Numeric annotations can also be used to indicate that a database element must not or need not be present to match the query.
-
Negated elements must not be present in the matching subgraph. Negated elements are specified by the numeric annotation
[0], indicating that exactly zero database elements can match this query element. -
Optional elements may be but are not required to be present in the matching subgraph. Optional elements are specified by the numeric annotation
[0..]or[0.., indicating that zero or more database elements can match this query element.j]
To be well formed, a query must remain a connected graph after removing any negated or optional elements.
The database fragment shown in Figure 4.24 includes information on movies and any awards they have won.
![Database fragment [Annot_DB04.xml]](images/Annot_DB04.png)
This fragment contains information on three movies, all directed by
Ron Howard. A Beautiful Mind was nominated for
and won the Academy Award for Best Picture for 2002;
Apollo 13 was nominated for the Academy
Award for Best Picture for 1995, but the Oscar was awarded to
Forrest Gump that year; and Far and
Away was not nominated for any Academy Awards.
Suppose we want to find all movies that have not been nominated for an Academy Award. One way to do so is to only match subgraphs that do not include any Academy Award objects.
![Query [Annot_DB04_Q01.qg2.xml]](images/Annot_DB04_Q01.png)
The [0] annotation on the
award object means that this query
matches only those movies that are not linked to any Academy Awards.
For now we’ll simply note that the [1..]
edge annotation is usually the correct annotation for edges adjacent
to negated or optional vertices and proceed accordingly. Additional
information on choosing the correct annotation for edges adjacent to
negated or optional vertices is included in the section
“Annotating edges adjacent to negative and optional
vertices” later in this chapter.
The result of executing this query on the database fragment shown in Figure 4.24 is shown below.
Only a single movie, Far and Away, matches the
query. Note that negated query elements do not appear in the query
results.
Because the query includes a [0]
annotation on the award vertex, there
can be no corresponding object in order to match
the query, and
therefore no award objects can appear in
the matching subgraphs.
Compare this query to a similar query where we don’t care whether the movie has been nominated for an Academy Award.
![Query [Annot_DB04_Q02.qg2.xml]](images/Annot_DB04_Q02.png)
In this example, the award vertex is
annotated with [0..], making matches to
this vertex optional. That is, subgraphs in the database will match
this query regardless of whether or not the corresponding movie has
been nominated for an Academy Award. The results
of executing the new query with the optional
award vertex are shown below.
This time, all three movies appear in the query results. The optional
award vertex lets the query match both
movies that have received nominations and movies that have not
received any nominations. Because Far and Away has
no nominations, its subgraph does not include any award
objects.
Although not yet supported by Proximity, QGraph queries can include negated and optional edges in addition to negated and optional vertices. Care must be taken to avoid disconnected queries; queries with negated or optional edges must also include required alternate paths to all query elements.
Figure 4.29 illustrates a query that
includes a negated edge. This query finds movies that have been
nominated for, but not won an Academy Award. The
won edge is annotated with
[0] indicating that matching subgraphs
must not include a matching link.
To prevent the
query from becoming disconnected, the nominated
edge is required. The [1..] edge annotation
means that the query only matches subgraphs that contain one or more
nominated
links connecting the movie to any
Academy Award objects.
The expected results, had we been able to execute this query in Proximity, are shown below.
Only one movie in our database fragment,
Apollo 13 has been nominated for but not
won an Academy Award.
Because edges adjacent to annotated vertices must themselves be
annotated, how are we to annotate the edge adjacent to a negated or
optional vertex? In general, the usual annotation of
[1..] remains appropriate. We require the
presence of the query edge to avoid having the query become
disconnected.
For example, an optional edge connected to an optional
vertex could result in a disconnected subgraph when the matching
vertex is present but the matching edge is not. Optional or negated
edges are usually inappropriate in most queries, although they may
be safely used if other elements in the query ensure that the query remains
connected after removing any optional or negated elements.
For example, consider the database fragment shown in Figure 4.31.
If we wanted to identify author-book pairs and
also include movie adaptations, if any, we might initially (and
incorrectly) create a query
that looks like that shown in Figure 4.32.
This query uses the improper annotation [0..] for
both the movie vertex and adjacent
adapted-to edge. Such a query can match
disconnected subgraphs. Consider the highlighted subset of elements
from this database shown in Figure 4.33.
The highlighted set of objects and links matches the query shown in
Figure 4.32.
The [0..]
annotation on the movie vertex and
[0..] annotation on the
adapted-to edge mean that the query is
allowed to match database structures that include a matching
movie object but that omit the matching
adapted-to link. This can
produce disconnected subgraphs like the highlighted elements shown
above, which clearly isn’t what was intended by the query.
To prevent the possibility of matching such disconnected subgraphs,
use the [1..] annotation on edges adjacent to a
vertex annotated with [0] or
[0..].