1. 关联-association
2. 集合-collection
association是用于一对一和多对一，而collection是用于一对多的关系
association-一对一 ,人和身份证的关系
<mapper namespace="com.glj.mapper.PersonMapper">
<resultMap type="com.glj.poji.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="selectPersonById"
javaType="com.gl.Card">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="age" column="age"/>
</association>
</resultMap>
<select id="selectPersonById" parameterType="int" resultMap="personMapper">
select * from tb_person where id = #{id}
</select>
collection 一对多和association的多对一关系
学生和班级的一对多的例子
<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
select * from tb_clazz where id = #{id}
</select>
<resultMap type="com.glj.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.gl.pojo.Student"
column="id" javaType="ArrayList"
fetchType="lazy" select="com.gl.mapper.StudentMapper.selectStudentByClazzId">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
</collection>
</resultMap>