This page is for application designers. It explains the concepts of ERDs (entity-relationship diagrams) and how they define your system's data structure.
| Insert excerpt | ||||||||
|---|---|---|---|---|---|---|---|---|
|
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.
| Panel | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||
|
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
Identifying an Attribute
This help page refers to an attribute using the form stream.attribute, for example: Teacher.Name, Student.Name or SchoolDept.Name.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
If you look at the School ERD example above, you can see that every stream has an attribute called UID, and that it has the
- 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.
| Insert excerpt | ||||||
|---|---|---|---|---|---|---|
|
Display Name
| Anchor | ||||
|---|---|---|---|---|
|
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
| Anchor | ||||
|---|---|---|---|---|
|
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
.
Attribute properties for:
ClassRoom
Department
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
Under-the-hood this is done using a direct SQL join.
Link to how to create a view. I feel as though this needs the attribute selector
| Tip |
|---|
The attributes that will display data on a grid view are those in:
|
Why You Cannot Show Data from 1:many Attributes
If you add an attribute from a stream that is related to the base stream via a 1:many relationship, PhixFlow does not show any data in the grid rows. This is because there may be many values, which cannot be displayed in a single cell of the grid.
| Tip |
|---|
In a future release, it will be possible to aggregate numerical data from a 1:many relationship. |
I am here - the following section should probably go .
Displaying Names Not Identifiers
| Tip |
|---|
In a future release, foreign keys will show the display name rather than the primary key identifier. |
Streams are related via primary keys. If a stream has a foreign key attribute and you use it in a grid view, the column displays the identifier. You can see this in the Teacher column below.
This can be put into the compiling views bit maybe
When creating the view, you need to choose a stream to be the base stream. Your choice depends on the records that you want to show, as it is the base stream's records that are displayed. In this case I want the records from the course stream, in which there is one record for each course, so when I create the view I start with this stream.
The left of the Attribute Selector / View Editor window lists related streams. By selecting the Teacher stream, I can add the Department attribute to the grid view.
For details of creating views and using the attribute selector see Compiling Views and Showing Data on a Screen.
Editing Data in a View
When you create a view that includes attributes from related streams, only data from the base stream is editable. For example, using the view in the example above, you could create a form to add a new course, with fields for:
- Department
- Course
- Classroom
- Teacher.
However, as the Department attribute is not present in the base Course stream, you cannot add the department information via the form.
Creating PhixFlow Streams From Existing Data
You can create streams in an ERD, but this is time consuming. If you already have data, PhixFlow can create streams from it.
You will need to add records to the streams.
table called CourseAttendee. Notice that CourseAttendee does not need a unique identifier.
- A course is attended by many students
- A student takes many courses.
Using ERDs in Views
ERD's are important because the streams, attributes and relationships define the logical structure of the data that your application uses. PhixFlow displays this data to application users via views. Most commonly this will be a grid view, which displays data in rows and columns.
The concept for a view is described below and illustrated on the right:
From related streams, select other attributes to add more columns to the grid.
| Note |
|---|
PhixFlow can only display data from a related primary key. |
Viewing ERD Data
| Anchor | ||||
|---|---|---|---|---|
|
| Insert excerpt | ||||||
|---|---|---|---|---|---|---|
|
| hidden | true |
|---|
9.0.0 todo - technical questions outstanding
- The Course grid has duplicate records, and I don't understand why.
- combination of columns - Its an SQL Join. Phixflow finds ALL the records in the related stream that have a matching key value. This is why the base stream should have a many to one relationships to the related table. This will only have ONE unique record - the primary key. For each record in the base table, PhixFlow goes to the join column, finds the ID in the related table and fetches back the appropriate attribute. This is a single value.
- Do not try to connect from your primary key in your base stream to a foreign key in a related stream . In this case PhixFlow will find multiple records. It cannot display multiple records in the single cell available.
- For the Views page -When aggregation comes in, link to Aggregate Functions
- Editability of fields: ask Gary about: "and only then if you haven't followed a one-to-many relationship." I think this might be the same issue as illustrated by the grids in 4.
explain why one-to-many / many-to-one are important. You might want to include worked examples showing the data you will get if you build a view starting with
- Teacher
- SchoolDept
So I created the grids and see blank columns. I think this relates to aggregation.
What about views other than grids
cards
fields
graphs - line, vertical and horizontal bar
For Creating PhixFlow Streams from Existing Data
Streams line...
See Creating Dashboards and Reports.
Using ERDs in Views
PhixFlow displays data to application users via views most commonly a grid view, which displays data in rows and columns.