Db4o aggregate query
Db4o is an object-oriented database primarily used for storing and retrieving objects in Java and. NET applications. It does not directly support traditional aggregate queries in relational databases, such as SUM, COUNT, AVG, etc. However, similar aggregation queries can be implemented using the query function of db4o.
The following is an example table structure and sample data:
Suppose we have a class called Person that represents information about a person, including name, age, and gender attributes.
public class Person {
private String name;
private int age;
private String gender;
//Constructor, Getter, and Setter methods omitted
}
Example data:
Person person1 = new Person("Alice", 25, "Female");
Person person2 = new Person("Bob", 30, "Male");
Person person3 = new Person("Charlie", 27, "Male");
Person person4 = new Person("David", 35, "Male");
//Store to database
ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded.newConfiguration(), "persons.db4o");
db.store(person1);
db.store(person2);
db.store(person3);
db.store(person4);
db.commit();
Now, we will introduce some common db4o query techniques for aggregate queries.
1. Query the average age of everyone:
ObjectSet<Person> result = db.query(new Predicate<Person>() {
@Override
public boolean match(Person person) {
return true;
}
});
int totalAge = 0;
int count = 0;
while (result.hasNext()) {
Person person = result.next();
totalAge += person.getAge();
count++;
}
double averageAge = (double) totalAge / count;
System.out.println("Average age: " + averageAge);
2. Query the number of males:
ObjectSet<Person> result = db.query(new Predicate<Person>() {
@Override
public boolean match(Person person) {
return person.getGender().equals("Male");
}
});
int count = 0;
while (result.hasNext()) {
result.next();
count++;
}
System.out.println("Number of males: " + count);
3. Query the name and age of the oldest person:
ObjectSet<Person> result = db.query(new Predicate<Person>() {
@Override
public boolean match(Person person) {
return true;
}
});
Person oldestPerson = null;
while (result.hasNext()) {
Person person = result.next();
If (oldestPerson==null | | person. getAge()>oldestPerson. getAge()){
oldestPerson = person;
}
}
if (oldestPerson != null) {
System.out.println("Oldest person: " + oldestPerson.getName() + ", Age: " + oldestPerson.getAge());
}
Although db4o does not directly support traditional aggregate queries, it is possible to achieve similar functionality through querying and traversing objects. The above examples are only for the purpose of explanation. In practical application, it may be necessary to make appropriate adjustments according to specific needs and Domain model.
