MyBatis 级联查询

级联是在resultMap标签中配置。级联不是必须的，级联的好处是获取关联数据十分便捷，但是级联过多会增加系统的复杂度，同事降低系统的性能，次增彼减，所以记录超过3层时，就不要考虑使用级联了，因为这样会造成多个对象的关联，导致系统的耦合、负责和难以维护。
MyBatis中的级联分3种：
1.鉴别器（discriminator）它是一个根据某些条件决定采用具体实现类级联的方案，比如体检根据性别来区分检查项目。
2.一对一（association）
3.一对多（collection）

表模型：

关系：
以雇员表为中心。
雇员表和工牌表一对一关系。
雇员表和员工任务表是一对多关系。
员工任务表和任务表是一对一关系。
每个雇员都会有一个体检表，随着雇员性别不同，会有不同的关联表。

1、实体类：
忽略getter、setter

(1)体检表-父类

public class HealthForm {    private Long id;    private Long empId;    private String heart;    private String liver;    private String spleen;    private String lung;    private String kidney;    private String note;    }

(2)女性体检表

public class FemaleHealthForm extends HealthForm{    private String uterus;}

(3) 男性体检表

public class MaleHealthForm extends HealthForm{    private String prostate;}

(4) 工牌表

public class Employee {    private Long id;    private String realName;    private SexEnum sex=null;    private Date brithday;    private String mobile;    private String email;    private String position;    private String note;    //一对一    private WorkCard workCard;    //一对多    private List<EmployeeTask> employeeTaskList=null;    }

(5) 任务表

public class Task {    private Long id;    private String title;    private String context;    private String note;}

(6) 雇员任务表

public class EmployeeTask {    private Long id;    private Long empId;    private Task task=null;    private String taskName;    private String note;}

(7) 雇员父类

public class Employee {    private Long id;    private String realName;    private SexEnum sex=null;    private Date brithday;    private String mobile;    private String email;    private String position;    private String note;    //一对一    private WorkCard workCard;    //一对多    private List<EmployeeTask> employeeTaskList=null;}

(8) 男雇员

public class MaleEmployee extends Employee{    private MaleHealthForm maleHealthForm=null;}

(9) 女雇员

public class FemaleEmployee extends Employee{    private FemaleHealthForm  femaleHealthForm=null;}

2、配置映射文件

<mapper namespace="com.bob.analyst.dao.TaskMapper">    <select id="getTask" parameterMap="long" resultType="com.bob.analyst.model.Task">     select id,title,context,note from tbl_task where id=#{id}    </select></mapper>

<mapper namespace="com.bob.analyst.model.WorkCard">    <select id="getWorkCardByEmpId" parameterType="long" resultType="com.bob.analyst.model.WorkCard">       select id,emp_id as empId,real_name as realName,       department,mobile,position,note from tbl_work_card where id = #{id}    </select></mapper>

<mapper namespace="com.bob.analyst.dao.EmployeeTaskMapper">      <resultMap type="com.bob.analyst.model.EmployeeTask" id="EmployeeTaskMap">        <id column="id" property="id"/>        <result column="emp_id" property="empId"/>        <result column="task_name" property="taskName"/>        <result column="note" property="note"/>        <!-- 一对一关联 -->        <association property="task" column="task_id" select="com.bob.analyst.dao.getTask"/>      </resultMap>      <select id="getEmployeeTaskByEmpId" resultMap="EmployeeTaskMap">        select id,emp_id,task_name,note from tbl_employee_task        where emp_id=#{empId}      </select></mapper>

<mapper namespace="com.bob.analyst.dao.EmployeeTaskMapper">      <select id="getMaleHealthForm" parameterType="long" resultMap=" com.bob.analyst.model.MaleHealthForm">        select id,heart,liver,spleen,lung,kidney,prostate,note from tbl_male_health_from        where emp_id=#{id}      </select></mapper>

<mapper namespace="com.bob.analyst.dao.FemaleHealthFormMapper">      <select id="getFemaleHealthForm" parameterType="long" resultMap="com.bob.analyst.model.FemaleHealthForm">        select id,heart,liver,spleen,lung,kidney,uterus,note from tbl_female_health_from        where emp_id=#{id}      </select></mapper>

<mapper namespace="com.bob.analyst.dao.EmployeeMapper">      <resultMap type="com.bob.analyst.model.Employee" id="employee">        <id column="id" property="id"/>        <result column="real_name" property="realName"/>        <result column="sex" property="sex" typeHandler="com.bob.analyst.util.SexEnum"/>        <result column="brithday" property="brithday"/>        <result column="mobile" property="mobile"/>        <result column="email" property="email"/>        <result column="position" property="position"/>        <result column="note" property="note"/>        <!-- 一对一关联 -->        <association property="workCard" column="id"         select="com.bob.analyst.dao.WorkCardMapper.getWorkCardByEmpId"/>        <!-- 一对多 -->        <collection property="employeeTaskList" column="id"         select="com.bob.analyst.dao.EmployeeTaskMapper.getEmployeeTaskByEmpId"></collection>        <!-- 鉴别器 -->        <discriminator javaType="long" column="sex">            <case value="1" resultMap="maleHealthFormMapper"></case>            <case value="2" resultMap="femaleHealthFormMapper"></case>        </discriminator>      </resultMap>      <resultMap type="com.bob.analyst.model.FemaleEmployee" id="femaleHealthFormMapper" extends="employee">        <association property="femaleHealthForm" column="id"         select="com.bob.analyst.dao.FemaleHealthFormMapper.getFemaleHealthForm"></association>      </resultMap>      <resultMap type="com.bob.analyst.model.MaleEmployee" id="maleHealthFormMapper" extends="employee">        <association property="maleHealthForm" column="id"         select="com.bob.analyst.dao.MaleHealthFormMapper.getMaleHealthForm"></association>      </resultMap>      <select id="getEmployee" parameterType="long" resultMap="employee">        select id,real_name as realName,sex,birthday,mobile,email,position,note from tbl_employee        where id=#{id}      </select></mapper>