MyBatis的关联关系一对一一对多多对多

一对一示例

一个妻子对应一个丈夫
数据库表设计时在妻子表中添加一个丈夫主键的作为外键

1 对应的JavaBean代码
虽然在数据库里只有一方配置的外键，但是这个一对一是双向的关系。
Husband实体类

public class Husband implements Serializable{
private int hid;
private String hname;
private Wife wife;
public int getHid() {
return hid;
}
public void setHid(int hid) {
this.hid = hid;
}
public String getHname() {
return hname;
}
public void setHname(String hname) {
this.hname = hname;
}
public Wife getWife() {
return wife;
}
public void setWife(Wife wife) {
this.wife = wife;
}
}

Wife 实体类

public class Wife implements Serializable{
private int wid;
private String wname;
private Husband husband;
public int getWid() {
return wid;
}
public void setWid(int wid) {
this.wid = wid;
}
public String getWname() {
return wname;
}
public void setWname(String wname) {
this.wname = wname;
}
public Husband getHusband() {
return husband;
}
public void setHusband(Husband husband) {
this.husband = husband;
}
public Wife(int wid, String wname, Husband husband) {
super();
this.wid = wid;
this.wname = wname;
this.husband = husband;
}
}

2 编写接口，定义方法

public interface HusbandMapper {
//根据id查询丈夫
public Husband queryHusbandById(int id);
//根据id查询丈夫和妻子
public Husband queryHusbandAndWife(int id);
}

3 编写实体类的映射文件 XXXMapper.xml

注意点：双向一对一关联有两种配置文件的写法
方式1：嵌套结果使用嵌套结果映射来处理重复的联合结果的子集封装联表查询的数据(去除重复的数据)
方式2：嵌套查询通过执行另外一个SQL映射语句来返回预期的复杂类型

<?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.szjx.mapper.HusbandMapper">
<select id="queryHusbandById" resultType="Husband" parameterType="int">
select * from husband where hid=#{hid}
</select>


<!-- 查询丈夫和妻子    双向一对一的关系映射-->
<!-- 两种方式  
   1、 嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集
           封装联表查询的数据(去除重复的数据)-->
<select id="queryHusbandAndWife" resultMap="husbandAndWife" parameterType="int">
select * from husband h left outer join wife w
on h.wid=w.wid where h.hid=#{hid}
</select>
<!-- 一对一双向关联   两种自定义返回集结果 -->
 <resultMap type="Husband" id="husbandAndWife">
<id property="hid" column="hid"/>
<result property="hname" column="hname"/>
<!--多表联合查询  必须要映射   不映射会报错-->
<association property="wife" javaType="Wife">
<id property="wid" column="wid"/>
<result property="wname" column="wname"/>
</association>
</resultMap> 

<!-- 方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
           SELECT   * FROM class WHERE c_id=1;
           SELECT   * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值 -->
     <select id="queryHusbandAndWife2" resultMap="husbandAndWife2" parameterType="int">
      select * from husband where hid=#{hid}
     </select>
     <resultMap type="Husband" id="husbandAndWife2">
      <id property="hid" column="hid"/>
      <result property="hname" column="hname"/>
      <association property="wife" javaType="Wife" select="getWife" column="wid"></association>
     </resultMap>
     <select id="getWife" resultType="Wife" parameterType="int">
      select * from wife where wid=#{wid}
     </select>
  
</mapper>

4编写测试

public class One2OneTest {
private SqlSession sqlSession;
private HusbandMapper mapper;
@Before
public void before(){
//获取session
sqlSession=DBTools.getSession();
mapper=sqlSession.getMapper(HusbandMapper.class);
}
@After
public void after(){
//提交事务
sqlSession.commit();
}
@Test
public void queryHusband(){
Husband h=mapper.queryHusbandById(1);//根据id查询丈夫
System.out.println(h);
}
@Test
public void queryHusbandAndWife(){
Husband h=mapper.queryHusbandAndWife(1);//嵌套结果    
System.out.println(h);
}
@Test
public void queryHusbandAndWife2(){
Husband h=mapper.queryHusbandAndWife2(1);//嵌套查询
System.out.println(h);
}
}

一对多

示例：一个人有多个手机
1数据库表的设计手机表作为子表有人表里的主键作为外键
MyBatis的关联关系一对一一对多多对多

2 实体类的设计
Person

public class Person {
private int pid;
private String pname;
private List<Phone> pList;
public int getPid() {
return pid;
}
public void setPid(int pid) {
this.pid = pid;
}
public String getPname() {
return pname;
}
public void setPname(String pname) {
this.pname = pname;
}
public List<Phone> getpList() {
return pList;
}
public void setpList(List<Phone> pList) {
this.pList = pList;
}
}

