mybatis 的一对多一对一 还有动态sql

 一对多查询

老师和学生：一个老师对应多个学生

package com.example.tkmappertest.vo;

import com.example.tkmappertest.entity.Student;
import com.example.tkmappertest.entity.Teacher;

import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class TeacherAndStudentVo {

    private int id;
    private String tname;
    private int tage;
    private Date createTime;
    //StudentVo这里面的id和外面的id不能相同要不就只能查询出来一条记录
    private List<StudentVo> listStu=new ArrayList<>();

    public int getId() {
        return id;
    }

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

    public String getTname() {
        return tname;
    }

    public void setTname(String tname) {
        this.tname = tname;
    }

    public int getTage() {
        return tage;
    }

    public void setTage(int tage) {
        this.tage = tage;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public List<StudentVo> getListStu() {
        return listStu;
    }

    public void setListStu(List<StudentVo> listStu) {
        this.listStu = listStu;
    }
}

跟据老师的id查出多个学生直接封装成复杂的实体类

 <resultMap id="resultMapName" type="com.example.tkmappertest.vo.TeacherAndStudentVo">
        <id property="id" column="id"/>
        <result column="tname" property="tname"/>
        <result column="tage" property="tage"/>
        <result column="create_time" property="createTime"/>
        <collection property="listStu"  ofType="com.example.tkmappertest.vo.StudentVo">
            <id property="sid" column="sid"/>
            <result column="name" property="name"/>
            <result column="age" property="age"/>
            <result column="create_time" property="createTime"/>
            <result column="t_id" property="tId"/>
        </collection>
    </resultMap>

    <select id="selectAllByTid" parameterType="int" resultMap="resultMapName">
        select
          t.id,
          t.tname,
          t.tage,
          t.create_time,
          s.id as sid,
          s.name,
          s.age,
          s.create_time,
          s.t_id
        from
          teacher t
       left join
          student s
        on
          t.id=s.t_id
        where
          t.id=#{tid}
    </select>

嵌套查询：

package com.example.tkmappertest.vo;

import com.example.tkmappertest.entity.Student;
import com.example.tkmappertest.entity.Teacher;

import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class TeacherAndStudentVo {

    private int id;
    private String tname;
    private int tage;
    private Date createTime;
    //StudentVo这里面的id和外面的id不能相同要不就只能查询出来一条记录
    private List<StudentVo> listStu=new ArrayList<>();

    public int getId() {
        return id;
    }

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

    public String getTname() {
        return tname;
    }

    public void setTname(String tname) {
        this.tname = tname;
    }

    public int getTage() {
        return tage;
    }

    public void setTage(int tage) {
        this.tage = tage;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public List<StudentVo> getListStu() {
        return listStu;
    }

    public void setListStu(List<StudentVo> listStu) {
        this.listStu = listStu;
    }
}

<resultMap id="resultMapName2" type="com.example.tkmappertest.vo.TeacherAndStudentVo">
        <id property="id" column="id"/>
        <result column="tname" property="tname"/>
        <result column="tage" property="tage"/>
        <result column="create_time" property="createTime"/>
        <!--column="id" 说明把上个id的值传到下个sql里面当参数
            column="{id=id,name=tname,age=tage}" 这个是可以把上个sql的多个值传入下个sql当参数相当于map
            select="selectStuById" 这个是指定下个sql的查询语句
        -->
        <collection property="listStu" column="id" select="selectStuById"  ofType="com.example.tkmappertest.vo.StudentVo">
            <result column="sid" property="sid"/>
            <result column="name" property="name"/>
            <result column="age" property="age"/>
            <result column="create_time" property="createTime"/>
            <result column="t_id" property="tId"/>
        </collection>
    </resultMap>

<!--这个是第一个sql查询老师的信息-->
<select id="selectAllByTid2" parameterType="int" resultMap="resultMapName2">
        select
          t.id,
          t.tname,
          t.tage,
          t.create_time
        from
          teacher t
        where
          t.id=#{tid}
    </select>
<!--这个是第二个查询的信息 查询这个老师下面所有的学生-->
<select id="selectAllByTid2" parameterType="int" resultMap="resultMapName2">
        select
          t.id,
          t.tname,
          t.tage,
          t.create_time
        from
          teacher t
        where
          t.id=#{tid}
    </select>

 动态sql

<insert id="insertIntoSelect" parameterType="com.example.tkmappertest.entity.Student">
        insert into student
        <trim prefix="(" suffix=")" suffixOverrides=",">
            <if test="id!='' and id!=null">
              id,
            </if>
            <if test="name!='' and name!=null">
                name,
            </if>
            <if test="age!='' and age!=null">
                age,
            </if>
            <if test="createTime!=null">
                create_time,
            </if>
            <if test="tId!=null and tId!=''">
                t_id,
            </if>
        </trim>
        values
        <trim prefix="(" suffix=")" suffixOverrides=",">
            <if test="id!='' and id!=null">
                #{id},
            </if>
            <if test="name!='' and name!=null">
                #{name},
            </if>
            <if test="age!='' and age!=null">
                #{age},
            </if>
            <if test="createTime!=null">
                #{createTime},
            </if>
            <if test="tId!=null and tId!=''">
                #{tId},
            </if>
        </trim>
    </insert>

trim的用法：

<trim prefix="(" suffix=")" suffixOverrides=",">

prefix就前缀  suffix 是后缀

suffixOverrides就是用逗号进行分隔
foreach的用法

 <insert id="insertAll" parameterType="com.example.tkmappertest.entity.Student">
        insert into student(id,name,age,create_time,t_id) values
      <foreach collection="list" item="stu" index="index" separator="," >
          (#{stu.id},#{stu.name},#{stu.age},#{stu.createTime},#{stu.tId})
      </foreach>
    </insert>

<foreach collection="list" item="stu" index="index" separator="," >

collection 说明是集合 如果只传个list这里面就写list 如果传过来的是个map那这就写map的健 如果加了@Param("") 这个注解那么这里面是什么collection就是什么

item 是从集合中拿出来的单个的对象 如果是字符串就可以直接#{item的名} 直接取如果是对象的话就得#{item的名.属性名}取

separator是用什么分隔两次循环

解决重复插入的问题：

<insert id="insertByName" parameterType="com.example.tkmappertest.entity.Student">
      insert into
        student(id,name,age,create_time)
        select
          #{id},
          #{name},
          #{age},
          #{createTime}
        from
          dual
        where
          not exists(select 1 from student where name=#{name})
</insert>