Tables
A table has attributes that represent aspects of the entity.
A table represents a thing or entity.and Attributes
The table name is displayed in the header.
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Choose a name that reflects the thing your table represents. |
Tables:
- School Department
- Class Room
- Teacher
- Course
- Student
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:
- StudentID
- Student Name
- Student Address
- Intake Year
Because the ERD defines the data, each attribute has a type for the data, such as string or integer.
To see the context menu for a table:
- in the table header, click
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Special Attribute: Primary Key
Each table must also have a special attribute called its primary key. PhixFlow automatically adds a primary key when you create a table. The primary key is called UID, which is short for Unique IDentifier because every data record will always have a unique value for this attribute. For example, several customers can have the same first or family name, or even both, so these attributes cannot be a UID. PhixFlow makes sure every data record has a unique value, by generating a unique number for the UID.
If an entity has something unique for each instance of it, you can use that as the UID. For example, in the UK a person has a unique National Insurance number. However, if you use something like this, PhixFlow cannot make sure all UIDs are unique.
Sometimes we want to have the same data in different tables. For example, the Order table may need to include the customer name and address. It's important not to duplicate data in PhixFlow, so instead you create a relationship between the Customer and Order tables. The order table can "get" the data it needs from the customer table. To make sure it is getting the correct data record, the relationship must be between one table's primary key/UID and an attribute in another table. PhixFlow automatically sets this as the foreign key.
The ERD's entities, attributes and relationships define the logical structure of the data that your application uses.PhixFlow uses the relationships between tables when you come to make views of data. You can create views that include include attributes from different tables, provided there is a relationship. When you add an attribute to a view, PhixFlow can provide a list of all the other attributes that you can use in the view.
Key Concepts
Table
A table represents a thing or entity. The table name is displayed in the header.
Choose a name that reflects the thing your table represents.
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:
- StudentID
- Student Name
- Student Address
- Intake Year
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 by a green key icon.
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The UID created by PhixFlow has a data type of integer, because PhixFlow can automatically generate a unique integer value for each new data record. If you want to use a different type of value, for example alpha-numeric, PhixFlow cannot create unique values for each record. You must ensure all records have unique values. |
Primary keys in different tables:
- DepartmentID
- ClassRoomID
- TeacherID
Relationships and Foreign Keys
Using Data Without Duplication
Sometimes we want to have the same data in different tables. For example, the Order table may need to include the customer name and address. It's important not to duplicate data in PhixFlow, so instead you 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 toCustomer and Order tables. The order table can "get" the data it needs from the customer table. To make sure it is getting the correct data record, the relationship must be between one table's primary key/UID and 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.
By convention, set the name of a relationship an action (verb) that follows the left-right flow of the diagram.
- a department→ has→ many teachers
- a teacher has one department
- a teacher → runs→ several courses
- classroom → is used by→ many courses
PhixFlow automatically sets this as the 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:
- EmployeeID (primary key)
- Name
- Address
- Phone
- Department = foreign key for DepartmentID
Department attributes
- DepartmentID - primary key
- Department Name
- Purpose
- Department Manager = foreign key for EmployeeID
Understanding Relationships
The ERD's entities, attributes and relationships define the logical structure of the data that your application uses.
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.
By convention, set the name of a relationship an action (verb) that follows the left-right flow of the diagram.
- a department→ has→ many teachers
- a teacher has one department
- a teacher → runs→ several courses
- classroom → is used by→ many courses
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 has a relationship to the Teacher table. This relationship is highlighted in blue.
- The Teacher table has a relationship from its TeacherID primary key to an attribute in the Course table. This relationship is highlighted in green.
By convention, set the name of a relationship an action (verb) that follows the left-right flow of the diagram.
- a department→ has→ many teachers
- a teacher has one department
- a teacher → runs→ several courses
- classroom → is used by→ many courses
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-many
To create a many-to-many relationship, you need an intermediate table that has foreign key attributes from each of the tables you want to have a relationship. 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
Data Structure and Views
PhixFlow uses the relationships between tables when you come to make views of data. You can create views that include include attributes from different tables, provided there is a relationship. When you add an attribute to a view, PhixFlow can provide a list of all the other attributes that you can use in the view.
Data: Tables, Attributes and
DataRecords
The ERD does not include the data records. If you think of an Excel spreadsheet, a table in an ERD includes the name of the worksheet and the titles of the columns. It does not show the data records from the rows.
You can add data records to your table: link??
For tables that have data records, you can see the records.
- Click on a table in the ERD to open its properties.
- In the properties toolbar, click
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Context menu
Jiras
?? Initial Cap only
Configure → Table properties
Hide Attributes - Hide attributes
Remove this item from the Diagram → Remove from ERD... model, screen, workflow, actionflow...
Permanently Delete → Delete everywhere
Inital cap only
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