6.实现关联表查询

6.实现关联表查询

6.1. 一对一关联

1). 提出需求

根据班级id查询班级信息(带老师的信息)

班级实体中含有老师实体

2). 创建表和数据

CREATE TABLE teacher(

t_id INT PRIMARY KEY AUTO_INCREMENT, 
t_name VARCHAR(20)
);
CREATE TABLE class(
c_id INT PRIMARY KEY AUTO_INCREMENT, 
c_name VARCHAR(20), 
teacher_id INT
);
ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);


INSERT INTO teacher(t_name) VALUES('LS1');
INSERT INTO teacher(t_name) VALUES('LS2');


INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1);
INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);


##直接查询class只能得到teacher_id得不到teacher的信息
##1.联表查询
SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;


##2.执行两次查询
SELECT * FROM class WHere c_id=1;    -- teacher_id=1

SELECT * FROM teacher WHERE t_id=1    -- 使用上面的teacher_id=1

3). 定义实体类：

public class Teacher {

         privateint id;

         privateString name;

}

public class Classes {

         privateint id;

         privateString name;

         privateTeacher teacher;

}

4). 定义sql映射文件ClassMapper.xml

<?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.mybatis.test5.ClassesMapper">

<!-- 根据班级id查询班级信息（带老师的信息）

   ##直接查询class只能得到teacher_id得不到teacher的信息 

   ##1.联表查询 

SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;

   ##2.执行两次查询 

SELECT * FROM class WHere c_id=1;  // teacher_id=1 

SELECT * FROM teacher WHERE t_id=1 // 使用上面的teacher_id=1

-->

    

    <!-- 

         方式一：联表查询：使用嵌套结果映射来处理重复的联合结果的子集

         封装联表查询的数据(去除重复的数据)

     select * from class c, teacher t where c.teacher_id=t.t_id and  c.c_id=1

     -->

    

    <select id="getClass" parameterType="int" resultMap="getClassMap">

     SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=#{id};

    </select>

    

    <resultMap type="com.mybatis.test5.Classes" id="getClassMap">

       <id property="id" column="c_id"/>

       <result property="name" column="c_name"/>

       <!-- 把联表查询得到的teacher属性值通过association标签封装到Teacher中 -->

       <association property="teacher" javaType="com.mybatis.test5.Teacher">

          <id property="id" column="t_id"/>

          <result property="name" column="t_name"/>

       </association>

    </resultMap>

    

    <!-- 方式二：嵌套查询：通过执行另一个sql映射语句来返回预期的复杂类型 

         SELECT * FROM class WHere c_id=1;  // teacher_id=1 

         SELECT * FROM teacher WHERE t_id=1 // 1是上一个查询得到的teacher_id的值

    -->

    

    <select id="getClass2" parameterType="int" resultMap="getClass2Map">

         SELECT * FROM class WHERE c_id=#{id};

    </select>

    

    <!-- 这句sql是由下面association标签的select="getTeacher"执行的

                用SELECT t_id id, t_name name解决字段名与实体类属性名不相同的冲突

                如果像上面用select* 要指定property对应的column如<result property="name" column="c_name"/>

     -->

    <select id="getTeacher" parameterType="int" resultType="com.mybatis.test5.Teacher">

         SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}

    </select>

    

    <resultMap type="com.mybatis.test5.Classes" id="getClass2Map">

       <id property="id" column="c_id"/>

       <result property="name" column="c_name"/>

       <association property="teacher" column="teacher_id" select="getTeacher">

       </association>

    

    </resultMap>

</mapper>