Database Reference
In-Depth Information
Identity, rolldown, and rollup are the same techniques for both an RDBMS and in Mon-
goDB:
•
Identity
. Identity is the closest to subtyping, because the subtyping symbol is re-
placed with a one-to-one relationship for each supertype/subtype combination. The
main advantage of identity is that all of the business rules at the supertype level and
at the subtype level remain the same as in the logical data model. That is, we can
continue to enforce relationships at the supertype or subtype levels as well as en-
force that certain fields be required at the supertype or subtype levels. Identity al-
lows us to still require the
printVersionWeightAmount
for print versions and the
eBookDownloadSize
for eBooks, for example. The main disadvantage of identity
is that it can take more time to retrieve data as it requires navigating multiple tables
to access both the supertype and subtype information.
•
Rolldown
. Rolldown means we are moving the attributes and relationships of the
supertype down to each of the subtypes. Rolling down can produce a more user-
friendly structure than identity or rolling up because subtypes are often more con-
crete concepts than supertypes, making it easier for the users of the data model to
relate to the subtypes. However, we are repeating relationships and fields, which
could reduce any user-friendliness gained from removing the supertype. In addi-
