使用db4o的QBE进行数据库的查询

最新推荐文章于 2023-01-19 18:35:27 发布

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#db4o #QBE #db4o查询

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本文介绍使用db4o数据库的QBE(Query By Example)方式进行数据查询的方法。通过具体的Person对象实例，演示了如何进行全表扫描、短格式查询及条件查询。此方式简化了查询过程，提高了开发效率。

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使用db4o的QBE进行数据库的查询，使用的对象是Person，在上一篇文件中已经写过

import com.db4o.Db4oEmbedded;
import com.db4o.ObjectContainer;
import com.db4o.ObjectSet;
import com.db4o.eight.study.bean.Person;

public class DB4oQueryUseQBE {
	
	final static String DB4OFILENAME = System.getProperty("user.dir")
			+ "/database.db4o";

	public static void QBEQuery(){
		ObjectContainer db = Db4oEmbedded.openFile(
				Db4oEmbedded.newConfiguration(), DB4OFILENAME);
		Person proto = new Person(null, null, null);
		try {
			ObjectSet<Person> result = db.queryByExample(proto);
			System.out.println(result.size());
			for (Person per : result) {
			    System.out.println("id = " + per.getId()+
			    		", name = "+ per.getName() + ", password = " + per.getPassword());
			}
		} finally {
			db.close();
		}
	}
	
	public static void shortQBEQuery() {
		ObjectContainer db = Db4oEmbedded.openFile(
				Db4oEmbedded.newConfiguration(), DB4OFILENAME);
		try {
			ObjectSet<Person> result = db.queryByExample(Person.class);
			for (Person per : result) {
			    System.out.println("id = " + per.getId()+
			    		", name = "+ per.getName() + ", password = " + per.getPassword());
			}
		}
		finally {
			db.close();
		}
	}
	
	public static void queryByCondition() {
		ObjectContainer db = Db4oEmbedded.openFile(
				Db4oEmbedded.newConfiguration(), DB4OFILENAME);
		try {
			Person person = new Person("000002", null, "654321");
			ObjectSet<Person> result = db.queryByExample(person);
			for (Person per : result) {
			    System.out.println("id = " + per.getId()+
			    		", name = "+ per.getName() + ", password = " + per.getPassword());
			}
		}
		finally {
			db.close();
		}
	}
	
	public static void main(String[] args) {
//		DB4oQueryUseQBE.QBEQuery();
//		DB4oQueryUseQBE.shortQBEQuery();
		DB4oQueryUseQBE.queryByCondition();
	}
}