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.

ERD

This is a high level view of an ERD diagram for a school

Table

This is an illustration of the elements which make up a Table in an ERD.

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, for example the Address attributes is a String. To change the attribute to reflect the nature of your data, click on the attribute's name to open it's properties, here you can edit its details including the data type.

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 Rollback Recordsets.

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 it's 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 suitable for a human. For this reason, when you create a table PhixFlow automatically adds an attribute called Name and ticks the Display Name property. The Display Name will be shown in place of the Primary Key when displayed on a View. Display Names do not need to be unique and should be something user friendly.

Lets look at the following example:

The SchoolDept table is created and PhixFlow has created both a UID, for example 1490 or 1550, and a Name, for example Mathematics.

To help users make sense of this PhixFlow will display the department's Name, when when any relational view makes reference to the SchoolDept using it's Primary Key. See Using Relational Views for more information. 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, or linked to, information in another table. The key benefit is you can access data in many different tables without needing to duplicate data.

In the 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 represents this relationships using a line that joins tables: the SchoolDept DeptID to the Teacher Department:

When you create screens that show data, PhixFlow uses the relationships from the ERD to provide options for displaying data from related attributes in the same grid, form, card, graph or chart; see Understanding Data Views. This means we only need to store the School DeptID against a teacher, but we have access to all of the SchoolDept details such as Name.

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.

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 instance of a value of DeptID. For example, the Maths Department UID could occur 10 times, once for each of the 10 maths teachers.

Although the relationship is between the primary key and a foreign key, when the foreign key is included in a screen, PhixFlow shows the attribute that has Display Name selected, which is the Name attribute by default. This gives the user a the meaningful string, rather than the primary key's identifier. For example, a table showing Teacher.Department will have data such as Science, rather than an ID like1490.

One-to-many and Many-to-one

PhixFlow draws a relationship line between a primary key and a foreign key. This represents a 1:many relationship. The "many" end of the line has several lines. For example, one teacher runs many courses.  A many:1 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-to-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.

Using ERDs in Views

PhixFlow displays data to application users via Views. Most commonly this will be a grid view, which displays data in table rows and columns.

The concept for a view is described below:

1. Start with a specific table, which is the base table for the view.
2. Select the attributes from the base table that you want to be the columns in the grid.

3. From related table, select other attributes to add more columns to the grid.

4. PhixFlow joins the related tables via their relationships.
   Under-the-hood this is done using a direct SQL join.

Why You Cannot Show Data from 1:many Attributes

If you add an attribute from a table that is related to the base table 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.

Creating PhixFlow Tables From Existing Data

You can create tables in an ERD, but if you already have data, PhixFlow can create tables from it. See Adding Content to an ERD for more information.

Displaying ERD Data

If you are working with tables that have records, to see the records:

1. In the ERD, click on a table to open its properties.
2. In the properties toolbar, click  More Options.
3. Select  Show view.
4. PhixFlow displays a the default grid view, listing all the records.

Also see Creating Dashboards and Reports for full details.