自关联

自关联是指，自己即充当一方，又充当多方。其实就是普通1:n 和 n:1的变形

1、一对多
1）表结构
-- 新闻栏目
CREATE TABLE `news` (
`id` int(5) NOT NULL AUTO_INCREMENT,
`name` varchar(10) NOT NULL,
`pid` int(5) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

2）实体类
// 新闻栏目：当前的新闻栏目被看作是一方，即父栏目
public class News {
private Integer id;
private String name;
private Set<News> childen;
//省略get set属性方法
@Override
public String toString() {
return "News [id=" + id + ", name=" + name + ", childen=" + childen
+ "]";
}
}
3）Dao接口
public interface INewsDao {
List<News> selectById(int id);//查询指定id栏目下面的所有子孙菜单

News selectById2(int id);//查询指定id栏目及下面的所有子孙菜单
}
4）mapper.xml文件
<!-- 查询指定id栏目下面的所有子孙菜单 -->
<resultMap type="News" id="newsMapper">
<id column="id" property="id"/>
<result column="name" property="name"/>

<collection property="childen"
ofType="News"
select="selectById"
column="id"/>
</resultMap>

<select id="selectById" resultMap="newsMapper">
select id,name from news where pid = #{xxx}
</select>


<!-- 查询指定id栏目 及 下面的所有子孙菜单 -->
<select id="selectChilden" resultMap="newsMapper">
select id,name from news where pid=#{ooo}
</select>

<resultMap type="News" id="newsMapper2">
<id column="id" property="id"/>
<result column="name" property="name"/>

<collection property="childen"
ofType="News"
select="selectChilden"
column="id" />
</resultMap>

<select id="selectById2" resultMap="newsMapper2">
select id,name from news where id=#{xxx}
</select>
标签属性可以参考：

MyBatis 关联查询 （一对多、多对一）

5）测试输出：
一、
[DEBUG] ==> Preparing: select id,name from news where pid = ?
[DEBUG] ==> Parameters: 1(Integer)
[TRACE] <== Columns: id, name
[TRACE] <== Row: 3, 内地娱乐
[DEBUG] ====> Preparing: select id,name from news where pid = ?
[DEBUG] ====> Parameters: 3(Integer)
[DEBUG] <==== Total: 0
[TRACE] <== Row: 4, 港台娱乐
[DEBUG] ====> Preparing: select id,name from news where pid = ?
[DEBUG] ====> Parameters: 4(Integer)
[DEBUG] <==== Total: 0
[DEBUG] <== Total: 2
News [id=3, name=内地娱乐, childen=[]]
News [id=4, name=港台娱乐, childen=[]]
二、
[DEBUG] ==> Preparing: select id,name from news where id=?
[DEBUG] ==> Parameters: 2(Integer)
[TRACE] <== Columns: id, name
[TRACE] <== Row: 2, 体育新闻
[DEBUG] ====> Preparing: select id,name from news where pid=?
[DEBUG] ====> Parameters: 2(Integer)
[TRACE] <==== Columns: id, name
[TRACE] <==== Row: 5, 篮球
[DEBUG] ======> Preparing: select id,name from news where pid = ?
[DEBUG] ======> Parameters: 5(Integer)
[TRACE] <====== Columns: id, name
[TRACE] <====== Row: 7, NBA
[DEBUG] ========> Preparing: select id,name from news where pid = ?
[DEBUG] ========> Parameters: 7(Integer)
[DEBUG] <======== Total: 0
[TRACE] <====== Row: 8, CBA
[DEBUG] ========> Preparing: select id,name from news where pid = ?
[DEBUG] ========> Parameters: 8(Integer)
[DEBUG] <======== Total: 0
[DEBUG] <====== Total: 2
[TRACE] <==== Row: 6, 足球
[DEBUG] ======> Preparing: select id,name from news where pid = ?
[DEBUG] ======> Parameters: 6(Integer)
[TRACE] <====== Columns: id, name
[TRACE] <====== Row: 9, 意甲
[DEBUG] ========> Preparing: select id,name from news where pid = ?
[DEBUG] ========> Parameters: 9(Integer)
[TRACE] <======== Columns: id, name
[TRACE] <======== Row: 12, AC米兰
[DEBUG] ==========> Preparing: select id,name from news where pid = ?
[DEBUG] ==========> Parameters: 12(Integer)
[TRACE] <========== Columns: id, name
[TRACE] <========== Row: 14, 皮尔洛
[DEBUG] ============> Preparing: select id,name from news where pid = ?
[DEBUG] ============> Parameters: 14(Integer)
[DEBUG] <============ Total: 0
[DEBUG] <========== Total: 1
[TRACE] <======== Row: 13, 国际米兰
[DEBUG] ==========> Preparing: select id,name from news where pid = ?
[DEBUG] ==========> Parameters: 13(Integer)
[DEBUG] <========== Total: 0
[DEBUG] <======== Total: 2
[TRACE] <====== Row: 10, 西甲
[DEBUG] ========> Preparing: select id,name from news where pid = ?
[DEBUG] ========> Parameters: 10(Integer)
[DEBUG] <======== Total: 0
[TRACE] <====== Row: 11, 中超
[DEBUG] ========> Preparing: select id,name from news where pid = ?
[DEBUG] ========> Parameters: 11(Integer)
[DEBUG] <======== Total: 0
[DEBUG] <====== Total: 3
[DEBUG] <==== Total: 2
[DEBUG] <== Total: 1
News [id=2, name=体育新闻, childen=[News [id=5, name=篮球, childen=[News [id=8, name=CBA, childen=[]], News [id=7, name=NBA, childen=[]]]], News [id=6, name=足球, childen=[News [id=11, name=中超, childen=[]], News [id=10, name=西甲, childen=[]], News [id=9, name=意甲, childen=[News [id=12, name=AC米兰, childen=[News [id=14, name=皮尔洛, childen=[]]]], News [id=13, name=国际米兰, childen=[]]]]]]]]

