上一篇文章介紹了基于Mybatis對數據庫的增、刪、改、查。這一篇介紹下關聯查詢（join query）。

三張表：user article blog

表的存儲sql文件：

/*Navicat MySQL Data TransferSource Server         : localhostSource Server Version : 50620Source Host           : localhost:3306Source Database       : mybatisTarget Server Type    : MYSQLTarget Server Version : 50620File Encoding         : 65001Date: 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');

配置文件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別名定義 -->    <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，每個xml配置文件對應一個接口 -->    <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類的定義和User.xml的配置見上一文章。

Article類定義：

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類中有一個User類。

Article.xml的配置：

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.mybatis.test.IArticleOperation">    <resultMap type="User" id="userResultMap">        <!-- 屬性名和數據庫列名映射 -->        <id property="id" column="user_id"  />        <result property="userName" column="user_userName"  />        <result property="userAge" column="user_userAge"  />        <result property="userAddress" column="user_userAddress"  />    </resultMap>        <!-- User join Article進行聯合查詢  (一對一)-->        <resultMap id="articleResultMap" type="Article">        <id property="id" column="article_id" />        <result property="title" column="article_title" />        <result property="content" column="article_content" />        <!-- 將article的user屬性映射到userResultMap -->        <association property="user" javaType="User" resultMap="userResultMap"/>      </resultMap>        <!-- 使用別名來映射匹配 -->    <select id="getUserArticles" parameterType="int" resultMap="articleResultMap">       select user.id user_id,user.userName user_userName,user.userAddress user_userAddress,       article.id article_id,article.title article_title,article.content article_content        from user,article        where user.id=article.userid and user.id=#{id}    </select>        <!-- 另一種聯合查詢  (一對一)的實現，但是這種方式有“N+1”的問題 -->   <!--  <resultMap id="articleResultMap" type="Article">        <id property="id" column="article_id" />        <result property="title" column="article_title" />        <result property="content" column="article_content" />        <association property="user" javaType="User" column="userid" select="selectUser"/>      </resultMap>        <select id="selectUser" parameterType="int" resultType="User">        select * from user where id = #{id}    </select> -->    </mapper>

Blog類定義：

package com.mybatis.test;import java.util.List;public class Blog {    private int id;    private String title;    private List<Article> articles;        public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }        public String getTitle() {        return title;    }    public void setTitle(String title) {        this.title = title;    }        public List<Article> getArticles() {        return articles;    }    public void setArticles(List<Article> articles) {        this.articles = articles;    }    }

Blog類中有一個List<Article>。

Blog.xml配置：

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.mybatis.test.IBlogOperation">    <resultMap id="userResultMap" type="User">        <id property="id" column="user_id"  />        <result property="userName" column="user_userName"  />        <result property="userAge" column="user_userAge"  />        <result property="userAddress" column="user_userAddress"  />    </resultMap>        <resultMap id="articleResultMap" type="Article">        <id property="id" column="article_id" />        <result property="title" column="article_title" />        <result property="content" column="article_content" />        <association property="user" javaType="User" resultMap="userResultMap"/>      </resultMap>        <resultMap id="blogResultMap" type="Blog">        <id property="id" column="blog_id" />        <result property="title" column="blog_title" />        <!-- 將article list屬性映射到collection -->        <collection property="articles" ofType="Article" resultMap="articleResultMap"/>    </resultMap>        <!-- select語句 -->    <select id="getBlogByID" parameterType="int" resultMap="blogResultMap">       select user.id user_id,user.userName user_userName,user.userAddress user_userAddress,       article.id article_id,article.title article_title,article.content article_content,        blog.id blog_id, blog.title blog_title       from user,article,blog        where user.id=article.userid and blog.id=article.blogid and blog.id=#{id}    </select>    </mapper>

IArticleOperation定義：

package com.mybatis.test;import java.util.List;public interface IArticleOperation {    public List<Article> getUserArticles(int userID);}

IBlogOperation定義：

package com.mybatis.test;public interface IBlogOperation {        Blog getBlogByID(int id);    }

