mybatis关联查询之一对一查询

一对一也就是 A 表的一条记录对应 B 表的一条记录，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，本节主要研究一对一的关系。

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>

3、编写数据库表对应的实体。

---

对于department表，对应实体如下：

package com.yefengyu.mybatis.entity;

public class Department
{
    private Integer id;
    
    private String deptName;

    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;
    }

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

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.Employee;


public interface EmployeeMapper
{
    Employee getEmployee(Integer id);
}

根据员工ID查询员工信息和部门信息

编写mapper映射文件（本节重点）

---

一对一查询mapper文件有三种编写方式，分别是：

1、级联属性法

<?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.EmployeeMapper">
    <resultMap id="employee" type="com.yefengyu.mybatis.entity.Employee">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="d_id" property="department.id"/>
        <result column="dept_name" property="department.deptName"/>
    </resultMap>

    <select id="getEmployee" resultMap="employee">
        select e.id id, e.last_name last_name, e.dept_id dept_id, d.id d_id,d.dept_name dept_name
        from employee e, department d
        where e.id = #{id} and e.dept_id = d.id
    </select>
</mapper>

2、association嵌套结果集

<?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">
    <!--
         使用association定义关联的单个对象的封装规则；
    -->
    <resultMap id="employee" type="com.yefengyu.mybatis.entity.Employee">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>

        <!--  association可以指定联合的javaBean对象
            property="department"：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="department" javaType="com.yefengyu.mybatis.entity.Department">
            <id column="d_id" property="id"/>
            <result column="dept_name" property="deptName"/>
        </association>

    </resultMap>

    <select id="getEmployee" resultMap="employee">
        select e.id id, e.last_name last_name, e.dept_id dept_id, d.id d_id,d.dept_name dept_name
        from employee e, department d
        where e.id = #{id} and e.dept_id = d.id
    </select>
</mapper>

3、association分步查询法

1）首先需要创建一个关于department的接口

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Department;


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

2）其次编写该接口的映射文件

<?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">
    <select id="

getDeptById

" resultType="com.yefengyu.mybatis.entity.Department">
        select id,dept_name from department where id = #{id}
    </select>
</mapper>

3）现在定义一个查询employee的接口（级联属性法和association嵌套结果集测试时已经建立该接口）

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Employee;


public interface EmployeeMapper
{
    Employee getEmployee(Integer id);
}

4）编写该接口对应的映射文件

<?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.EmployeeMapper">
    <resultMap id="employee" type="com.yefengyu.mybatis.entity.Employee">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <!-- 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="department"
                     select="com.yefengyu.mybatis.mapper.DepartmentMapper.getDeptById"
                     column="dept_id">
        </association>
    </resultMap>

    <select id="getEmployee" resultMap="employee">
        select * from employee where id = #{id}
    </select>
</mapper>

延迟加载：

每次查询Employee对象的时候，都将部门信息一起查询出来。但是想要部门信息在我们使用的时候再去查询，如果不需要就不查询，怎么实现？分步查询的基础上加两个配置：

<!--显示的指定每个我们需要更改的配置的值，即使他是默认的。防止版本更新带来的问题  -->
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>

Mybatis仅支持association关联对象和collection关联集合对象的延迟加载，association指的就是一对一，collection指的就是一对多查询。

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/EmployeeMapper.xml"/>
        <mapper resource="mapper/DepartmentMapper.xml"/>
    </mappers>
</configuration>

测试

---

通用测试代码：

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
    {
        EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
        Employee employee = mapper.getEmployee(1);
        System.out.println(employee);
    }
    finally
    {
        sqlSession.close();
    }
}

准对于延迟加载的特殊测试，对于上面测试代码，如果只打印员工名

System.out.println(employee.getLastName());

那么不会查询部门表

Created connection 1776957250.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]
==>  Preparing: select * from employee where id = ? 
==> Parameters: 1(Integer)
<==    Columns: id, last_name, dept_id
<==        Row: 1, tom, 1
<==      Total: 1
tom
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]
Returned connection 1776957250 to pool.

如果打印部门相关信息，例如通用测试的代码（由于实体都重写了toString方法，因此打印employee的时候也打印了department，所以也是使用了部门信息），此时的日志为：

Created connection 1776957250.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]
==>  Preparing: select * from employee where id = ? 
==> Parameters: 1(Integer)
<==    Columns: id, last_name, dept_id
<==        Row: 1, tom, 1
<==      Total: 1
==>  Preparing: select id,dept_name from department where id = ? 
==> Parameters: 1(Integer)
<==    Columns: id, dept_name
<==        Row: 1, A
<==      Total: 1
Employee{id=1, lastName='tom', department=Department{id=1, deptName='A'}}
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]
Returned connection 1776957250 to pool.