# 你知道 Mybatis 框架如何实现级联关系吗？
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## 级联关系实现的方式
- 一对一
- 一对多
## 案例实操
### 一对一关系
实现方式 resultType/resultMap
**接口方法**
~~~ java
/**
* 一对一关联查询 resultType
* @param userId
* @return
*/
UserDto queryUserCardInfoForResultType(@Param("userId")int userId);
~~~
**sql 查询**
~~~ xml
<select id="queryUserCardInfoForResultType" resultType="userDto">
SELECT
u.id,
u.user_name AS userName,
u.user_pwd AS userPwd,
u.flag,
u.cid,
c.number
FROM
USER u
LEFT JOIN card c ON u.cid = c.id
WHERE
u.id = #{userId}
</select>
~~~
#### resultType **实现**
~~~ xml
<select id="queryUserByIdResultType" parameterType="int" resultType="com.xxx.dto.UserDto">
SELECT
u.id,
u.user_name AS userName,
u.create_time,
u.flag,
u.cid AS cid,
c.number
FROM
User u
LEFT JOIN card c u ON u.cid =c.id
WHERE
u.id = #{id}
</select>
~~~
#### resultMap 实现
~~~ xml
<!--自定义映射关系实现关联 -->
<resultMap id="baseUser" type="com.xxx.mybatis.pojo.User">
<id column="id" property="id" /> <!-- 主键 -->
<!-- 普通例 -->
<result column="user_name" property="userName" />
<result column="true_name" property="trueName" />
<result column="role_name" property="roleName"/>
</resultMap>
<select id="queryUserInfoByIdV2" parameterType="int" resultMap="baseUser" >
SELECT u.id,u.user_name,u.true_name,l.role_name
FROM t_user u JOIN t_user_role r
ON (u.id = r.user_id) JOIN t_role l
on (r.role_id = l.id)
WHERE u.id = #{id}
</select>
~~~
#### resultMap + association实现
**resultMap** **映射定义**
~~~ xml
<resultMap id="baseUserRole" type="userRole">
<id column="id" property="id"></id>
<result column="role_id" property="roleId"></result>
<association property="role" column="role_id" select="queryRoleById"/>
</resultMap>
<resultMap id="baseRole" type="role">
<id column="id" property="id"></id>
<result column="role_name" property="roleName"/>
<result column="role_remark" property="roleRemark"/>
</resultMap>
~~~
**查询**
~~~ xml
<select id="queryUserRoleInfoById" parameterType="int" resultMap="baseUserRole">
SELECT id, role_id FROM t_user_role WHERE id = #{id}
</select>
<select id="queryRoleById" parameterType="int" resultMap="baseRole">
SELECT id, role_name , role_remark FROM t_role WHERE id = #{role_id}
</select>
~~~
### 一对多关系
实现方式:resultMap 实现
resultType 有局限，无法去重，需手动处理。
**ResultMap 定义 **
~~~ xml
<resultMap id="baseGoods" type="goods">
<id column="id" property="id"/>
<result column="name" property="goodsName"/>
<result column="promotePrice" property="price"/>
<collection property="ixxxs" column="id" select="queryGoodsIxxxsById"/>
</resultMap>
<resultMap id="baseGoodsImnage" type="GoodsIxxxs">
<id column="id" property="id"></id>
<result column="goods_skuid" property="goodsSkuid" ></result>
<result column="goods_img" property="ixxxUrl" ></result>
</resultMap>
~~~
查询
~~~ xml
<select id="queryGoodsById" parameterType="int" resultMap="baseGoods" useCache="true">
select id, name , promotePrice FROM yg_goods WHERE id = #{id}
</select>
<select id="queryGoodsIxxxsById" parameterType="int" resultMap="baseGoodsImnage" useCache="true">
SELECT id , goods_skuid , goods_img FROM yg_goods_imgage where goods_skuid = #{id}
</select>
~~~
## 扩展
### sql
sql 元素用来定义一个**可以复用的 SQL 语句段**，供其它语句调用。比如：
~~~ xml
<sql id="User_columns">userId, userName, password</sql>
<!--用include 引用-->
<select id="findUserById" resultMap="RM_User" >
select <include refid="User_columns"/>
from user where userId = #{userId}
</select>
~~~
/sql>
<!--用include 引用-->
<select id="findUserById" resultMap="RM_User" >
select <include refid="User_columns"/>
from user where userId = #{userId}
</select>
~~~
###
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