域对象（就是domain下面的类）之间的关联关系：
按照多重性可分为一对一、一对多、多对一和多对多，即数据库中的表之间的关系
根据关联关系，所以以右为主，所以一对一、多对一的处理方式是一样的，一对多和多对多一样，而左边决定抽象方法的返回类型。
举例：一个年级的班级信息表与学生信息表
如果用一张表来存储的话，每条学生数据都有班级字段，并且都是写“xx班级”；
但如果将学生信息和班级信息拆分，而学生表仍有班级Id字段，虽然看起来并没有什么区别，
但是这样就会减少占用的存储空间，毕竟数字肯定是比"xx班级"的字节数小；
一旦数据多了，差距就出现了，而且有利于数据的维护，比如需要改班级信息，两张表的情况下，只需要改班级表就好；但如果在一张表中，需要改与班级相关的每天学生信息。
但两张表的查询要比一张表麻烦一点，
所以就用resultMap来做结果映射：就是在多表查询时，用来根据需求，配置查询结果
============================================================
*对一
员工实体类
public class Employee {
private Long id;
private String name;
//多对一配置:多个员工属于一个部门
private Department department;
//省略getter与setter,toString
...
}
部门实体类
public class Department {
private Long id;
private String name;
//省略getter与setter,toString
...
}
映射文件
1、嵌套结果：查出多表的全部字段，然后选自己想要的
<mapper namespace="cn.itsource.many2one.mapper.EmployeeMapper">
<!--
resultMap:完成结果映射
1. id代表映射名称（随便取），给别的sql调用
2. type:映射对应返回的对象类型
-->
<resultMap id="employeeMap" type="cn.itsource.mybatis.many2one.domain.Employee">
<id property="id" column="id" /><!--id标签表示主键，这里用result也可，但规范一点-->
<result property="name" column="name" />
<!--association ：联系 -->
<association property="department" column="dept_id" javaType="cn.itsource.many2one.domain.Department">
<id property="id" column="did"/>
<result property="name" column="dname" />
</association>
</resultMap>
<!--
resultMap：通过id，引用结果映射
1.把查询结果弄成自己想要的实体类，必需使用结果映射
2.多张表查询时，如果出现相同的列名，一定要取别名，不然默认赋值出现column值的第一列
-->
<select id="findAll" resultMap="employeeMap">
select e.id,e.name,d.id did,d.name dname from
employee e left join department d
on d.id = e.dept_id
</select>
</mapper>
查询结果
2. Nested query : first check a table, then use the column of the foreign key as a parameter, execute another sql, go to another table to check
EmployeeMapper.xml
<mapper namespace="cn.itsource.many2one.mapper.EmployeeMapper">
<resultMap id="employeeMap" type="cn.itsource.many2one.domain.Employee">
<id property="id" column="id" />
<result property="name" column="name" />
<!-- select:指向要查询的sql，并执行（如果在同一个文件中可以省略前面的namespace）
所以xml之间调用sql，是直接根据命名空间+id，而不是去找接口哦~
column：就是把当前查出来的这一列，作为参数，传给select指向的sql-->
<association property="department" column="dept_id"
select="cn.itsource.many2one.mapper.DepartmentMapper.findById">
</association>
</resultMap>
<!-- resultMap：引用结果映射 -->
<select id="findAll" resultMap="employeeMap">
select * from employee
</select>
</mapper>
DepartmentMapper.xml: For standardization, the sql for which table is operated should be written in which mapper.xml file
<mapper namespace="cn.itsource.many2one.mapper.DepartmentMapper">
<!--根据id获取相应的部门数据-->
<select id="findById" resultType="cn.itsource.many2one.domain.Department">
select * from department where id=#{id}
</select>
</mapper>
================================================= =========
* To many
employee entity class
public class Employee {
private Long id;
private String name;
//省略getter与setter,toString
...
}
Departmental entity
public class Department {
private Long id;
private String name;
//一对多配置(一个部门对应多个员工)
private List<Employee> employees = new ArrayList<>();
//省略getter与setter,toString
...
}
The same two processing methods are basically the same.
The difference is that the connection is collection — ofType , because *to-many is "many", of course
, it is received by collection; and one/many-to-one, in fact, collection can also be used , Because the collection can hold multiple, so of course you can install one,
but for the sake of standardization, use different
myBatis will automatically put the data queried from the second table into the corresponding list, so it is displayed like this
ClassInfo{
id=1, name='0531班级', students=[Student{
id=7, name='陈佳星', age=22}, Student{
id=8, name='李鹏', age=25}]}
ClassInfo{
id=2, name='0532班级', students=[Student{
id=9, name='郑杨', age=24}, Student{
id=10, name='高小满', age=19}, Student{
id=11, name='张鑫', age=21}]}
ClassInfo{
id=3, name='0533班级', students=[Student{
id=12, name='廖家乐', age=28,}, Student{
id=13, name='母杨帆', age=22}]}