2、多对一
1）实体类有变化：
去掉了private Set<News> childen;其实应该留着，删掉只是为了便于理解多对一。

添加了private News parent; 肯定只有一个父类栏目

2个成员变量都存在的话，就一个类实现多对一和一对多

// 新闻栏目：当前的新闻栏目被看作是多方，即子栏目
public class News {
private Integer id;
private String name;
private News parent;
//省略get set属性方法
@Override
public String toString() {
return "News [id=" + id + ", name=" + name + ", parent=" + parent + "]";
}
}
2）Dao接口
public interface INewsDao {
News selectById(int id);//查询指定id栏目及其所有父辈菜单
}
3）mapper.xml文件
<!-- 查询指定id栏目及其所有父辈菜单 -->
<resultMap type="News" id="newsMapper">
<id column="id" property="id"/>
<result column="name" property="name"/>

<association property="parent"
javaType="News"
select="selectById"
column="pid" />
</resultMap>

<select id="selectById" resultMap="newsMapper">
select id,name,pid from news where id = #{xxx}
</select>
每一个子栏目，肯定只会存在一个父栏目。所以是一对一的关系，用<association />
4）测试输出：
测试输出：
[DEBUG] ==> Preparing: select id,name,pid from news where id = ?
[DEBUG] ==> Parameters: 12(Integer)
[TRACE] <== Columns: id, name, pid
[TRACE] <== Row: 12, AC米兰, 9
[DEBUG] ====> Preparing: select id,name,pid from news where id = ?
[DEBUG] ====> Parameters: 9(Integer)
[TRACE] <==== Columns: id, name, pid
[TRACE] <==== Row: 9, 意甲, 6
[DEBUG] ======> Preparing: select id,name,pid from news where id = ?
[DEBUG] ======> Parameters: 6(Integer)
[TRACE] <====== Columns: id, name, pid
[TRACE] <====== Row: 6, 足球, 2
[DEBUG] ========> Preparing: select id,name,pid from news where id = ?
[DEBUG] ========> Parameters: 2(Integer)
[TRACE] <======== Columns: id, name, pid
[TRACE] <======== Row: 2, 体育新闻, 0
[DEBUG] ==========> Preparing: select id,name,pid from news where id = ?
[DEBUG] ==========> Parameters: 0(Integer)
[DEBUG] <========== Total: 0
[DEBUG] <======== Total: 1
[DEBUG] <====== Total: 1
[DEBUG] <==== Total: 1
[DEBUG] <== Total: 1
News [id=12, name=AC米兰, parent=News [id=9, name=意甲, parent=News [id=6, name=足球, parent=News [id=2, name=体育新闻, parent=null]]]]