案例

一个博客系统中，用户可以任意发表博文（Post），用户还可以对博文进行评论（Comment）。于是在这个系统中，存在以下的关系：

在数据库中，Author，Post和Comment存在于不同的表中。但Comment会使用外键关联Post，而Post又会外键关联Author。

create table author (id int primary key,name varchar(20)
);create table post (id int primary key,authorId int,title varchar(100),created timestamp,content text,foreign key (authorId) REFERENCES author(id)
);create table comment (id int primary key,postId int,content text,created timestamp,foreign key (postId) REFERENCES post(id)
);

在java类中，Author, Post和Comment分别对应的java类型为：

public class Post {private int id;private Author author;private String title;private String content;private Timestamp created;private List<Comment> comments;// getters and setters
}public class Author {private int id;private String name;// getters and setters
}public class Comment {private int id;private int postId;private String content;private Timestamp created;// getters and setters
}

下面使用Mybatis的联合查询，创建一个Post实例，其中comments会自动填充到Post的comments表中。

联合查询

首先给出mybatis的配置：

<?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><typeAliases><typeAlias type="com.test.Post" alias="Post" /><typeAlias type="com.test.Author" alias="Author" /><typeAlias type="com.test.Comment" alias="Comment" /></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://localhost:3306/joey" /><property name="username" value="mysql" /><property name="password" value="mysql" /></dataSource></environment></environments><mappers><mapper resource="com/test/my/BlogMapper.xml" /></mappers>
</configuration>

以及SessionManager工厂类：

public class SessionManager {private static SqlSessionFactory sessionFactory;public static synchronized SqlSessionFactory getSessionFactory() {if (sessionFactory != null) return sessionFactory;String resource = "com/lux/stat/dest/mybatis-conf.xml";try {sessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource));} catch (IOException e) {e.printStackTrace();}return sessionFactory;}
}

还有BlogMapper接口：

public interface BlogMapper {public Post getPostById(int id);public List<Post> getPosts();
}

最后是测试用main函数：

public class Test {public static void main(String[] args) {// initData();SqlSession session = SessionManager.getSessionFactory().openSession();BlogMapper mapper = session.getMapper(BlogMapper.class);Post post = mapper.getPostById(1);System.out.println(post.getAuthor().getName());System.out.println(post.getComments().size());System.out.println(post.getComments().get(0).getContent());List<Post> posts = mapper.getPosts();post=posts.get(0);System.out.println(post.getAuthor().getName());System.out.println(post.getComments().size());System.out.println(post.getComments().get(0).getContent());session.close();}
}

Mybatis联合查询提供了两种方式

方式一，嵌套查询

BlogMapper.xml

<?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.test.my.BlogMapper"><resultMap id="postResult" type="Post"><association property="author" column="authorId" javaType="Author" select="getAuthorById"/><collection property="comments" javaType="ArrayList" column="id" ofType="Comment" select="getCommentsById"/></resultMap><select id="getPostById" resultMap="postResult">select id, authorId, title, content, created from post where id=#{param1}</select><select id="getPosts" resultMap="postResult">select id, authorId, title, content, created from post</select><select id="getAuthorById" resultType="Author">select id,  name from author where id=#{param1}</select><select id="getCommentsById" resultType="Comment">select id,  content, created, postId from comment where postId=#{param1}</select>
</mapper>

此类方式的缺点是效率低。组合最后的实例，必须要执行多条语句。像本例的getPosts()方法，如果posts数目为n，那么执行sql的条数应该为1+n+n。

方式二，JOIN语句

BlogMapper.xml

<?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.test.my.BlogMapper"><resultMap id="postResult" type="Post"><id property="id" column="id" /><result property="title" column="title" /><result property="content" column="content" /><result property="created" column="created" /><association property="author" javaType="Author"><id property="id" column="authorId" /><result property="name" column="name" /></association><collection property="comments" javaType="ArrayList" ofType="Comment"><id property="id" column="commentId" /><result property="content" column="comment" /><result property="created" column="commentCreated" /></collection></resultMap><select id="getPosts" resultMap="postResult">select A.id, A.title, A.content, A.created, B.id as authorId, B.name, C.id as commentId, C.content as comment, C.created as commentCreatedfrom post A join author B on A.authorid=B.idjoin comment C on A.id = C.postId</select><select id="getPostById" resultMap="postResult">select A.id, A.title, A.content, A.created, B.id as authorId, B.name, C.id as commentId, C.content as comment, C.created as commentCreatedfrom post A join author B on A.authorid=B.idjoin comment C on A.id = C.postIdwhere A.id=#{param1}</select>
</mapper>

使用此种方法，可以一条join语句便组装好post实例。