31. 31ระบบฐานข้อมูล
Composite Entities
Also known as bridge entities
Composed of the primary keys of each of the entities to be
connected
May also contain additional attributes that play no role in the
connective process
35. 35ระบบฐานข้อมูล
RELATIONSHIP Strength
Existence dependence
Entity’s existence depends on the existence of one or more
other entities
Existence independence
Entity can exist apart from one or more related entities
Weak (non-identifying) relationships
One entity is not existence-independent on another entity
Strong (Identifying) Relationships
Related entities are existence-dependent
40. 40ระบบฐานข้อมูล
Relationship Strength and Weak Entities
Weak entity meets two conditions
Existence-dependent:
Cannot exist without entity with which it has a relationship
Has primary key that is partially or totally derived from the
parent entity in the relationship
52. 52ระบบฐานข้อมูล
Total participation (แสดงแทนด้วยเส้นคู่): ทุกๆ entity ใน entity set
participates อย่างน้อยเท่ากับ 1 relationship ใน relationship set (บางครั้ง
เรียก Mandatory)
Partial participation: บาง entities อาจจะไม่ participate กับ relationship
ใน relationship set เลยก็ได้(บางครั้งเรียก Optional)
ตัวอย่าง การมีส่วนร่วมในความสัมพันธ์ แบบอื่น
53. 53ระบบฐานข้อมูล
The generalization hierarchy depicts the parent-child
relationship.
The supertype contains the shared attributes, while
the subtype contains the unique attributes.
A subtype entity inherits its attributes and its
relationships from the supertype entity.
Entity Supertypes and Subtypes
55. 55ระบบฐานข้อมูล
The supertype entity set is usually related to several
unique and disjointed (nonoverlapping) subtype
entity sets.
The supertype and its subtype(s) maintain a 1:1
relationship.
Entity Supertypes and Subtypes