Database Reference
In-Depth Information
This model shows the two corresponding physical entities representing relational database
tables, based upon the prior logical order entities:
The physical entities also contain database-specific information, such as the format and
length of an attribute (
orderStatusDescription
is 50 characters), and whether the data ele-
ment is required to have a value (
orderNumber
is not null and is therefore required to have
a value, but
orderActualDeliveryDate
is null and therefore not required to have a value).
An entity instance is an example of an entity. So
Order 4839-02
is an instance of
Order
,
for example. An entity instance at the RDBMS level is called a record.
MONGODB DOCUMENT = RDBMS RECORD (ENTITY INSTANCE AT PHYSICAL LEVEL)
