Implementing Object DB aggregation queries using Java
To use Object DB for various aggregation queries in Java, it is necessary to first add the dependencies of Object DB, then establish a connection to the database, and write corresponding query code.
Add Maven dependency for Object DB:
<dependencies>
<dependency>
<groupId>com.objectdb</groupId>
<artifactId>objectdb</artifactId>
<version>2.8.1</version>
</dependency>
</dependencies>
Establish database connection:
EntityManagerFactory emf = Persistence.createEntityManagerFactory("objectdb://localhost/mydb.odb");
EntityManager em = emf.createEntityManager();
Here are some common examples of aggregation queries:
1. Total number of statistical records:
Query query = em.createQuery("SELECT COUNT(*) FROM Entity");
Long count = (Long) query.getSingleResult();
System. out. println ("Total records:"+count);
2. Sum:
Query query = em.createQuery("SELECT SUM(field) FROM Entity");
Double sum = (Double) query.getSingleResult();
System. out. println ("Total of fields:"+sum);
3. Take the maximum value:
Query query = em.createQuery("SELECT MAX(field) FROM Entity");
Double max = (Double) query.getSingleResult();
System. out. println ("Maximum value of field:"+max);
4. Take the minimum value:
Query query = em.createQuery("SELECT MIN(field) FROM Entity");
Double min = (Double) query.getSingleResult();
System. out. println ("Minimum value of field:"+min);
5. Calculate the average value:
Query query = em.createQuery("SELECT AVG(field) FROM Entity");
Double avg = (Double) query.getSingleResult();
System. out. println ("Average value of fields:"+avg);
6. Group statistics:
Query query = em.createQuery("SELECT field, COUNT(*) FROM Entity GROUP BY field");
List<Object[]> results = query.getResultList();
for (Object[] result : results) {
String fieldValue = (String) result[0];
Long count = (Long) result[1];
System. out. println ("Field:"+fieldValue+", Quantity:"+count);
}
Note: In these examples, 'Entity' is the name of the entity class in the database, and 'field' is a field in the entity class.
Complete code example:
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
import java.util.List;
public class AggregationQueryExample {
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("objectdb://localhost/mydb.odb");
EntityManager em = emf.createEntityManager();
//Total number of statistical records
Query query1 = em.createQuery("SELECT COUNT(*) FROM Entity");
Long count = (Long) query1.getSingleResult();
System. out. println ("Total records:"+count);
//Summation
Query query2 = em.createQuery("SELECT SUM(field) FROM Entity");
Double sum = (Double) query2.getSingleResult();
System. out. println ("Total of fields:"+sum);
//Take the maximum value
Query query3 = em.createQuery("SELECT MAX(field) FROM Entity");
Double max = (Double) query3.getSingleResult();
System. out. println ("Maximum value of field:"+max);
//Take the minimum value
Query query4 = em.createQuery("SELECT MIN(field) FROM Entity");
Double min = (Double) query4.getSingleResult();
System. out. println ("Minimum value of field:"+min);
//Calculate Average
Query query5 = em.createQuery("SELECT AVG(field) FROM Entity");
Double avg = (Double) query5.getSingleResult();
System. out. println ("Average value of fields:"+avg);
//Group statistics
Query query6 = em.createQuery("SELECT field, COUNT(*) FROM Entity GROUP BY field");
List<Object[]> results = query6.getResultList();
for (Object[] result : results) {
String fieldValue = (String) result[0];
Long groupCount = (Long) result[1];
System. out. println ("Field:"+fieldValue+", Quantity:"+groupCount);
}
em.close();
emf.close();
}
}
Please modify the sample code based on your own database and entity classes.
