【Mybatis】collection解决一对多关联查询 - 好文

在上上篇中 <http://blog.csdn.net/zjy15203167987/article/details/79455455>
我们用级联属性的方式把数据查出来封装到Employee中，还有另一种方式实现数据的封装。
接下来小编将介绍collection标签的相关知识，包括嵌套查询和分段查询两种方式。

现在有员工和部门两张表，前两篇文章介绍的都是根据员工查部门，本篇文章介绍由部门查所有员工

首先请参照上上篇中 <http://blog.csdn.net/zjy15203167987/article/details/79455455>
步骤1，2，3 , 4，此处从第5步开始

一、嵌套查询

5.新增DepartmentMapper.java接口方法
public interface DepartmentMapper { public Department getDeptByIdPlus(Integer
id); }
6.新增DepartmentMapper.xml文件，配置信息如下
<?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.atguigu.mybatis.dao.DepartmentMapper">   <resultMap
type="com.atguigu.mybatis.bean.Department" id="MyDept"> <id column="did"
property="id"/> <result column="dept_name" property="departmentName"/>  <collection property="emps"
ofType="com.atguigu.mybatis.bean.Employee">  <id
column="eid" property="id"/> <result column="last_name" property="lastName"/>
<result column="email" property="email"/> <result column="gender"
property="gender"/> </collection> </resultMap>  <select id="getDeptByIdPlus"
resultMap="MyDept" > SELECT d.id did, d.dept_name dept_name, e.id, e.last_name
last_name, e.email email, e.gender gender FROM tbl_dept d LEFT JOIN
tbl_employee e ON d.id = e.d_id WHERE d.id = #{id} </select> </mapper>
7.新增测试方法
@Test public void test06() throws IOException{ SqlSessionFactory
sqlSesssionFactory = getSqlSesssionFactory(); SqlSession openSession =
sqlSesssionFactory.openSession(); try { DepartmentMapper mapper =
openSession.getMapper(DepartmentMapper.class); Department deptByIdPlus =
mapper.getDeptByIdPlus(2); System.out.println(deptByIdPlus);
System.out.println(deptByIdPlus.getEmps()); } finally { openSession.close(); } }
8.控制台信息

二、分段查询

5.新增EmployeeMapperPlus.xml文件，配置信息如下
<?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.atguigu.mybatis.dao.EmployeeMapperPlus"> <select
id="getEmpsByDeptId" resultType="com.atguigu.mybatis.bean.Employee"> select *
from tbl_employee where d_id=#{deptId} </select> <mapper/>

6.DepartmentMapper.xml文件，配置信息
<?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.atguigu.mybatis.dao.EmployeeMapperPlus"> <resultMap
type="com.atguigu.mybatis.bean.Department" id="MyDeptStep"> <id column="id"
property="id"/> <id column="dept_name" property="departmentName"/> <collection
property="emps"
select="com.atguigu.mybatis.dao.EmployeeMapperPlus.getEmpsByDeptId"
column="{deptId=id}" fetchType="lazy"></collection> </resultMap>  <select id="getDeptByIdStep"
resultMap="MyDeptStep"> select id,dept_name departmentName from tbl_dept where
id=#{id} </select> <mapper/>

7.新增测试方法
@Test public void test06() throws IOException{ SqlSessionFactory
sqlSesssionFactory = getSqlSesssionFactory(); SqlSession openSession =
sqlSesssionFactory.openSession(); try { DepartmentMapper mapper =
openSession.getMapper(DepartmentMapper.class); Department deptByIdStep =
mapper.getDeptByIdStep(1); System.out.println(deptByIdStep); } finally {
openSession.close(); } }
8.控制台信息：分步查询效果

热门工具换一换