mybatis关联查询(一对多和多对一)

mybatis关联查询

DBUtil mybatis工具类

public class DBUtil {
    private static SqlSessionFactory factory = null;
    static {
        try {
            String mybatis_config = "mybatis-config.xml";
            InputStream in = Resources.getResourceAsStream(mybatis_config);
            factory = new SqlSessionFactoryBuilder().build(in);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
	// 获取SqlSession
    public static SqlSession getSqlSession() {
        return factory.openSession(true);
    }
	// 获取mapper
    public static <T> T getMapper(Class<T> mapper) {
        return getSqlSession().getMapper(mapper);
    }
}

mybatis一对多查询

resultMap格式:

cloumn属性可以是表字段名或者别名

<resultMap id="唯一的标识" type="映射的pojo对象">    
    <id column="表的主键字段" jdbcType="字段类型" property="映射pojo对象的主键属性" />    
    <result column="表字段名或者别名" jdbcType="字段类型" property="映射到pojo对象的一个属性"/> 
     <!--多个标签<result>...-->
    <collection property="pojo的集合属性名" ofType="集合中的pojo对象">        
        <id column="主键字段" jdbcType="字段类型" property="集合中pojo对象的主键属性" />        
        <result column="表字段名或者别名" jdbcType="字段类型" property="集合中的pojo对象的属性" /> 
        <!--多个标签<result>...-->
    </collection>
</resultMap>

案例

一个老师对应多个学生, 查询老师的时候把老师

学生类:

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private Integer id;
    private String name;
}

老师类:

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private Integer id;
    private String name;
    List<Student> students;
}

老师dao接口

public interface TeacherMapper {
    List<Teacher> findAllTeacher();
}

方试一: 案结果嵌套处理（联表查询）

TeacherMapper.xml

<select id="findAllTeacher" resultMap="teacherStudent">
    select
    t.id as t_id,
    t.name as t_name,
    s.id as s_id,
    s.name as s_name
    from teacher t
    left join student s on
    s.t_id = t.id
</select>

<resultMap id="teacherStudent" type="teacher">
    <id column="t_id" property="id"/>
    <result column="t_name" property="name"/>
    <collection property="students" ofType="student">
        <id column="s_id" property="id"/>
        <result column="s_name" property="name"/>
    </collection>
</resultMap>

方式二: 按查询嵌套处理

TeacherMapper.xml

<select id="findAllTeacher" resultMap="teacherStudent">
    select * from teacher;
</select>

<select id="findStudentById" resultType="student">
    select * from student where t_id = #{id};
</select>

<resultMap id="teacherStudent" type="teacher">
    <id column="id" property="id"/>
    <result column="name" property="name"/>
    <collection property="students" javaType="ArrayList" ofType="student" select="findStudentById" column="id"/>
</resultMap>

方式一方式二测试均如下:

@Test
public void test1() {
    TeacherMapper mapper1 = DBUtil.getMapper(TeacherMapper.class);
    List<Teacher> teachers = mapper1.findAllTeacher();
    for (Teacher teacher : teachers) {
        System.out.println(teacher.getName());
        for (Student student : teacher.getStudents()) {
            System.out.println("\t" + student);
        }
    }
}

结果:

张老师
	Student(id=1, name=张三)
	Student(id=2, name=李四)
	Student(id=3, name=王五)
	Student(id=4, name=赵六)
王老师
	Student(id=5, name=李七)
	Student(id=6, name=冯八)

mybatis多对一查询

resultMap格式:

<resultMap id="唯一的标识" type="映射的pojo对象">    
    <id column="表的主键字段" jdbcType="字段类型" property="映射pojo对象的主键属性" />    
    <result column="表字段名或者别名" jdbcType="字段类型" property="映射到pojo对象的一个属性"/> 
     <!--多个标签<result>...-->
    <association property="pojo的集合属性名" javaType="饮用配型">        
        <id column="主键字段" jdbcType="字段类型" property="集合中pojo对象的主键属性" />        
        <result column="表字段名或者别名" jdbcType="字段类型" property="集合中的pojo对象的属性" /> 
        <!--多个标签<result>...-->
    </association>
</resultMap>

案例

查询学生, 并查询学生的老师

学生类:

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private Integer id;
    private String name;
    private Teacher teacher;
}

老师类:

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private Integer id;
    private String name;
}

学生dao层

public interface StudentMapper {
    List<Student> findAllStudent();
}

方式一: 案结果嵌套处理（联表查询）

StudentMapper.xml

<select id="findAllStudent" resultMap="studentTeacher">
    select
    s.id as s_id,
    s.name as s_name,
    t.id as t_id,
    t.name as t_name
    from student s left join teacher t on
    s.t_id = t.id;
</select>

<resultMap id="studentTeacher" type="student">
    <id column="s_id" property="id"/>
    <result column="s_name" property="name"/>
    <association property="teacher" javaType="teacher">
        <id column="t_id" property="id"/>
        <result column="t_name" property="name"/>
    </association>
</resultMap>

方式二: 按查询结果嵌套

StudentMapper.xml

<select id="findAllStudent" resultMap="studentTeacher">
    select * from student;
</select>

<select id="findTeacherById" resultType="teacher">
    select * from teacher where id = #{t_id};
</select>

<resultMap id="studentTeacher" type="student">
    <id column="id" property="id"/>
    <result column="name" property="name"/>
    <association property="teacher" column="t_id" javaType="teacher" select="findTeacherById"/>
</resultMap>

方式一方式二测试均如下:

@Test
public void test1() {
    StudentMapper mapper = DBUtil.getMapper(StudentMapper.class);
    List<Student> students = mapper.findAllStudent();
    for (Student student : students) {
        System.out.println(student);
    }
}

结果:

Student(id=1, name=张三, teacher=Teacher(id=1, name=张老师))
Student(id=2, name=李四, teacher=Teacher(id=1, name=张老师))
Student(id=3, name=王五, teacher=Teacher(id=1, name=张老师))
Student(id=4, name=赵六, teacher=Teacher(id=1, name=张老师))
Student(id=5, name=李七, teacher=Teacher(id=2, name=王老师))
Student(id=6, name=冯八, teacher=Teacher(id=2, name=王老师))