The Context
Spira has three editions (SpiraTest, SpiraTeam, and SpiraPlan). Since each edition builds on the previous edition with more features, there are various tables that are exposed as the available tables in SpiraDocs. The relationship exposed here can be derived from the primary_keys (unique to the table) and the foreign_keys (relationship to another table). If all the tables are represented in a single model, it will be complex and unreadable. So, some higher level model simplification is required.
Simplified ERD Model Description
Given below is the higher level relationship between the groups categorizing a set of tables. The description of each group is given beneath the diagram.

Group Name | Summary Description |
Essential PPM | A collection of entities representing how the workspaces are organized as products, programs, and portfolios. Products derive properties from templates. |
Essential Traceability | A collection of entities representing how essential artifact traceability is maintained using requirements, testcases, and incidents. |
Scenario Traceability | A collection of entities that allow scenario level traceability using test sets, configurations, and automation hosts. |
Artifact Connections | A collection of entities that allow track documents, associations, and comments. |
Team Accountability | A collection of entities that use tasks and risks. |
Product Auditability | A collection of entities that use baselines and history tracking. |
Program Functions | A collection of entities for program and portfolio level information to support scaled agile functions. |
Essential PPM
The model below represents the entities and their relationships with other entities in this group.

Essential Traceability
The model below represents the entities and their relationships with other entities in this group.

Scenario Traceability
The model below represents the entities and their relationships with other entities in this group.

Artifact Associations
The model below represents the entities and their relationships with other entities in this group.

Team Accountability
The model below represents the entities and their relationships with other entities in this group.

Product Auditability
The model below represents the entities and their relationships with other entities in this group.

Program Mappings
The model below represents the entities and their relationships with other entities in this group.

Summary
While the above ER representation is given, it is a snapshot in time! As Spira evolves, additional entities may be added. Please consult SpiraDocs for the latest tables as well as the detailed attributes available for every entity.