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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. The lines connecting the
tables show the relationships between them.
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Tables and Attributes
The table name is displayed in the header.
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
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To refer to an attribute, it's a good idea include the table name. For example, the attribute Name occurs in several tables, so you can specify: Teacher.Name, Student.Name or SchoolDept.Name. |
Primary Key UID
Each table must have a special attribute that contains a unique value for every record. For example, several teachersand 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
The UID:
is by default is automatically generated by PhixFlow so that there, 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
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If your data already has a unique attribute, you can use that as the UID. For example, the Teacher table could have a National Insurance attribute. However, if you decide:
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 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.
Usually, you want to use the foreign key attribute name, rather than UID, when the data is shown in a view. PhixFlow automatically ticks the Dispay Name property for a foreign key, as shown in the properties screenshot on the right.
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.
Why ERDs are Important
The ERD's tables, attributes and relationships define the logical structure of the data that your application uses.
However, a table has attributes that you may not want an application user to see, such as UIDs or audit data. When you are creating views to display data in your application, you may want to combine attributes from different table s into one view. For example, your application might want to show:
- Course Name
- Department Name
- Teacher Name
- Number of Students
Each of these attributes comes from a different table. The relationships that you create between tables in an ERD make it possible for PhixFlow to create views using attributes from different tables. For example:
Course Table - 1 relationship to Teacher Table
- Course.Teacher to Teacher.UID and display name Teacher.Name
Course Table - 2 relationships to Department Table (Tech check: this is true??)
- Course.Teacher to Teacher.UID
- Teacher.Department to SchoolDept.UID to SchoolDept.Name.
A Note About Data Records
An ERD defines the table, attribute and relationships. It does not show any data records. If you think of an Excel spreadsheet, an ERD shows the name of the worksheet (table) and the titles of the columns (attributes) but not the rows of data.
You can:
- either define an ERD before adding any data to PhixFlow. In this case you create the tables, attributes and relationships; see Defining Data Structures using ERDs
- or you can load data from external sources first. You can then drag tables into the ERD and simply add the relationships; see Setting Up Data for an overview and links.
If you are working with tables that have records, to see them:
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.