Article类定义:

com.mybatis.test;
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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。

Article.xml的配置:

version="1.0" encoding="UTF-8" ?>
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<!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类定义:

com.mybatis.test;
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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类中有一个List

Blog.xml配置:

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<?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>

Mybatis的“N+1查询问题”

示例:

id=”blogResult” type=”Blog”>
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<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:

id="blogResult" type="Blog">
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<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。