ENTITY-RELATIONSHIP DATA MODEL - Database Design and Development: An Essential Guide for IT Professionals - page 224

ProductNo

ProdDesc

PRODUCT

ProductNo ProductNo

has

has

PRICE
REVISION

COST
REVISION

Unitcost

EffectiveDate UnitPrice EffectiveDate

Figure 7-16 Time-dependent model components.

OrderNo OrderDate

OrderAmount

ORDER

contains

OrderNo

ORDER
DETAIL

UnitPrice

NoOfUnits

I temNo

ItemDesc

Figure 7-17 Identifying relationship.

for its existence. This implies that you cannot have an order detail entity unless a
corresponding order entity exists. Without an order entity an order detail entity
cannot exist in the database. It is meaningless to think of stray and loose order detail
entities existing in the database. Figure 7-17 shows the relationships between two
entity types presented in Figure 7-5. Notice the notations used in the diagram to
distinguish the components.
Note the following points about the two entity types:

Weak entity type. An entity type that is existence-dependent on another entity
type. ORDER DETAIL is a weak entity type. It depends on ORDER entity.