mybatis关联查询之一对多查询

一对多，是最常见的一种设计。就是 A 表的一条记录，对应 B 表的多条记录，且 A 的主键作为 B 表的外键。这主要看以哪张表为中心，下面的测试数据中，从employee 表来看，一个员工对应一个部门，是一对一关系，如果从部门角度来看，则是一对多的关系，一个部门对应多个员工，本节主要研究一对多的关系。

查询部门的时候将部门对应的所有员工信息也查询出来

数据表建立

---

新建数据表department，有两个字段，插入两条数据如下：

id	dept_name
1	CIA
2	FSB

新建数据表employee，有三个字段，其中dept_id是外键，关联department表的主键id。插入数据如下：

id	last_name	dept_id
1	Tom	1
2	Jerry	2
3	Neo	1
4	Cypher	2

新建maven工程，添加依赖，主要是mybatis和mysql

---

<dependencies>
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.4.6</version>
    </dependency>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.47</version>
    </dependency>
</dependencies>

编写数据库表对应的实体。

---

对于department表，对应实体如下：注意增加一个包含了Employee集合。

package com.yefengyu.mybatis.entity;

import java.util.List;


public class Department
{
    private Integer id;

    private String deptName;

    private List<Employee> employees;

    public Integer getId()
    {
        return id;
    }

    public void setId(Integer id)
    {
        this.id = id;
    }

    public String getDeptName()
    {
        return deptName;
    }

    public void setDeptName(String deptName)
    {
        this.deptName = deptName;
    }

    public List<Employee> getEmployees()
    {
        return employees;
    }

    public void setEmployees(List<Employee> employees)
    {
        this.employees = employees;
    }

    @Override
    public String toString()
    {
        return "Department{" +
               "id=" + id +
               ", deptName='" + deptName + '\'' +
               ", employees=" + employees +
               '}';
    }
}

对于employee表，实体如下，注意在Employee实体中把外键直接变成对于Department对象的引用。

package com.yefengyu.mybatis.entity;

public class Employee
{
    private Integer id;

    private String lastName;

    private Department department;

    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 Department getDepartment()
    {
        return department;
    }

    public void setDepartment(Department department)
    {
        this.department = department;
    }

    @Override
    public String toString()
    {
        return "Employee{" +
               "id=" + id +
               ", lastName='" + lastName + '\'' +
               ", department=" + department +
               '}';
    }
}

编写mapper接口

---

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Department;


public interface DepartmentMapper
{
    public Department getDeptById(Integer id);
}

根据部门ID查询部门信息和对应的所有员工信息。

编写mapper映射文件（本节重点）

---

collection嵌套结果集方法：

<?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.yefengyu.mybatis.mapper.DepartmentMapper">
    <resultMap id="dept" type="com.yefengyu.mybatis.entity.Department">
        <id column="d_id" property="id"/>
        <result column="dept_name" property="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="employees" ofType="com.yefengyu.mybatis.entity.Employee" javaType="java.util.ArrayList">
            <id column="id" property="id"/>
            <result column="last_name" property="lastName"/>
        </collection>
    </resultMap>
    <select id="getDeptById" resultMap="dept">
        select e.id id, e.last_name last_name, e.dept_id dept_id, d.id d_id,d.dept_name dept_name
        from department d
        left join employee e
        on e.dept_id = d.id
        where d.id = #{id} 
    </select>
</mapper>

新建一个mybatis全局配置文件，详细信息见官网

---

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
        <setting name="logImpl" value="STDOUT_LOGGING" />
    </settings>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatis?characterEncoding=utf8&amp;allowMultiQueries=true"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/DepartmentMapper.xml"/>
    </mappers>
</configuration>

测试

---

public static void main(String[] args)
    throws IOException
{
    InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml");
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
    SqlSession sqlSession = sqlSessionFactory.openSession();
    try
    {
        DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);
        Department dept = mapper.getDeptById(1);
        System.out.println(dept);
    }
    finally
    {
        sqlSession.close();
    }
}

