上一篇文章《Mybatis实现数据的增删改查（CRUD）》先容了基于Mybatis对数据库的增、删、改、查。这一篇先容下关联查询（join query）。

三张表：user article blog

表的存储sql文件：

/*
Navicat MySQL Data Transfer

Source Server         : localhost
Source Server Version : 50620
Source Host           : localhost:3306
Source Database       : mybatis

Target Server Type    : MYSQL
Target Server Version : 50620
File Encoding         : 65001

Date: 2014-10-19 18:27:31
*/

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for `user`
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `userName` varchar(50) DEFAULT NULL,
  `userAge` int(11) DEFAULT NULL,
  `userAddress` varchar(200) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES ('1', 'summer', '30', 'shanghai');
INSERT INTO `user` VALUES ('2', 'test1', '22', 'suzhou');
INSERT INTO `user` VALUES ('3', 'test1', '29', 'some place');
INSERT INTO `user` VALUES ('4', 'lu', '28', 'some place');
INSERT INTO `user` VALUES ('5', 'xiaoxun', '27', 'nanjing');

-- ----------------------------
-- Table structure for `article`
-- ----------------------------
DROP TABLE IF EXISTS `article`;
CREATE TABLE `article` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `userid` int(11) NOT NULL,
  `title` varchar(100) DEFAULT NULL,
  `content` text,
  `blogid` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of article
-- ----------------------------
INSERT INTO `article` VALUES ('1', '1', 'test_title_1', 'test_content_1', '1');
INSERT INTO `article` VALUES ('2', '1', 'test_title_2', 'test_content_2', '1');
INSERT INTO `article` VALUES ('3', '1', 'test_title_3', 'test_content_3', '2');
INSERT INTO `article` VALUES ('4', '1', 'test_title_4', 'test_content_4', '2');
INSERT INTO `article` VALUES ('5', '2', 'test_title_5', 'test_content_5', '2');

-- ----------------------------
-- Table structure for `blog`
-- ----------------------------
DROP TABLE IF EXISTS `blog`;
CREATE TABLE `blog` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(200) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of blog
-- ----------------------------
INSERT INTO `blog` VALUES ('1', 'xiaoxun_blog');
INSERT INTO `blog` VALUES ('2', 'zhang_blog');

设置文件Configuration.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!-- mybatis别名界说 -->
    <typeAliases> 
        <typeAlias alias="User" type="com.mybatis.test.User"/> 
        <typeAlias alias="Article" type="com.mybatis.test.Article"/> 
        <typeAlias alias="Blog" type="com.mybatis.test.Blog"/>
    </typeAliases> 

    <environments default="development">
        <environment id="development">
        <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
            <property name="driver" value="com.mysql.jdbc.Driver"/>
            <property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatis" />
            <property name="username" value="root"/>
            <property name="password" value="admin"/>
            </dataSource>
        </environment>
    </environments>
    
    <!-- mybatis的mapper文件，每个xml设置文件对应一个接口 -->
    <mappers>
        <mapper resource="com/mybatis/test/User.xml"/>
        <mapper resource="com/mybatis/test/Article.xml"/>
        <mapper resource="com/mybatis/test/Blog.xml"/>
    </mappers>
</configuration>

User类的界说和User.xml的设置见上一文章。

Article类界说：

package com.mybatis.test;

public class Article {

    private int id;
    private User user;
    private String title;
    private String content;
    
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    
    public User getUser() {
        return user;
    }
    public void setUser(User user) {
        this.user = user;
    }
    
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    
    public String getContent() {
        return content;
    }
    public void setContent(String content) {
        this.content = content;
    }
    
}

Article类中有一个User类。