Implementing db4o aggregate queries using Java

Db4o is an open source Object database based on Java. It provides a concise and efficient way to persist and query Java objects. The following are the steps to implement various aggregation queries in db4o using Java, as well as the relevant Maven coordinates and sample code: Step 1: Add Maven dependency for db4o Add the following Maven coordinates to the pom.xml file of the project: <dependency> <groupId>com.db4o</groupId> <artifactId>db4o-full-java5</artifactId> <version>8.0.249.16098</version> </dependency> Step 2: Create an object model Define the object model that needs to be persisted in Java code. For example, we can create a class called 'Person': public class Person { private String name; private int age; //Omitting constructors and getter/setter methods } Step 3: Connect to the db4o database Create a db4o Object database and connect to the specified file path. In this example, we save the database file in a file named 'database. db4o': ObjectContainer db = Db4oEmbedded.openFile("database.db4o"); Step 4: Insert Data Insert some data into the database. For example: Person person1 = new Person("Alice", 25); Person person2 = new Person("Bob", 30); db.store(person1); db.store(person2); Step 5: Execute Aggregation Query You can use db4o's query API to perform various data aggregation queries. The following are several commonly used examples of aggregation queries: 1. Count Query query = db.query(); query.constrain(Person.class); int count = query.execute().size(); System.out.println("Total count: " + count); 2. Sum Query query = db.query(); query.constrain(Person.class); int sum = 0; List<Person> results = query.execute(); for (Person person : results) { sum += person.getAge(); } System.out.println("Total age sum: " + sum); 3. Average Query query = db.query(); query.constrain(Person.class); double sum = 0; List<Person> results = query.execute(); for (Person person : results) { sum += person.getAge(); } double average = sum / results.size(); System.out.println("Average age: " + average); 4. Max Query query = db.query(); query.constrain(Person.class); int maxAge = 0; List<Person> results = query.execute(); for (Person person : results) { if (person.getAge() > maxAge) { maxAge = person.getAge(); } } System.out.println("Max age: " + maxAge); 5. Min Query query = db.query(); query.constrain(Person.class); int minAge = Integer.MAX_VALUE; List<Person> results = query.execute(); for (Person person : results) { if (person.getAge() < minAge) { minAge = person.getAge(); } } System.out.println("Min age: " + minAge); Step 6: Close database connection After using the database, it is necessary to close the database connection to release resources: db.close(); The above are the basic steps and sample code for implementing various aggregation queries in db4o using Java. According to specific needs, more complex aggregation query operations can be performed using db4o's more query APIs.