一对一,一对多,多对一,自关联

1.SQL片段
<sql id="columns">
stuname,stuage
</sql>
<select>
select <include refid="columns"></include> from student
</select>
2.一对多 Dad------>Son

两种方案：单条SQL

<resultMap id="DadMapper" type="Dad">
<id property="fatherid" column="fatherid"></id>
<collection property="sons" ofType="Son">
<id property="sonid" column="sonid"></id>
<result property="sonname" column="sonname"/>
</collection>
</resultMap>
<select resultMap="DadMapper">
select sonname,fathername from son,dad where son.fatherid=dad.fatherid
and dad.fatherid=#{fatherid}
</select>

多条SQL
多对一：两种方案：单条SQL
多条SQL
自关联：无限级菜单
DB表结构
Category表名
cid cname pid
1 服装 0
2 儿童服装 1
3 儿童男装 2

xml 思想递归
：使用的是一对多一个分类关联一个分类集合。
<resultMap id="CategoryMapper" type="Category">
<id column="cid" property="cid"></id>
<result column="cname" property="cname"></result>
<collection property="cates" ofType="Category" select="getChildrenListByPid" column="cid">
</collection>
</resultMap>

<select id="getChildrenListByPid" resultMap="CategoryMapper">
select * from category where pid=#{pid}
</select>
dao层：
List<Category> List<List<List<Category>>>

多对多：老师和学生