Mybatis关联查询(嵌套查询).docx

Mybatis关联查询（嵌套查询）
表的存储sql文件：
<?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别名定义 -->
<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配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>

表的存储sql文件：
<?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">
<!-- 属性名和数据库列名映射 -->
<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进行联合查询 (一对一)-->
<resultMap id="articleResultMap" type="Article">
<id property="id" column="article_id" />
<result property="title" column="article_title" />
<result property="content" column="article_content" />
<!-- 将article的user属性映射到userResultMap -->
<association property="user" javaType="User" resultMap="userResultMap"/>
</resultMap>

<!-- 使用别名来映射匹配 -->
<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>

<!-- 另一种联合查询 (一对一)的实现，但是这种方式有“N+1”的问题 -->
<!-- <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.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" />
<!-- 将article list属性映射到collection -->
<collection property="articles" ofType="Article" resultMap="articleResultMap"/>
</resultMap>

<!-- select语句 -->
<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定义：
package com.mybatis.test;

import java.util.List;

public interface IArticleOperation {

public List<Article> getUserArticles(int userID);
}
IBlogOperation定义：
package com.mybatis.test;

public interface IBlogOperation {

Blog getBlogByID(int id);

}
Test类：
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 {
//通过配置文件初始化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);
mit();
System.out.println("新增用户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);
mit();
}
} finally {
session.close();
}
}

/**
* 删除后要commit.
*
* @param id
*/
public void deleteUser(int id) {
SqlSession session = sqlSessionFactory.openSession();
try {
IUserOperation userOperation = session
.getMapper(IUserOperation.class);
userOperation.deleteUser(id);
mit();
} 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() + "用户名："
+ article.getUser().getUserName() + "用户地址："
+ 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() + "用户名："
+ article.getUser().getUserName() + "用户地址："
+ 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查询问题”
<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>
有两个查询语句：一个来加载博客，另外一个来加载作者，而且博客的结果映射描述了“selectAuthor”语句应该被用来加载它的 author 属性。
其他所有的属性将会被自动加载，假设它们的列和属性名相匹配。
这种方式很简单，但是对于大型数据集合和列表将不会表现很好。问题就是我们熟知的“N+1 查询问题”。概括地讲，N+1 查询问题可以是这样引起的：
你执行了一个单独的 SQL 语句来获取结果列表（就是“+1”）。
对返回的每条记录，你执行了一个查询语句来为每个加载细节（就是“N”）。
这个问题会导致成百上千的 SQL 语句被执行。这通常不是期望的。
可以采用关联的嵌套结果来解决这个问题：
<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 这是结果映射的 ID，可以映射关联的嵌套结果到一个合适的对象图中。这是一种替代方法来调用另外一个查询语句。