Mybatis關聯查詢（嵌套查詢）
阿新 • • 發佈：2018-04-16
IT 簡單 art 聯合查詢 類定義 sele tex 映射 htm 
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三張表：user article blog
表的存儲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‘);
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：
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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：
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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：
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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，但是對於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
參考：
http://www.mybatis.org/mybatis-3/zh/sqlmap-xml.html
Mybatis關聯查詢（嵌套查詢）