Understanding ERDs

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.

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.

PhixFlow cannot ensure unique values for a primary key where:

the name is UID but its data type is not integer
the name is not UID

If your data already has a unique attribute, you can use that as the primary key. For example, the Teacher table could have a National Insurance attribute, which would be unique to a teacher. In this case, you must set up your own processes to make sure that the records in a table all have unique values in the primary key.

Display Name

Typically a primary key is not user-friendly and you do not want to display it. Instead, you want to display something more that makes 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

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.

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.