IDEF1X, Crow’s Foot, and Chen: Reading the Same ER Model

Quick Compare

I. IDEF1X Notation (keys and dependency)

IDEF1X distinguishes identifying relationships, where the migrated parent key participates in the child’s primary key, from non-identifying relationships, where it does not. A non-identifying child key may be natural or surrogate; it need not be a single generated ID.

IDEF1X symbols and keys

• Square corners: identifier-independent entity. This does not mean every parent relationship is optional.
• Rounded corners: identifier-dependent entity, a child in at least one identifying relationship. A dependent entity can also have other non-identifying relationships.
• Key divider: primary-key attributes appear above it; other attributes, including foreign keys outside the primary key, appear below it.
• Alternate key: another candidate key, often marked AK in tools. A non-unique index or tool-specific IE (“inversion entry”) label is not a candidate key.
• Child dot: without a cardinality qualifier, the parent may have zero, one, or many children. A P qualifier means one or more; Z means zero or one.

How the rule reads in IDEF1X

• Parent: OFFICE; Child: EMPLOYEE.
• Identifying line ⇒ OFFICE_ID becomes part of EMPLOYEE’s PK.
• Non-identifying line ⇒ EMPLOYEE keeps its own PK; OFFICE_ID is an FK only.

II. Crow’s Foot Notation (cardinality and optionality)

Crow’s Foot uses end-markers for maximum (|=one, <=many) and minimum (|=at least one, O=zero allowed). Start from one entity instance and read the markers at the opposite end to count its related instances. Then reverse direction.

• OFFICE→EMPLOYEE: O< at EMPLOYEE side = zero or many employees per office.
• EMPLOYEE→OFFICE: || at OFFICE side = each employee must belong to exactly one office.

III. Chen Notation (entities, diamonds, attributes)

Chen shows entities as rectangles, relationships as diamonds (e.g., assign), and attributes as ovals. Cardinality (1/N) and participation (total/partial) appear near lines/ends. For this example, the OFFICE-to-relationship connection is a single line (partial participation), while EMPLOYEE has a double line (total participation). The 1/N labels specify the maximum counts. Do not add Crow’s Foot circles to this Chen diagram. An attribute such as since_date may be attached as an oval to the relationship diamond.

IDEF1X: Identifying Does Not Mean Mandatory

A dashed relationship can still require a parent. In the shared example, the dashed line has no parent-end diamond: every employee needs an office, even though office_id is outside the employee primary key. For an identifying relationship, the migrated primary-key attributes cannot be null.

Always read the diagram’s declared notation and tool legend. Subtype symbols, colors, and index labels may be tool extensions; they should not be presented as universal “IEEE” conventions without a specific standard or product reference.

From ERD to Relational Tables

The following Oracle SQL examples illustrate two alternative key designs. They are not migrations to run sequentially against an existing application and have not been executed against a live Oracle database in this revision.

Non-identifying example: employee uses a surrogate primary key; the office reference is a separate, non-null foreign key.

-- Parent
CREATE TABLE office (
  office_id   NUMBER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  office_code VARCHAR2(30) UNIQUE NOT NULL,
  name        VARCHAR2(100) NOT NULL
);

-- Child
CREATE TABLE employee (
  employee_id NUMBER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  office_id   NUMBER NOT NULL,
  last_name   VARCHAR2(60) NOT NULL,
  first_name  VARCHAR2(60) NOT NULL,
  CONSTRAINT fk_employee_office
    FOREIGN KEY (office_id) REFERENCES office(office_id)
);

Identifying (child identity depends on parent): parent PK participates in child PK.

CREATE TABLE employee_ident (
  office_id   NUMBER NOT NULL,
  emp_no      NUMBER NOT NULL,
  last_name   VARCHAR2(60) NOT NULL,
  first_name  VARCHAR2(60) NOT NULL,
  CONSTRAINT pk_employee_ident PRIMARY KEY (office_id, emp_no),
  CONSTRAINT fk_employee_ident_office
    FOREIGN KEY (office_id) REFERENCES office(office_id)
);

In the identifying alternative, emp_no is unique within an office, so two offices can use the same number. Moving an employee to another office changes this composite primary key and may affect references to that employee. Choose this design only when that identity rule matches the business. In both examples, an office can exist without employees; a foreign key on employee does not require an office to have child rows.

Cheat Sheet: Reading the Same Rule

• IDEF1X: solid = identifying; dashed = non-identifying; watch which keys migrate and where.
• Crow’s Foot: |/< (max), |/O (min). The || next to OFFICE means “exactly one office per employee.”
• Chen: check 1/N markers and line styles for total vs partial participation; relationship attributes hang off the diamond.

Common Pitfalls

• Forgetting optionality (mixing up O vs |).
• Treating a conceptual M:N relationship as an implementation with a single foreign key. A direct M:N is valid in a conceptual model; resolve it through an associative table for the relational schema.
• Confusing identifying with mandatory: identifying affects keys; mandatory affects minimum participation.
• Dropping valid business-key uniqueness constraints when adding surrogate keys.

What to Use When

• Crow’s Foot: day-to-day design and reviews; concise handoff to implementation.
• Chen: pedagogy, discovery workshops, or when relationship attributes must be explicit.
• IDEF1X: regulated/standards-driven environments needing precise key/relationship semantics.

Reference and further reading

The IDEF1X symbols above follow the historical FIPS 184 IDEF1X specification, particularly sections 3.5, 3.8, and 3.9. For the course introduction, return to the three ER model notations; continue with entities, attributes, and relationships.