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This page is for data modellers or application designers. It explains how to use an ERD to define your system's data and its relationships.

Overview

PhixFlow ERDs are simplified Entity-Relationship Diagrams. "Entities" are things in the world that you want to represent. In PhixFlow, things in the world are represented by tables.

A table has attributes that represents aspects of the entity. For example, you might want to record the contact details for a person. An ERD for this has a table called "Person" that has the attributes "First Name", "Family Name, "Address" and "Telephone Number". Because the ERD defines the data, each attribute sets the type of data it requires. For example, "Name" has a type of "string", whilst Telephone number has a type of  "number".

Each table must also have a special attribute called its primary key. PhixFlow automatically adds a primary key called UID when you create a table. The UID, or Unique IDentifier, always has a unique value. For example, a person might have the same first name or family name or even both, so these attributes cannot be a UID. PhixFlow makes sure every data record has a unique value, by generating a unique number for the UID.

If an entity has something unique for each instance of it, you can use that as the UID. For example, in the UK a person has a unique National Insurance number. However, if you use something like this, PhixFlow cannot make sure all UIDs are unique. 

Sometimes we want to have the same data different tables. For example, you might want to have people's names in the Customer, Employee and Mail-List tables. The relationships between tables on an ERD show how tables can get this data from the Person table, so that is not duplicated. A relationship connects the primary key in one table to an attribute in another. 

Person UID → Employee Name???

The ERD's entities, attributes and relationships define the logical structure of the data that your application uses.

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. 

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Example ERD

The following example shows an ERD for a school. Each box represents a different table. "I feel it would be useful to have a narrative here, School it made up of Teachers and Class Rooms...."

Key Concepts

TermMeansSchool Example

Table


A table represents a thing or entity. The table name is displayed in the header.

Choose a name that reflects the thing your table represents.

Tables:

  • School Department
  • Class Room
  • Teacher
  • Course
  • Student
Attributes

The table has a list of attributes. If you display the table data in a grid, the attributes are the column headers. They name what the data is, and sets the type (date, integer, string etc.) and any format details. Provide an example?

Student table has attributes:

  • StudentID
  • Student Name
  • Student Address
  • Intake Year
Primary key

PhixFlow automatically generates an attribute that is the unique identifier, UID. This is the table's primary key that is used to uniquely identify each record in the table. The records for this attribute will all have a unique value. You may want to rename UID to something more descriptive, although it's a good idea to include ID in the name. ID is a reserved word.

Primary keys are indicated using by a green key icon.

We recommend that a primary key attribute is an integer, because PhixFlow can automatically generate a unique integer value for each new data record. Who said this? I have need UIDs that are alpha numeric e.g. at IHS they prefixed their UIDs with a character to depict the type C1234. Just want to check this is true.

Primary keys in different tables:

  • DepartmentID
  • ClassRoomID
  • TeacherID
Relationship

You can create a relationship between the primary key in one table, to an attribute in another table.  On the diagram the relationship is shown as an arrow from a primary key to an attribute in another table. Relationships always point from a primary key to a non-primary attribute. 

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 → runs→ several courses
  • classroom → is ued by→ many courses
Foreign key

Relationships show that the data in a primary key in one table also appears in an attribute in another table. The attribute may have a different name, but it must represent the same data. This attribute is a foreign key. Do we need to say it needs to represent the same data type and parameter e.g. Integer, precision 10?

Foreign keys are indicated using a grid icon.

When you create a relationship in an ERD, PhixFlow automatically sets the foreign key status for the attribute.

Employee attributes:

  • EmployeeID (primary key)
  • Name
  • Address
  • Phone
  • Department = foreign key for DepartmentID

Department attributes

  • DepartmentID - primary key
  • Department Name
  • Purpose
  • Department Manager = foreign key for EmployeeID

Understanding Relationships

Relationships between tables have a direction that depends on the table on which you are focused. For example, looking at the Teacher table, shown below, you can see 2 relationships.

  • The school DepartmentID primary key has a relationship to the Teacher table. This relationship is highlighted in blue.
  • The Teacher table has a relationship from its TeacherID primary key to an attribute in the Course table. This relationship is highlighted in green.

One-to-many and Many-to-one

PhixFlow always draws a relationship arrow from a primary key to another, non-primary, attribute 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-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 multiple courses. This relationship is shown using the intermediate table called CourseAttendee.

  • A course→ is attended by→ many students
  • A student→ takes → many courses


Tables and Data Records

The ERD does not include the 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 add data records to your table: link??

For tables that have data records, you can see the records.

  1. Click on a table in the ERD to open its properties.
  2. In the properties toolbar, click  More Options.
  3. Select tbc




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