Generalization hierarchies

Concepts

Hierarchies describe a supertype and its specialised subtypes.

A subtype can be the supertype of another hierarchy.

Identity maps each subtype to a table with the inherited key.

Rolldown maps each leaf subtype to a table.

Rollup combines subtype fields in the root table.

Shared-primary-key foreign keys can be recognised as identity layouts.

Transformation changes are compared and rendered as ordered migrations.

Lint identifies missing discriminators, cycles and incompatible inherited fields.

Use a hierarchy declaration with category and subtype lines.

JSON version 6 carries hierarchy declarations and target transformations.

Category nodes connect the supertype to its subtypes.

The browser editor can create, edit, synchronise and delete hierarchies.

Multiple inheritance and custom discriminator expressions are not supported.