Mybatis一对一，一对多，自关联

先说一对一，是通过association标签，有两种方式：

1.

<association property="userInfo" resultMap="com.weida18.mapper.UserInfoMapper.UserInfo"/>

或者

<association property="userInfo" column="userId" javaType="com.weida18.entity.UserInfo">
<id property="userId" column="userId"/>
<result property="idNumber" column="idNumber"/>
<result property="realName" column="realName"/>
<result property="birthday" column="birthday"/>
<result property="sex" column="sex"/>
</association>

这种方式需要使用关联查询，即select a.*,b.* from A a left join B b on a.userid = b.userid where ...........。

2.

<association property="userInfo" column="id" select="com.weida18.mapper.UserInfoMapper.findUserInfoByUserId<span style="font-family: Arial, Helvetica, sans-serif;">/>

这种方式不需要表关联，在查询时先查询user，再拿着id自动去查userinfo。

一对多关系：

<!-- 表关联 n:n -->
<collection property="tags" column="id"
select="com.weida18.mapper.VideoTagMapper.findTagByVideoId"/>

自关联：

<span>	</span><resultMap id="VideoColumn" type="com.weida18.entity.VideoColumn">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="parentId" column="parentId"/>
<result property="topFlag" column="topFlag"/>

<association property="column" column="parentId"
select="com.weida18.mapper.VideoColumnMapper.findVideoColumnById"/>

</resultMap>