RelationalSample
Entity relationship diagram
Entities, their attributes and the rules that connect them, drawn in crow’s-foot notation. The blueprint for a normalised, transactional database.
Try this
- 01Pick a sentence in the side panel and find its line: the symbol next to each entity says how many of it the other side can have.
- 02Click the line between Customer and Address, then the one between Customer and Order: bar plus crow’s foot versus circle plus crow’s foot. One is mandatory, the other optional.
- 03Notice Order line has a two-part key (order_id + line_no): it cannot exist without its order.
Notation
- Entity
- A thing the business keeps records about (customer, order). Becomes a table.
- Primary key (PK)
- The column or columns that identify one row.
- Foreign key (FK)
- A column that points at another entity’s primary key.
- Bar ( | )
- Exactly one: mandatory participation.
- Circle ( o )
- Zero: the relationship is optional on that side.
- Crow’s foot
- Many. Combined with a bar it reads “one or more”, with a circle “zero or more”.
Use it when
- Designing an operational (OLTP) database
- Agreeing business rules with stakeholders before building
- Documenting an existing schema
Avoid it when
- Designing analytical models for BI (use dimensional modelling)
- Highly connected data where the relationships are the point (consider a graph)