Example ERD
The following example shows an ERD for a school. Each box represents a different table. "I feel it would be useful to have a narrative here, School it made up of Teachers and Class Rooms...."
Key Concepts
| Term | Means | School Example | ||
|---|---|---|---|---|
Table | A table represents a thing or entity. The table name is displayed in the
| Tables:
| ||
| Attributes | The table has a list of attributes. If you display the table data in a grid, the attributes are the column headers. They name what the data is, and sets the type (date, integer, string etc.) and any format details. Provide an example?
| Student table has attributes:
| ||
| Primary key | PhixFlow automatically generates an attribute that is the unique identifier, UID. This is the table's primary key that is used to uniquely identify each record in the table. Primary keys are indicated
| Primary keys in different tables:
| ||
| Relationship | You can create a relationship between the primary key in one table, to an attribute in another table. On the diagram the relationship is shown as an arrow from a primary key to an attribute in another table. Relationships always point from a primary key to a non-primary attribute.  When you are designing screens, you can create views to display attributes from a table AND from other, related tables. Between two tables with multiple relationships, each connection must have a unique name. We need to review this as I am not sure we give the user an option to name a relationship? | By convention, set the name of a relationship an action (verb) that follows the left-right flow of the diagram.
| ||
| Foreign key | Relationships show that the data in a primary key in one table also appears in an attribute in another table. The attribute may have a different name, but it must represent the same data. This attribute is a foreign key . Do we need to say it needs to represent the same data type and parameter e.g. Integer, precision 10? Foreign keys are indicated using a grid icon.  When you create a relationship in an ERD, PhixFlow automatically sets the foreign key status for the attribute. | Employee attributes:
Department attributes
|
Understanding Relationships
Relationships between tables have a direction that depends on the table on which you are focused. For example, looking at the Teacher table, shown below, you can see 2 relationships.
- The school DepartmentID primary key connects into the Teacher table. This inbound relationship, called has-teachers, is highlighted in blue.
- The Teacher table connects from its TeacherID its TeacherID primary key to an attribute in the Course table. This outbound relationship, called runs, is highlighted in green.
One-to-many and Many-to-one
PhixFlow always draws a relationship arrow from a primary key to another, non-primary, attribute foreign key. This represents a one-to-many relationship, for example, one teacher→ runs→ many courses.
A many-to-one relationship is implied when you read a relationship in the opposite direction. For example, a department has many teachers, and several teachers work for one department.
Many-to-manymany
To create a many-to-many relationship, you need an intermediate table that has foreign key attributes from the tables you want to connect. For example, many students take multiple courses. This relationship is shown using the intermediate table called CourseAttendee.
- A course→ is attended by→ many students
- A student→ takes → many courses