知识点：查询一个实体类，并查出这个类下面的集合

Employee.java实体类

package com.hand.mybatis.bean;
public class Employee {
    private Integer eId;
    private String eName;
    private Integer gender;
    private String email;
    private Department dept;
   
    public Employee() {
        super();
    }  
    public Employee(Integer eId,String eName, Integer gender, String email) {
        super();
        this.eId=eId;
        this.eName = eName;
        this.gender = gender;
        this.email = email;
    }

    public Integer geteId() {
        return eId;
    }
    public void seteId(Integer eId) {
        this.eId = eId;
    }
   
    public String getEName() {
        return eName;
    }
    public void setEname(String ename) {
        this.eName = ename;
    }
    public Integer getGender() {
        return gender;
    }
    public void setGender(Integer gender) {
        this.gender = gender;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    } 
    public Department getDept() {
        return dept;
    }
    public void setDept(Department dept) {
        this.dept = dept;
    }
    @Override
    public String toString() {
        return “Employee [eId=” + eId + “, ename=” + eName + “, gender=” + gender + “, email=” + email + “]”;
    }

}

 Department.java实体类

package com.hand.mybatis.bean;
import java.util.List;

public class Department {
    private Integer id;
    private String departName;
    private List<Employee> empList;
    
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getDepartName() {
        return departName;
    }
    public void setDepartName(String departName) {
        this.departName = departName;
    }
    
    public List<Employee> getEmpList() {
        return empList;
    }
    public void setEmpList(List<Employee> empList) {
        this.empList = empList;
    }
     
    @Override
    public String toString() {
        return “Department [id=” + id + “, departName=” + departName + “]”;
    }
   
}

DepartmentMapper.java接口

package com.hand.mybatis.dao;
import com.hand.mybatis.bean.Department;

public interface DepartmentMapper {

    Department getDeptById(Integer did);
    
    //根据部门id，查询部门信息，同时将部门下的所有员工信息查询出来
     Department getDeptByIdPlus(Integer did);
    
     //分步查询部门信息
     Department getDeptByIdStep(Integer did);
}

DepartmentMapper.xm l映射文件

 <?xml version=”1.0″ encoding=”UTF-8″?>
<!DOCTYPE mapper
PUBLIC “-//mybatis.org//DTD Mapper 3.0//EN”
“http://mybatis.org/dtd/mybatis-3-mapper.dtd”>
<mapper namespace=”com.hand.mybatis.dao.DepartmentMapper”>

   <!– Department getDeptById(Integer did); –>
   <select id=”getDeptById”  resultType=”com.hand.mybatis.bean.Department”>
       SELECT did id,deptname departName FROM dept WHERE did=#{did}
   </select>
   
    <!–1.查询部门的时候将部门对应的所有员工信息也查询出来
    –>
   
    <resultMap type=”com.hand.mybatis.bean.Department” id=”departmentMap“>
        <result property=”id” column=”did”/>
        <result property=”departName” column=”deptname”/>
        <!–collection定义关联集合类型的属性的封装规则
            ofType:指定集合里面元素的类型  –->
        <collection property=”empList” ofType=”com.hand.mybatis.bean.Employee”>
         <!– 定义这个集合元素的封装规则 –>
          <id column=”eid” property=”eId”/>
          <result column=”ename” property=”eName”/>
          <result column=”email” property=”email”/>
          <result column=”gender” property=”gender”/>
        </collection>
    </resultMap>
      
    <!– Department getDeptByIdStep(Integer did); –>   接口一：根据部门did查询部门信息，员工信息   
    <select id=”getDeptByIdPlus” resultMap=”departmentMap“>
        SELECT d.did,d.deptname,e.eid,e.ename,e.email,e.gender
        FROM dept d
        LEFT JOIN emp e
        ON d.did=e.did
        WHERE d.did=#{did}
    </select>
    
    
    <!--2. select_resultMap关联查询collection分步查询 –>
    
    <resultMap type=”com.hand.mybatis.bean.Department” id=”departmentMapPlus“>
      <id column=”did” property=”id”/>
      <result column=”deptname” property=”departName”/>
      <collection property=”empList”
          select=”com.hand.mybatis.dao.EmployeeMapper.getEmpByDeptId”
          column=”{dId=did}” fetchType=”lazy”>//可以多列传值
      </collection>
    </resultMap>

   <!–Department getDeptByIdStep(Integer did); –>接口二：根据部门id分步查询员工信息
   <select id=”getDeptByIdStep” resultMap=”departmentMapPlus“>
      SELECT did,deptname
      from dept
      WHERE did=#{did}
   </select>
 
  <!–  扩展，多列的值传递过去，将多列的值封装map传递
       column=”{key1=column1,key2=column2}”
       fetchType=”lazy”:表示使用延迟加载
       – lazy:延迟
       – eager:立即–>
 
</mapper>

 代码：https://github.com/shuaishuaihand/mybatis.git