# Data model diagram

The same data gets modelled three times at different levels of commitment, and mixing the levels is the usual cause of unproductive modelling meetings.

Draw the level you need, say which level it is, and keep them as separate diagrams.

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## When to reach for one

* Early domain modelling, before anybody has chosen a database.
* Explaining to a business audience without column types on the page.
* Documenting the gap between the domain model and what the tables actually look like.

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## How to draw it in Capable

| **Route** | **Use when**                                   |
| --------- | ---------------------------------------------- |
| Mermaid   | Conceptual and logical models, quickly.        |
| DBML      | Physical models where types and keys matter.   |
| draw.io   | Presentation versions for a business audience. |

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## A worked example

A conceptual model: entities and relationships, no types:

![Data model diagram](https://help.gocapable.com/images/att1417543695.svg)

The source, which you can paste into a new diagram and edit:

```
erDiagram
  CUSTOMER ||--o{ SUBSCRIPTION : holds
  SUBSCRIPTION ||--|| PLAN : "is on"
  SUBSCRIPTION ||--o{ INVOICE : generates
  INVOICE ||--o{ PAYMENT : "settled by"
```

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## The three levels

| **Level**  | **Contains**                        | **Audience**              |
| ---------- | ----------------------------------- | ------------------------- |
| Conceptual | Entities and relationships          | Business and product      |
| Logical    | Attributes, keys, normalisation     | Analysts and engineers    |
| Physical   | Tables, types, indexes, constraints | Whoever runs the database |

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## A few things that catch people out

* Say which level a diagram is. An unlabelled data model gets read at the wrong level.
* Conceptual models have no keys and no types. Adding them makes it a logical model.
* Physical models belong next to the code; conceptual ones belong next to the requirements.

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## Related

[Star schemaA fact table with dimensions around it.](https://help.gocapable.com/diagrams/star-schema.html)

[Data lineage diagramSource, transformation, dashboard.](https://help.gocapable.com/diagrams/data-lineage-diagram.html)

[ExamplesWorking diagrams you can copy.](https://help.gocapable.com/diagrams/examples.html)

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_Get the cardinality right and the rest follows._