Test類：

package com.mybatis.test;import java.io.Reader;import java.util.List;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;public class Test {    private static SqlSessionFactory sqlSessionFactory;    private static Reader reader;    static {        try {            //通過配置文件初始化sqlSessionFactory            reader = Resources.getResourceAsReader("Configuration.xml");            sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);        } catch (Exception e) {            e.printStackTrace();        }    }    public static SqlSessionFactory getSession() {        return sqlSessionFactory;    }    public void getUserByID(int userID) {        SqlSession session = sqlSessionFactory.openSession();        try {            IUserOperation userOperation = session                    .getMapper(IUserOperation.class);            User user = userOperation.selectUserByID(userID);            if (user != null) {                System.out.println(user.getId() + ":" + user.getUserName()                        + ":" + user.getUserAddress());            }        } finally {            session.close();        }    }    public void getUserList(String userName) {        SqlSession session = sqlSessionFactory.openSession();        try {            IUserOperation userOperation = session                    .getMapper(IUserOperation.class);            List<User> users = userOperation.selectUsersByName(userName);            for (User user : users) {                System.out.println(user.getId() + ":" + user.getUserName()                        + ":" + user.getUserAddress());            }        } finally {            session.close();        }    }    /**     * 增加后要commit     */    public void addUser() {        User user = new User();        user.setUserAddress("place");        user.setUserName("test_add");        user.setUserAge(30);        SqlSession session = sqlSessionFactory.openSession();        try {            IUserOperation userOperation = session                    .getMapper(IUserOperation.class);            userOperation.addUser(user);            session.commit();            System.out.println("新增用戶ID：" + user.getId());        } finally {            session.close();        }    }    /**     * 修改后要commit     */    public void updateUser() {        SqlSession session = sqlSessionFactory.openSession();        try {            IUserOperation userOperation = session                    .getMapper(IUserOperation.class);            User user = userOperation.selectUserByID(1);            if (user != null) {                user.setUserAddress("A new place");                userOperation.updateUser(user);                session.commit();            }        } finally {            session.close();        }    }    /**     * 刪除后要commit.     *      * @param id     */    public void deleteUser(int id) {        SqlSession session = sqlSessionFactory.openSession();        try {            IUserOperation userOperation = session                    .getMapper(IUserOperation.class);            userOperation.deleteUser(id);            session.commit();        } finally {            session.close();        }    }    public void getUserArticles(int userid) {        SqlSession session = sqlSessionFactory.openSession();        try {            IArticleOperation articleOperation = session                    .getMapper(IArticleOperation.class);            List<Article> articles = articleOperation.getUserArticles(userid);            for (Article article : articles) {                System.out.println(article.getTitle() + ":"                        + article.getContent() + "用戶名："                        + article.getUser().getUserName() + "用戶地址："                        + article.getUser().getUserAddress());            }        } finally {            session.close();        }    }    public void getBlogArticles(int blogid) {        SqlSession session = sqlSessionFactory.openSession();        try {            IBlogOperation blogOperation = session                    .getMapper(IBlogOperation.class);            Blog blog = blogOperation.getBlogByID(blogid);            System.out.println(blog.getTitle() + ":");            List<Article> articles = blog.getArticles();            for (Article article : articles) {                System.out.println(article.getTitle() + ":"                        + article.getContent() + "用戶名："                        + article.getUser().getUserName() + "用戶地址："                        + article.getUser().getUserAddress());                /*System.out.println(article.getTitle() + ":"                        + article.getContent());*/            }        } finally {            session.close();        }    }    public static void main(String[] args) {        try {            Test test = new Test();            // test.getUserByID(1);            // test.getUserList("test1");            // test.addUser();            // test.updateUser();            // test.deleteUser(6);            //test.getUserArticles(1);            test.getBlogArticles(1);        } catch (Exception e) {            System.out.println(e.getMessage());        }    }}

Mybatis的“N+1查詢問題”

示例：

    <resultMap id=”blogResult” type=”Blog”>        <association property="author" column="blog_author_id"            javaType="Author" select=”selectAuthor” />    </resultMap>        <select id=”selectBlog” parameterType=”int” resultMap=”blogResult”>        SELECT * FROM BLOG WHERE ID = #{id}    </select>        <select id=”selectAuthor” parameterType=”int” resultType="Author">        SELECT * FROM AUTHOR WHERE ID = #{id}    </select>

有兩個查詢語句：一個來加載博客，另外一個來加載作者，而且博客的結果映射描述了“selectAuthor”語句應該被用來加載它的 author 屬性。

其他所有的屬性將會被自動加載，假設它們的列和屬性名相匹配。

這種方式很簡單，但是對于大型數據集合和列表將不會表現很好。問題就是我們熟知的“N+1 查詢問題”。概括地講，N+1 查詢問題可以是這樣引起的：

你執行了一個單獨的 SQL 語句來獲取結果列表（就是“+1”）。

對返回的每條記錄，你執行了一個查詢語句來為每個加載細節（就是“N”）。

這個問題會導致成百上千的 SQL 語句被執行。這通常不是期望的。

可以采用關聯的嵌套結果來解決這個問題：

    <resultMap id="blogResult" type="Blog">        <id property=”id” column="blog_id" />        <result property="title" column="blog_title" />        <association property="author" column="blog_author_id"            javaType="Author" resultMap=”authorResult” />    </resultMap>    <resultMap id="authorResult" type="Author">        <id property="id" column="author_id" />        <result property="username" column="author_username" />        <result property="password" column="author_password" />        <result property="email" column="author_email" />        <result property="bio" column="author_bio" />    </resultMap>

resultMap 這是結果映射的 ID，可以映射關聯的嵌套結果到一個合適的對象圖中。這是一種替代方法來調用另外一個查詢語句。

代碼下載：http://download.csdn.net/detail/luxiaoxun/8056559

Source Code：https://github.com/luxiaoxun/MybatisDemo