This page is for data modellers or application designers. It explains how to use a relationship diagram to understand how your system's data is interconnected.

TODO: Check for info in these links

• https://www.sisense.

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• com/en-gb/glossary/entity-relationship-diagram/
• https://www.smartdraw.com/entity-relationship-diagram/
• https://www.guru99.com/er-diagram-tutorial-dbms.html

Overview

Use an entity relationship diagram are commonly used to draw out the relationships between data elements in a database. It helps you to visualise how the systems information is connected.

The components of a diagram

• Each "entity" is displayed as a box,  This is equivalent to a stream in PhixFlow
• Entities have attributes
• Entities usually have one attribute that is a unique Primary key.
  Foreign keys are linked to the primary key in another entity
• Relationships show how entities are connected
• Attributes
• between streams - entities ??
• between attributes. 

Relationships can be 1-to-1, 1-to-many or many-to-many. TODO - self-connected?

Creates

Contains

Hasto visualise how your system's information is connected. PhixFlow relationship diagrams are similar to simplified entity-releationship diagrams (ERDs).  In PhixFlow, entities are represented by streams and their attributes. You can use relationship diagrams to draw the relationships between existing streams, or you can create the diagram and, at the same time, create the streams it represents.

For example, a simple relationship diagram can represent a company and its employees. A company has attributes, such as a name, address, turnover. In a PhixFlow stream, a company must also have an attribute that identifies the company uniquely. A company also has employees. In a relationship diagram this looks like:

Company  → employs → People
UniqueID
Name
Address
Turnover

Relationships can work both ways, for example a person "works for" the company. The relationships are usually expressed as an action (verb) that follows the left-right flow of the diagram. For example:

• either company → employs → people
• or people→ workfor → company
• but not company → workfor→ people

The relationship connects identifiers in each stream. A unique identifier in a stream is it's primary key. It uniquely identifies the row (data record) in the stream. In some cases a unique attribute from one stream is represented in another stream - this is a foreign key . It uniquely identifies a row in a different stream.

For example Person has attributes

• Name
• Address
• Employer - this contains the unique identifier of the company and is therefore a foreign key.

Relationships can be

• 1-1: primary key to primary key
• 1-to-many: primary key to foreign key
• many-to-1: foreign key to primary key
• many-to-many: foreign key to foreign key.

PhixFlow imposes no restrictions on naming relationships, but the relationship name must be unique. We recommend that you use an action word that represents the relationship, avoiding "has" or "contains".

The components of a diagram:

• Each stream is displayed as a box 
• The box also lists all the attributes
• Primary keys and foreign keys are highlighted in green
• Relationships connect stream attribute primary and foreign keys.
• You can collapse a stream box to show only the attributes that are primary or foreign keys.  

TODO Invoice example 

• Invoice has the totals information.  has attributes: custID, total, date etc. 
• Parent object - there are many
• And it shows the invoice lines e.g. call breakdown  - these are not the data records for the invoice
• Children with their own attributes: invoice id - 
• Relationship is btwn ID field invoice - and the invoice id
• Receipt is an invoice - individual lines - info

To show the process flow of data, use a BPMN diagramTODO purpose.

Other affected pages: repository, toolbars

Properties

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_standard_settings _standard_settings nopanel true

Basic Settings

Drawing Entity-Relationship Diagrams

To create an ERD, in the application repository, right-click ERD and select 

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_add _add nopanel true

. PhixFlow opens the properties for diagram, where you can add the name and a description for it. When you press Return or save a new properties tab, PhixFlow opens an empty diagram in the workspace on the left.

Adding Entities

To open an existing ERD:

• either, in the Repository, right-click the name and select Display.
• or, in the properties toolbar, click the ERD icon (TODO add).

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As streams can contain many attributes, you can collapse the view to show only primary and foreign keys. TODO ADD screenshot

Adding Relationships

To add a relationship, click on one attribute and drag the mouse pointer to an attribute in another stream. PhixFlow extends a red arrow line. Stop dragging, or click on an attribute name to connect the two. PhixFlow displays an attribute in bold when it has focus.

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You cannot create a relationship between attributes if they are not a primary or foreign key. If you add a relationship between attributes, PhixFlow automatically sets them to be one of these.

ERDs and Relational Views

TODO - Add how ERDs and Relational vies interact.

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• I ended up with 3 streams that did not appear until the diagram is saved - can I reproduce this
• relationship end points can be drawn a long way from the mouse. 
• I can untick keys and it keeps the relationship
• When entities are below each other PhixFlow gets confused.

Toolbar

TODO Same as Model Window Toolbar Icons

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