Phone 类

public class Phone {
private int tid;
private String tname;
private Person person;
public int getTid() {
return tid;
}
public void setTid(int tid) {
this.tid = tid;
}
public String getTname() {
return tname;
}
public void setTname(String tname) {
this.tname = tname;
}
public Person getPerson() {
return person;
}
public void setPerson(Person person) {
this.person = person;
}
}

一对多演示两个操作 1批量操作 2查询操作

1批量操作

定义 mapper接口方法和 xxxMapper.xml 映射文件

public interface PhoneMapper {
//批量保存手机
public int batchSavePhone(@Param("phones") List<Phone> phones);
}

PhoneMapper.xml

<?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.szjx.mapper.PhoneMapper">
<insert id="batchSavePhone">
insert into phone values
<!-- 注意点  collection 的集合 是mapper接口中定义的@Param("phones")的值   ，两个必须映射对应，否则就会报错 -->
<foreach collection="phones" item="phone" separator=",">
<!--collection 为用于遍历的元素（必选），支持数组、List、Set  -->
      <!-- item 表示集合中每一个元素进行迭代时的别名. -->
      <!--separator表示在每次进行迭代之间以什么符号作为分隔 符.  -->
(null,#{phone.tname},#{phone.person.pid})
</foreach>
</insert>
</mapper>

4批量插入操作功能实现

public class One2ManyTest {
    private SqlSession sqlSession;
    private PhoneMapper mapper;
    @Before
    public void before(){
        //获取session
        sqlSession=DBTools.getSession();
        mapper=sqlSession.getMapper(PhoneMapper.class);
    }
    @After
    public void after(){
        //提交事务
        sqlSession.commit();
    }
    @Test
    public void batchSavePhone(){//批量保存数据
        //在进行批量插入操作时 ，需要到 主表的主键 ，这时候可以将主表的数据从数据库中查询出来 在进行批量插入的操作 。
        Person person=new Person();//这里就new一个对象作为主表对象 并设置主键的id
        person.setPid(1);
        List<Phone> phones=new ArrayList<>();
        for(int i=0;i<10;i++){
            Phone phone=new Phone(i,"ipone"+i,person);
            phones.add(phone);
        }
        System.out.println(phones);
        mapper.batchSavePhone(phones);
    }
}

2一对多查询功能的实现

3 定义 mapper接口方法和 xxxMapper.xml 映射文件
PersonMapper

public interface PersonMapper {
//添加人
public int savePerson(Person person);
//根据id查询人的资料
public Person findPersonById(int id);
//根据id查询人与手机的资料
public Person findPersonAndPhone(int id);

PersonMapper.xml

<?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.szjx.mapper.PersonMapper">
<!-- 保存操作 -->
<insert id="savePerson" parameterType="Person">
insert into person values(null,#{pname})
</insert>
<!-- 根据ID查询人员信息   返回值 类型是 Person类  -->
<select id="findPersonById" parameterType="int" resultType="Person">
select * from person where pid=#{pid}
</select>
<!-- 根据id查询人员和手机号   自定义返回结果集   -->
<select id="findPersonAndPhone" parameterType="int" resultMap="personAndPhone">
select * from person p left outer join phone t
on p.pid= t.pid where p.pid=#{pid}
</select>
<!-- 自定义返回结果集  方式1:嵌套结果   -->
<resultMap type="Person" id="personAndPhone">
<id property="pid" column="pid"/>
<result property="pname" column="pname"/>
<!-- 注意点  集合的属性值  ：必须是实体类中 集合的成员变量名 -->
<collection property="pList" ofType="Phone">
<id property="tid" column="tid"/>
<result property="tname" column="tname"/>
</collection>
</resultMap>
</mapper>

一对多查询功能实现

public class One2ManyTest2 {
private SqlSession sqlSession;
private PersonMapper mapper;
@Before
public void before(){
//获取session
sqlSession=DBTools.getSession();
mapper=sqlSession.getMapper(PersonMapper.class);
}

@After
public void after(){
//提交事务
sqlSession.commit();
}
@Test
public void findPersonById(){
//根据id查询人员信息
Person person=mapper.findPersonById(1);
System.out.println(person.getPname());
}
@Test
public void findPersonAndPhoneById(){
//根据id查询人员和手机信息
Person person=mapper.findPersonAndPhone(1);
System.out.println(person);
}
}

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