mybatis学习（5）：关联查询的几种方式

文章末尾附上Employee.java  和 Department.java
方式一：联合查询：级联属性封装结果集
    <resultMap type="com.atguigu.mybatis.bean.Employee" id="MyDifEmp">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="gender" property="gender"/>
        <result column="did" property="dept.id"/>
        <result column="dept_name" property="dept.departmentName"/>
    </resultMap>

<!--  接口：public Employee getEmpAndDept(Integer id);-->
    <select id="getEmpAndDept" resultMap="MyDifEmp">
        SELECT e.id id,e.last_name last_name,e.gender gender,e.d_id d_id,
        d.id did,d.dept_name dept_name FROM tbl_employee e,tbl_dept d
        WHERE e.d_id=d.id AND e.id=#{id}
    </select>

方式二：使用association定义关联的单个对象的封装规则；
    <resultMap type="com.atguigu.mybatis.bean.Employee" id="MyDifEmp2">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="gender" property="gender"/>
        
        <!--  association可以指定联合的javaBean对象
        property="dept"：指定哪个属性是联合的对象
        javaType:指定这个属性对象的类型[不能省略]
        -->
        <association property="dept" javaType="com.atguigu.mybatis.bean.Department">
            <id column="did" property="id"/>
            <result column="dept_name" property="departmentName"/>
        </association>
    </resultMap>

<!--  public Employee getEmpAndDept(Integer id);-->
    <select id="getEmpAndDept" resultMap="MyDifEmp2">
        SELECT e.id id,e.last_name last_name,e.gender gender,e.d_id d_id,
        d.id did,d.dept_name dept_name FROM tbl_employee e,tbl_dept d
        WHERE e.d_id=d.id AND e.id=#{id}
    </select>

方式三：使用association进行分步查询：
        1、先按照员工id查询员工信息
        2、根据查询员工信息中的d_id值去部门表查出部门信息
        3、部门设置到员工中；
     <!--  id  last_name  email   gender    d_id   -->
     <resultMap type="com.atguigu.mybatis.bean.Employee" id="MyEmpByStep">
         <id column="id" property="id"/>
         <result column="last_name" property="lastName"/>
         <result column="email" property="email"/>
         <result column="gender" property="gender"/>
         <!-- association定义关联对象的封装规则
             select:表明当前属性是调用select指定的方法查出的结果
             column:指定将哪一列的值传给这个方法
             
             流程：使用select指定的方法（传入column指定的这列参数的值）查出对象，并封装给property指定的属性
          -->
         <association property="dept" 
             select="com.atguigu.mybatis.dao.DepartmentMapper.getDeptById"
             column="d_id">
         </association>
     </resultMap>
     <!--  public Employee getEmpByIdStep(Integer id);-->
     <select id="getEmpByIdStep" resultMap="MyEmpByStep">
         select * from tbl_employee where id=#{id}
     </select>

可以使用懒加载方式

Employee.java  和 Department.java

package com.atguigu.mybatis.bean;

import org.apache.ibatis.type.Alias;

@Alias("emp")
public class Employee {
	
	private Integer id;
	private String lastName;
	private String email;
	private String gender;
	private Department dept;
	
	public Employee() {
		super();
	}
	
	public Employee(Integer id, String lastName, String email, String gender) {
		super();
		this.id = id;
		this.lastName = lastName;
		this.email = email;
		this.gender = gender;
	}
	
	
	

	public Department getDept() {
		return dept;
	}

	public void setDept(Department dept) {
		this.dept = dept;
	}

	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getLastName() {
		return lastName;
	}
	public void setLastName(String lastName) {
		this.lastName = lastName;
	}
	public String getEmail() {
		return email;
	}
	public void setEmail(String email) {
		this.email = email;
	}
	public String getGender() {
		return gender;
	}
	public void setGender(String gender) {
		this.gender = gender;
	}
	@Override
	public String toString() {
		return "Employee [id=" + id + ", lastName=" + lastName + ", email="
				+ email + ", gender=" + gender + "]";
	}
	
	

}

package com.atguigu.mybatis.bean;

import java.util.List;

public class Department {
	
	private Integer id;
	private String departmentName;
	private List<Employee> emps;
	
	
	
	public List<Employee> getEmps() {
		return emps;
	}
	public void setEmps(List<Employee> emps) {
		this.emps = emps;
	}
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getDepartmentName() {
		return departmentName;
	}
	public void setDepartmentName(String departmentName) {
		this.departmentName = departmentName;
	}
	@Override
	public String toString() {
		return "Department [id=" + id + ", departmentName=" + departmentName
				+ "]";
	}
	
	

}