结果如下：

Created connection 1938056729.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@73846619]
==>  Preparing: select e.id id, e.last_name last_name, e.dept_id dept_id, d.id d_id,d.dept_name dept_name from department d left join employee e on e.dept_id = d.id where d.id = ? 
==> Parameters: 1(Integer)
<==    Columns: id, last_name, dept_id, d_id, dept_name
<==        Row: 1, Tom, 1, 1, CIA
<==        Row: 3, Neo, 1, 1, CIA
<==      Total: 2
Department{id=1, deptName='CIA', employees=[Employee{id=1, lastName='Tom', department=null}, Employee{id=3, lastName='Neo', department=null}]}
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@73846619]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@73846619]

上面查询虽然可以查询出数据，但是数据过多则会有性能问题，因此好的做法是分步查询。

分步查询

---

1、新增查询员工的接口，特别注意是根据部门id来查询

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Employee;


public interface EmployeeMapper
{
    Employee getEmpByDeptId(Integer deptId);
}

2、编写对应的mapper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.yefengyu.mybatis.mapper.EmployeeMapper">
    <select id="getEmployee" resultType="com.yefengyu.mybatis.entity.Employee">
        select * from employee where dept_id = #{id}
    </select>
</mapper>

3、编写查询部门的接口

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Department;


public interface DepartmentMapper
{
    public Department getDeptById(Integer id);
}

4、编写对应的映射文件

<?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.yefengyu.mybatis.mapper.DepartmentMapper">
    <resultMap id="dept" type="com.yefengyu.mybatis.entity.Department">
        <id column="id" property="id"/>
        <result column="dept_name" property="deptName"/>
        <!-- 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
        -->
        <collection property="employees" select="com.yefengyu.mybatis.mapper.EmployeeMapper.getEmployee"
                    column="id" fetchType="lazy">
        </collection>
    </resultMap>
    <select id="getDeptById" resultMap="dept">
        select id ,dept_name from department where id = #{id}
    </select>
</mapper>

5、测试结果

Created connection 1694556038.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6500df86]
==>  Preparing: select id ,dept_name from department where id = ? 
==> Parameters: 1(Integer)
<==    Columns: id, dept_name
<==        Row: 1, CIA
<==      Total: 1
==>  Preparing: select * from employee where dept_id = ? 
==> Parameters: 1(Integer)
<==    Columns: id, last_name, dept_id
<==        Row: 1, Tom, 1
<==        Row: 3, Neo, 1
<==      Total: 2
Department{id=1, deptName='CIA', employees=[Employee{id=1, lastName='Tom', department=null}, Employee{id=3, lastName='Neo', department=null}]}
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6500df86]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6500df86]

如果测试代码的打印改为：

System.out.println(dept.getDeptName());

那么结果如下，不会查询员工信息。

Created connection 1694556038.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6500df86]
==>  Preparing: select id ,dept_name from department where id = ? 
==> Parameters: 1(Integer)
<==    Columns: id, dept_name
<==        Row: 1, CIA
<==      Total: 1
CIA
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6500df86]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@6500df86]
Returned connection 1694556038 to pool.

全局配置与局部配置

---

1、全局配置

lazyLoadingEnabled：延迟加载的全局开关。当开启时，所有关联对象都会延迟加载。 特定关联关系中可通过设置 fetchType 属性来覆盖该项的开关状态。
默认值：false

aggressiveLazyLoading：当开启时，任何方法的调用都会加载该对象的所有属性。 否则，每个属性会按需加载（参考 lazyLoadTriggerMethods)。
默认值：false （在 3.4.1 及之前的版本默认值为 true），现在新版本可以不用关注此设置。

<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>

2、局部配置fetchType

<association property="" fetchType="eager"></association>
<collection property="" fetchType="lazy"></collection>

• lazy：延迟
• eager：立即

3、区别（查询部门信息，不查看员工信息时）

全局	局部	是否延迟
不开启	不开启	否
不开启	lazy	是
不开启	eager	否
开启	不开启	是
开启	lazy	是
开启	eager	否