【Mybatis】collection解决一对多关联查询

在上上篇中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">

	<!-- collection嵌套结果集的方式，定义关联的集合类型元素的封装规则-->
	
	<!-- 嵌套结果集的方式，使用collection标签定义关联的集合类型的属性封装规则 -->
	<resultMap type="com.atguigu.mybatis.bean.Department" id="MyDept">
		<id column="did" property="id"/>
		<result column="dept_name" property="departmentName"/>
		<!-- 
		collection定义关联结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="emps" ofType="com.atguigu.mybatis.bean.Employee">
			<!-- 定义这个集合中元素的封装规则 -->
			<id column="eid" property="id"/>
			<result column="last_name" property="lastName"/>
			<result column="email" property="email"/>
			<result column="gender" property="gender"/>
		</collection>
	
	</resultMap>
	
	<!-- public Department getDeptByIdPlus(Integer id); -->
	<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>
	<!-- public Department getDeptByIdStep(Integer id); -->
	<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