在MyBatis中，如果我们想对一对一或者一对多的多表进行查询，该如何处理呢？
MyBatis提供了下面两个标签来处理一对一、多对一、一对多的映射关系：
association: 处理一对一、多对一
collection: 处理一对多
一对一
每个人都有身份证，人和身份证的关系是一对一的。假设我们有下面两个class:
public class Card implements Serializable {private Integer id;private String code;// 省略get、set方法
}public class User implements Serializable {private Integer id;private String name;private Integer age;private Card card;//省略get、set方法
}
下面是Mapper和接口的实现：
package com.yrk.mapper;
import com.yrk.pojo.Card;
public interface CardMapper {Card selectCardById(Integer id);
}
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.yrk.mapper.CardMapper"><select id="selectCardById" parameterType="int" resultType="com.yrk.pojo.Card">select * from tb_card where id = #{id} </select>
</mapper>
package com.yrk.mapper;
import com.yrk.pojo.Person;
public interface PersonMapper {Person selectPersonById(Integer id);
}
<?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.yrk.mapper.PersonMapper"><resultMap type="com.yrk.pojo.Person" id="personMapper"><id property="id" column="id"/><result property="name" column="name"/><result property="age" column="age"/><association property="card" column="card_id" select="com.yrk.mapper.CardMapper.selectCardById"javaType="com.yrk.pojo.Card"></association></resultMap><select id="selectPersonById" parameterType="int" resultMap="personMapper">select * from tb_person where id = #{id}</select>
</mapper>
一对多
假设我们有班级和学生两个实体类，一个班级有多个学生，一个学生只会在一个班级：
package com.yrk.pojo;import java.io.Serializable;
import java.util.List;public class Clazz implements Serializable{private Integer id;private String code;private String name;//班级与学生是一对多的关系private List<Student> students;//省略set/get方法
}
public class Student implements Serializable {private Integer id;private String name;private Clazz clazz;//省略set/get方法
}
班级和学生是一对多的关系，在MyBatis中我们是用collection来实现一对多的映射：
<mapper namespace="com.yrk.mapper.ClazzMapper"><select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">select * from tb_clazz where id = #{id}</select><resultMap type="com.yrk.pojo.Clazz" id="clazzResultMap"><id property="id" column="id"/><result property="code" column="code"/><result property="name" column="name"/><!-- property: 指的是集合属性的值, ofType：指的是集合中元素的类型 --><collection property="students" ofType="com.yrk.pojo.Student"column="id" javaType="ArrayList"fetchType="lazy" select="com.yrk.mapper.StudentMapper.selectStudentByClazzId"><id property="id" column="id"/><result property="name" column="name"/></collection></resultMap>
</mapper>
package com.yrk.mapper;
import com.yrk.pojo.Clazz;
public interface ClazzMapper {Clazz selectClazzById(Integer id);
}
<mapper namespace="com.yrk.mapper.StudentMapper"><select id="selectStudentById" parameterType="int" resultMap="studentResultMap">select * from tb_clazz c,tb_student s where c.id = s.id and s.id = #{id}</select><select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">select * from tb_student where clazz_id = #{id}</select><resultMap type="com.yrk.pojo.Student" id="studentResultMap"><id property="id" column="id"/><result property="name" column="name"/><association property="clazz" javaType="com.yrk.pojo.Clazz"><id property="id" column="id"/><result property="code" column="code"/><result property="name" column="name"/></association></resultMap>
</mapper>
package com.yrk.mapper;
import com.yrk.pojo.Student;
public interface StudentMapper {Student selectStudentById(Integer id);
}
)
