第十三章MyBatis高级映射

news2025/7/10 18:29:21

多对一映射

创建数据表

student是主表
class_id关联class表的id

class表
在这里插入图片描述
student表

创建pojo

Class类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Class {

  private Long id;
  private String name;
  private List<Student> students;
}

Student类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {

  private Long id;
  private String name;
  private Long classId;
  private Class clazz;
}

映射方式

1. 级联映射

StudentMapper写入如下代码

Student selectByIdResult(Long id);

StudentMapper.xml写入如下代码

<sql id="studentSql">
    s.id,
    s.name,
    s.class_id,
    c.id as classId,
    c.name as className
</sql>

<resultMap id="studentResultResultMap" type="Student">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="classId" column="classId"/>
    <result property="clazz.id" column="classId"/>
    <result property="clazz.name" column="className"/>
</resultMap>

<select id="selectByIdResult" resultMap="studentResultResultMap" >
    select
    <include refid="studentSql"/>
    from student s left join class c on s.class_id = c.id
    where s.id=#{id}
</select>

2. association映射

StudentMapper写入如下代码

Student selectByIdAssociation(Long id);

StudentMapper.xml写入如下代码

<sql id="studentSql">
    s.id,
    s.name,
    s.class_id,
    c.id as classId,
    c.name as className
</sql>
<resultMap id="studentAssociationResultMap" type="Student">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="classId" column="class_id"/>
    <association property="clazz" javaType="Class">
        <id property="id" column="classId"/>
        <result property="name" column="className"/>
    </association>
</resultMap>
<select id="selectByIdAssociation" resultMap="studentAssociationResultMap">
    select 
    <include refid="studentSql"/>
    from student s left join class c on s.class_id=c.id
    where s.id = #{id}
</select>

3. 分步查询

上述两种方法都是一条sql完成
分步查询需要俩条sql，可复用支持延迟加载

StudentMapper.xml写入如下代码

Student selectByIdStep(Long id);

ClassMapper写入如下代码

Class selectById(Long id);

StudentMapper.xml写入如下代码

<sql id="studentSql">
    id,
    name,
    class_id
</sql>
<resultMap id="studentStepResultMap" type="Student">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="classId" column="class_id"/>
    <association property="clazz" select="org.example.mapper.ClassMapper.selectById" column="class_id"/>
</resultMap>
<select id="selectByIdStep" resultMap="studentStepResultMap">
    select 
    <include refid="studentSql"/>
    from student where id = #{id}
</select>

ClassMapper.xml写入如下代码

<sql id="classSql">
    id,
    name
</sql>
<select id="selectById"  resultType="Class">
    select
    <include refid="classSql"/>
    from class
    where id=#{id}
</select>

上述俩种查询每次都要查询2条sql，有时我们不需要查询2条。并且联表查询会出现笛卡尔现象，性能很差

这可以是用延迟加载来提高性能，用到属性的时候查询不用就不会查询

局部设置 fetchType=“lazy” 。只会作用域这条sql

<resultMap id="studentStepResultMap" type="Student">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="classId" column="class_id"/>
    <association property="clazz" fetchType="lazy" select="org.example.mapper.ClassMapper.selectById" column="class_id"/>
</resultMap>

全局开启延迟加载

<settings>
    <setting name="lazyLoadingEnabled" value="true"/>
</settings>

局部关闭使用 fetchType=“eager”

    <resultMap id="studentStepResultMap" type="Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="classId" column="class_id"/>
        <association property="clazz" fetchType="eager" select="org.example.mapper.ClassMapper.selectById" column="class_id"/>
    </resultMap>

一对多映射

创建pojo

Class类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Class {

  private Long id;
  private String name;
  private List<Student> students;
}

Student类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {

  private Long id;
  private String name;
  private Long classId;
  private Class clazz;
}

映射方式

1. collection映射

ClassMapper写入如下代码

Class selectCollection(Long id);

ClassMapper.xml写入如下代码

<sql id="classSql">
    c.id,
    c.name,
    s.id as studentId,
    s.name as studentName,
    s.class_id as classId
</sql>
<resultMap id="selectCollectionResultMap" type="Class">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <collection property="students" ofType="Student">
        <id property="id" column="studentId"/>
        <result property="name" column="studentName"/>
        <result property="classId" column="classId"/>
    </collection>
</resultMap>
<select id="selectByIdCollection" resultMap="selectCollectionResultMap">
    select
    <include refid="classSql"/>
    from class c left join student s on c.id=s.class_id
    where c.id=#{id}
</select>

2. 分步查询

ClassMapper写入如下代码

Class selectByIdStep(Long id);

StudentMapper写入如下代码

Student selectById(Long classId);

ClassMapper.xml写入如下代码

<sql id="classSql">
    id,
    name
</sql>
<resultMap id="selectStepResultMap" type="Class">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <collection property="students" select="org.example.mapper.StudentMapper.selectById" column="id"/>
</resultMap>
<select id="selectByIdStep" resultMap="selectStepResultMap">
    select
    <include refid="classSql"/>
    from class where id=#{id}
</select>

StudentMapper.xml写入如下代码

<sql id="studentSql">
    id,
    name,
    class_id
</sql>
<select id="selectById" resultType="Student">
    select
    <include refid="studentSql"/>
    from student where class_id = #{classId}
</select>