Mybatis联表查询

1 使用注解方式

1.1 一对一

ClazzModel.java （返回前端的模型）

public class ClazzModel {

    private Long id;
    private String name;
    private String description;
    private Grade grade; //联表
    private User user;// 联表
    //setter、getter

}

ClazzMapper.java

    @Select("select * from poll_clazz")
    @Results({
        //一对一  one = @One()
        @Result(property="id",column="id"),//不加这个查出来的id会为null
        @Result(property = "grade",column = "grade_id",one = @One(select="com.pibigstar.dao.GradeMapper.selectByPrimaryKey")),
        @Result(property = "user",column = "user_id",one = @One(select="com.pibigstar.dao.UserMapper.selectByPrimaryKey"))
    })
    List<ClazzModel> findAll();

1.2 一对多

QuestionModel.java

public class QuestionModel{
    private Long id;
    private String name;
    private String questionType;
    private List<Options> options;
    //省略 setter、getter方法
}

QuestionMapper.java

    @Select("select * from poll_questions")
    @Results({
         // 一对多  many = @Many()
         @Result(property="id",column="id"),//不加这个查出来的id会为null
@Result(property="options",column="id",many=@Many(select="com.pibigstar.dao.OptionsMapper.findAllByQuestionId"))
    })
    List<QuestionModel> findAll();

1.3 多对多

业务介绍： 有一张问卷表（questionnaire），一张题库表（question），一张问卷与题库的桥表（qq）

希望能查出所有的问卷，每个问卷有对应的题

思路：我们先查出所有的问卷，然后通过问卷id 在桥表中查找出此问卷中的所有问题

QuestionnaireModel.java

public class QuestionnaireModel{
    private Long id;
    private String name;
    private String description;
    private List<QuestionModel> questions;
    //省略setter、getter方法
}

QuestionnaireMapper.java

    @Select("select * from poll_questionnaire")
    @Results({
        @Result(property="id",column="id"),
          @Result(property="questions",column="id",many=@Many(select="com.pibigstar.dao.QQMapper.selectAllQuestions")),
    })
    List<QuestionnaireModel> findAllModel();

QQMapper.java

  @Select("select * from poll_questions where id in (select questions_id from poll_qq where questionnaire_id=#{id})")
    @Results({
        @Result(property="id",column="id"),
        @Result(property="options",column="id",many=@Many(select="com.pibigstar.dao.OptionsMapper.findAllByQuestionId"))
    })
    List<QuestionModel> selectAllQuestions(Long id);

1.4 注意：

分步查询有时候非常浪费数据库资源，我们可以采用懒加载方式，当有需要的时候才进行分步查询
使用：fetchType=FetchType.LAZY

    @Select("select * from poll_clazz")
    @Results({
        //一对一  one = @One()
        @Result(property="id",column="id"),//不加这个查出来的id会为null
        @Result(property = "grade",column = "grade_id",one = @One(fetchType=FetchType.LAZY,select="com.pibigstar.dao.GradeMapper.selectByPrimaryKey")),

    List<ClazzModel> findAll();

2 使用xml配置方式

2.1 一对一

<?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.briup.apps.poll.dao.extend.ClazzVMMapper">

  <resultMap id="clazzVMResultMap" type="com.briup.apps.poll.bean.extend.ClazzVM">
    <id column="id" jdbcType="BIGINT" property="id" />
    <!-- 联表查询 association  -->
    <association column="grade_id" property="grade" select="com.briup.apps.poll.dao.GradeMapper.selectByPrimaryKey"></association>
    <association column="charge_id" property="charge" select="com.briup.apps.poll.dao.UserMapper.selectByPrimaryKey"></association>
  </resultMap>

   <select id="selectById" parameterType="java.lang.Long" resultMap="clazzVMResultMap">
    select * from poll_clazz where id=#{id};
  </select>

</mapper>

2.2 一对多

 <resultMap type="com.User" id="userAndItemsResultMap">
        <!-- 用户信息 -->
        <id column="user_id" property="id"/>
        <result column="username" property="username"/>
        <result column="sex" property="sex"/>
        <result column="address" property="address"/>
        <!-- 一个用户对应多个订单 -->
        <collection property="ordersList" ofType="com.Orders">
                 <id column="id" property="id"/>
                 <result column="user_id" property="userid"/>
                <result column="number" property="number"/>
                <result column="createtime" property="createTime"/>
                <result column="note" property="note"/>
                <!-- 一个订单对应多个订单明细 -->
                <collection property="orderdetails" ofType="com.OrderDetail">
                    <id column="orderdetail_id" property="id"/>
                    <result column="items_id"   property="itemsId"/>
                    <result column="items_num"  property="itemsNum"/>
                    <result column="orders_id"  property="ordersId"/>
                    <!-- 一个订单明细对应一个商品 -->
                    <association property="items" javaType="com.Items">
                        <id column="items_id" property="id"/>
                        <result column="items_name" property="itemsName"/>
                        <result column="items_detail" property="detail"/>
                        <result column="items_price" property="price"/>
                    </association>
                </collection>
         </collection>
    </resultMap>