【三】MyBatis的SQL映射文件 ----- 一对多的关联查询
一对多关联查询的解决方案也有两种:嵌套结果和嵌套查询
在上一篇博客中已经讲了一对一的关联查询,在MySQL数据库中已经存在了班级表 class 和教师表 teacher。为了满足一对多关联查询的要求,还需要创建一张学生表 student。这样,在查询班级信息的同时,可以得到教师和学生的信息。
创建 student 表的 SQL 语句如下:
CREATE TABLE student( s_id INT PRIMARY KEY AUTO_INCREMENT, s_name VARCHAR(20), class_id INT ); INSERT INTO student(s_name, class_id) VALUES(‘xs_A‘, 1); INSERT INTO student(s_name, class_id) VALUES(‘xs_B‘, 1); INSERT INTO student(s_name, class_id) VALUES(‘xs_C‘, 1); INSERT INTO student(s_name, class_id) VALUES(‘xs_D‘, 2); INSERT INTO student(s_name, class_id) VALUES(‘xs_E‘, 2); INSERT INTO student(s_name, class_id) VALUES(‘xs_F‘, 2);
重写班级实体类 Class 如下:
SQL映射文件如下:
<?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="day03.bean.classMapper"> <!-- 方式一:嵌套结果 --> <select id="getClass" parameterType="int" resultMap="getClassMap"> SELECT * FROM class c,student s,teacher t WHERE c.c_id = s.class_id AND c.teacher_id = t.t_id AND c.c_id = #{id} </select> <resultMap type="Class" id="getClassMap"> <id property="id" column="c_id" /> <result property="name" column="c_name" /> <association property="teacher" javaType="Teacher"> <id property="id" column="t_id" /> <result property="name" column="t_name" /> </association> <!-- ofType:指定集合元素的类型 --> <collection property="students" ofType="Student"> <id property="id" column="s_id" /> <result property="name" column="s_name" /> <result property="classId" column="class_id" /> </collection> </resultMap> <!-- 方式二:嵌套查询:通过执行另外一个SQL映射语句来返回预期的复杂类型 SELECT * FROM class WHERE c_id=1; SELECT * FROM teacher WHERE t_id=1 //值1 是上一个查询得到的teacher_id字段的值 SELECT * FROM student WHERE class_id=1 //值1是第一个查询得到的c_id字段的值 --> <select id="getClass2" parameterType="int" resultMap="getClassMap2"> SELECT * FROM class WHERE c_id = #{id} </select> <select id="getTeacher" parameterType="int" resultType="Teacher"> SELECT t_id id,t_name name FROM teacher WHERE t_id = #{id} </select> <select id="getStudent" parameterType="int" resultType="Student"> SELECT s_id id,s_name name,class_id classId FROM student WHERE s_id = #{id} </select> <resultMap type="Class" id="getClassMap2"> <id property="id" column="c_id" /> <result property="name" column="c_name" /> <result/> <association property="teacher" javaType="Teacher" select="getTeacher" column="teacher_id"/> <collection property="students" select="getStudent" column="c_id" ofType="Student"/> </resultMap> </mapper>
其他代码请自己补充即可。
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