Querying object structures requires navigation. An example object
structure was shown on the navigating objects
page (new
window). An example of an ODMG OQL query follows. This query appears in the
published ODMG
standard (new window). For those of you
who know SQL, you will notice that it is similar to an SQL query, but with
object extensions. The similarity occurs because OQL is based on SQL-92.
select c.address
from Persons p,
p.children c where p.address.street = "Main Street" and
count(p.children) >= 2 and
c.address.city != p.address.city
The "dot" notation used in the query traverses the data structure
shown on the navigating objects page (new
window). The query inspects all children of all Persons to find
people who live on Main Street with at least two children. It returns only those
addresses of children who do not live in the same city as their parents. It
navigates from the Person class using the child reference to another instance of
the Person class and then to the Address and City classes.
ODMG OQL is not the only way to write these object queries. Similar queries can be
written using the other query languages used by some of the object database
products.
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based on 7 reviews.
based on 5 reviews.
based on 7 reviews.
