Database Reference
In-Depth Information
Figure 2. Projection and de-projection operations
owner (instance of concept Person). Thus references are not simply a navigational tool but rather allow
us to represent partial order which is a formal basis for representing data semantics.
Two operations of
projection
and
de-projection
are defined in COM by using partially ordered
structure of elements. Projection is applied to a set of elements and returns all their greater elements
along the specified dimension. Since greater elements are represented by references, this definition can
be formulated as follows: projection of a set is a set of all elements referenced by some dimension. For
example (Figure 2, left), in order to find all owners of the bank accounts we can project these accounts
up to the collection of owners:
(Accounts)
->
owner
->
(Persons)
The set of accounts and owners can be restricted so that the selected accounts are projected to the
selected persons:
(Accounts | balance > 1000)
->
owner
->
(Persons | age < 30)
This query returns all young owners of the accounts with large balance.
De-projection is applied to a set of elements and returns all their lesser elements along the specified
dimension. In terms of references, de-projection is defined as follows: de-projection of a set is a set of all
elements referencing them by some dimension. For example (Fig. 2, right), in order to find all accounts
of the persons we can de-project these persons down to the collection of accounts:
(Persons)
<-
owner
<-
(Accounts)
The owners and accounts can be restricted and then this query returns all accounts with large balance
belonging to young owners:
(Persons | age < 30)
<-
owner
<-
(Accounts | balance > 1000)
