases, you must set up your own processes to make sure no duplicate values are present in custome UID attributes. |
Relationships and Foreign Keys
Sometimes we want to have the same data in different tables. For example, both the Teacher and ClassRoom tables need- 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
The ERD's entities, attributes and relationships define the logical structure of the data that your application uses.
If you want to highlight the relationships that affect a table, click the table to select it. The following screenshot shows the Teacher table is selected.
- Blue lines indicate relationships to a foreign key in the selected table
- Green lines indicate relationships from the primary key in the selected table.
Why Relationships are Important
It's important not to duplicate data.
In an application screen, you can add grids showing data from tables. However, there are attributes from a table that you do not want to include in a view, such as the UIDs, and there may be attributes in other tables that you do want to include in a view. For example, your application might want to show:
- Course Name
- Department Name
- Teacher Name
- Number of Students
PhixFlow can add names from different tables into the same view using the relationships. For example:
Course Table - 1 step to Teacher Table
- Course via Teacher foreign key to Teacher Name
Course Table - 2 steps to Department Table
- Course via Teacher foreign key to Teacher Table
- Teacher Table via Department Foreign key to SchoolDept Table (and therefore department names).
When you are designing screens, you can create views to display attributes from a table AND from other, related tables.
By convention, set the name of a relationship an action (verb) that follows the left-right flow of the diagram.
- a department→ has→ many teachers
- a teacher has one department
- a teacher → runs→ several courses
- classroom → is used by→ many courses
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
Data Structure and Views
PhixFlow uses the relationships between tables when you come to make views of data. You can create views that include include attributes from different tables, provided there is a relationship. When you add an attribute to a view, PhixFlow can provide a list of all the other attributes that you can use in the view.
If you display the table data in a grid, the attributes are the column headers.
Data: Tables, Attributes and Records
This page does not mention data records. The ERD defines the table, attribute and relationships. It does now show any data records. If you think of an Excel spreadsheet, a table in an ERD includes the name of the worksheet and the titles of the columns. It does not show the data records from the rows.
You can draw an ERD before adding any data to PhixFlow, or you can load data from external sources and then drag tables into and ERD to define the relationships between tables.
How to load data or add it to empty tables: link??
If your tables have records, to see them:
- student takes many courses.
Viewing ERD Data
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Jiras
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Configure → Table properties
Hide Attributes - Hide attributes
Remove this item from the Diagram → Remove from ERD... model, screen, workflow, actionflow...
Permanently Delete → Delete everywhere
Inital cap only
ConContext menu
To see the context menu for a table:
in the table header, clickUsing ERDs in Views
PhixFlow displays data to application users via views most commonly a grid view, which displays data in rows and columns.