你知道 Mybatis 框架如何实现级联关系吗？

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

/** 

* 一对一关联查询 resultType 

* @param userId 

* @return 

*/ 

UserDto queryUserCardInfoForResultType(@Param("userId")int userId); 

sql 查询

<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

<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

<resultMap id="baseUser" type="com.xxx.mybatis.pojo.User">
    <id column="id"  property="id" /> 
    
    <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

<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

<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 **

<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

<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：

<sql id="User_columns">userId, userName, passwordsql>

<select id="findUserById" resultMap="RM_User" >    
    select <include refid="User_columns"/>
    from user where userId = #{userId}
select>