package pojo;

/**
 * 學(xué)生信息
 * 沒有cid這個屬性，后期會有特殊的方式來處理這個關(guān)系字段
 */
public class Student {
    private Integer sid;
    private String sname;
    private Clazz clazz;

    public Clazz getClazz() {
        return clazz;
    }

    public void setClazz(Clazz clazz) {
        this.clazz = clazz;
    }

    @Override
    public String toString() {
        return "Student{" +
                "sid=" + sid +
                ", sname='" + sname + '\'' +
                ", clazz=" + clazz +
                '}';
    }

    public Student() {
    }

    public Student(Integer sid, String sname) {
        this.sid = sid;
        this.sname = sname;
    }

    public Integer getSid() {
        return sid;
    }

    public void setSid(Integer sid) {
        this.sid = sid;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }
}

<?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="mapper.StudentMapper">

    <!-- 級聯(lián)屬性映射 -->

    <resultMap id="studentResultMap" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <result property="clazz.cid" column="cid"/>
        <result property="clazz.cname" column="cname"/>
    </resultMap>
    <select id="selectById" resultMap="studentResultMap">
        SELECT
            s.sid, s.sname, c.cid, c.cname
        FROM
            t_student_plus s
                LEFT JOIN
            t_clazz c ON s.cid = c.cid
        WHERE
            s.sid = #{sid}
    </select>

</mapper>

@Test
    public void testSelectBySid(){
        StudentMapper mapper = SqlSessionUtil.openSession().getMapper(StudentMapper.class);
        Student student = mapper.selectBySid(1);
        System.out.println(student);
    }

  <resultMap id="studentResultMapAssociation" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <!-- 嵌套映射，將 clazz 對象中的 cid 和 cname 映射 -->
        <association property="clazz" javaType="Clazz">
            <id property="cid" column="cid"/>
            <result property="cname" column="cname"/>
        </association>
    </resultMap>
    <select id="selectByIdAssociation" resultMap="studentResultMap">
        SELECT
            s.sid, s.sname, c.cid, c.cname
        FROM
            t_student_plus s
                LEFT JOIN
            t_clazz c ON s.cid = c.cid
        WHERE
            s.sid = #{sid}
    </select>

  <resultMap id="studentResultMapByStep" type="Student">
        <id property="sid" column="sid"/>
        <id property="sname" column="sname"/>
       <association property="clazz"
                            select="mapper.ClazzMapper.selectByCid"
                            column="cid"/>
    </resultMap>

<!--兩條sql語句，完成多對一的分布查詢-->
<!--    這是第一步，根據(jù)id值查詢學(xué)生的所有信息，這些信息當中含有班級id(cid-->
<select id="selectByIdStep1" resultMap="studentResultMapByStep" >
    select sid,sname,cid from t_student_plus where sid = #{sid}
</select>

public interface ClazzMapper {

    /**
     * 分布查詢第二步
     * 根據(jù)cid獲取Clazz信息
     * @param cid
     * @return
     */
    Clazz selectByCid(Integer cid);
}

<?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="mapper.ClazzMapper">
<select id="selectByCid" resultType="Clazz">
    select cid,cname from t_clazz where cid = #{cid}
</select>
</mapper>

  <resultMap id="studentResultMapByStep" type="Student">
        <id property="sid" column="sid"/>
        <id property="sname" column="sname"/>
       <association property="clazz"
                            select="mapper.ClazzMapper.selectByCid"
                            column="cid"
                            fetchType="lazy"/> <!--添加延遲-->
    </resultMap>

<!--兩條sql語句，完成多對一的分布查詢-->
<!--    這是第一步，根據(jù)id值查詢學(xué)生的所有信息，這些信息當中含有班級id(cid-->
<select id="selectByIdStep1" resultMap="studentResultMapByStep" >
    select sid,sname,cid from t_student_plus where sid = #{sid}
</select>

package pojo;

import java.util.List;

/**
 * 班級信息
 */
public class Clazz {

    public Clazz() {
    }

    private Integer cid;
    private String cname;
    private List<Student> stus;

    public List<Student> getStus() {
        return stus;
    }

    @Override
    public String toString() {
        return "Clazz{" +
                "cid=" + cid +
                ", cname='" + cname + '\'' +
                ", stus=" + stus +
                '}';
    }

    public void setStus(List<Student> stus) {
        this.stus = stus;
    }

    public Integer getCid() {
        return cid;
    }

    public void setCid(Integer cid) {
        this.cid = cid;
    }

    public String getCname() {
        return cname;
    }

    public void setCname(String cname) {
        this.cname = cname;
    }

    public Clazz(Integer cid, String cname) {
        this.cid = cid;
        this.cname = cname;
    }
}

public interface ClazzMapper {

    /**
     * 根據(jù)cid獲取Clazz信息
     * @param cid
     * @return
     */
    Clazz selectByCid(Integer cid);

    /**
     * 根據(jù)班級編號查詢班級信息。同時班級中所有的學(xué)生信息也要查詢。
     * @param cid
     * @return
     */
    Clazz selectClazzAndStusByCid(Integer cid);
}

<resultMap id="clazzResultMap" type="Clazz">
  <id property="cid" column="cid"/>
  <result property="cname" column="cname"/>
  <collection property="stus" ofType="Student">
    <id property="sid" column="sid"/>
    <result property="sname" column="sname"/>
  </collection>
</resultMap>

<select id="selectClazzAndStusByCid" resultMap="clazzResultMap">
  select * from t_clazz c join t_student s on c.cid = s.cid where c.cid = #{cid}
</select>