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描述下场景，比如一个人有多个qq号（假设一个人可以有重复的qq号）数据库结构，有两张表：
people表
id name
1 jackpeople_qq表
id people_id qq
1 1 123456
2 1 234567
3 1 456789
4 1 123456实体类：import java.io.Serializable;
import java.util.List; public class People implements Serializable{ private static final long serialVersionUID = -5935066186174346694L; private Long id; private String name; private List<String> qqs; public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public List<String> getQqs() { return qqs; } public void setQqs(List<String> qqs) { this.qqs = qqs; } @Override public String toString() { return "People [id=" + id + ", name=" + name + ", qqs=" + qqs + "]"; }
} mapper接口：import com.hnpicheng.mybatisissue.domain.People; public interface PeopleMapper { People selectPeopleById( Long id);
} 测试代码：import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.hnpicheng.mybatisissue.domain.People;
import com.hnpicheng.mybatisissue.mapper.PeopleMapper; public class App
{ public static void main( String[] args ) { ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring-mybatis.xml"); PeopleMapper peopleMapper = context.getBean(PeopleMapper.class); People p = peopleMapper.selectPeopleById(1L); System.out.println(p); }
} PeopleMapper.xml根据业务需要，如果不需要将重复数据查出来：
那么可以使用<?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.hnpicheng.mybatisissue.mapper.PeopleMapper"> <resultMap id="peopleResultMap" type="People"> <id property="id" column="id" /> <result property="name" column="name" /> <collection property="qqs" ofType="string" javaType="list"> <result column="qq" /> </collection> </resultMap> <select id="selectPeopleById" resultMap="peopleResultMap"> select p.*,pq.qq from people p left join people_qq pq on p.id = pq.people_id where p.id = #{id} </select>
</mapper> 测试结果，优点只要查一次，对于需要排重的查询业务，可以用这个方法：DEBUG [main] - ==> Preparing: select p.*,pq.qq from people p left join people_qq pq on p.id = pq.people_id where p.id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <== Total: 4
People [id=1, name=jack, qqs=[123456, 234567, 456789]] 若果需要将重复数据查询出来，那么可以使用以下配置<?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.hnpicheng.mybatisissue.mapper.PeopleMapper">
<resultMap id="peopleResultMap" type="People"> <id property="id" column="id" /> <result property="name" column="name" /> <collection property="qqs" column="id" select="selectQQByPeopleId"> 这里的column是指调用selectQQByPeopleId传递的参数<result column="qq" /> </collection> </resultMap> <select id="selectPeopleById" resultMap="peopleResultMap"> select * from people where id = #{id} </select> <select id="selectQQByPeopleId" resultType="string"> select qq from people_qq where people_id = #{id} </select>
</mapper> 测试结果：DEBUG [main] - ==> Preparing: select * from people where id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <== Total: 1
DEBUG [main] - ==> Preparing: select qq from people_qq where people_id = ?
DEBUG [main] - ==> Parameters: 1(Long)
DEBUG [main] - <== Total: 4
People [id=1, name=jack, qqs=[123456, 234567, 456789, 123456]]
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