Mybatis学习笔记-一对多关联
需求：根据classId查询对应的班级信息,包括学生,老师
Student实体类
public class Student {
private int id;
private String name;
//...
}
Classes 实体类
public class Classes {
private int id;
private String name;
private Teacher teacher;
private List<Student> students;
//...
}
Student实体类
public class Student {
private int id;
private String name;
//...
}
方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.c_id=s.class_id and c.c_id=#{id}
<select id="getClass" parameterType="int" resultMap="ClassResultMap3">
select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.c_id=s.class_id and c.c_id=#{id}
</select>
<resultMap type="com.mybatis.test04.Classes" id="ClassResultMap3">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="com.mybatis.test04.Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
<!-- ofType指定students集合中的对象类型 -->
<collection property="students" ofType="com.mybatis.test04.Student">
<id property="id" column="s_id"/>
<result property="name" column="s_name"/>
</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 //1 是上一个查询得到的teacher_id的值
SELECT * FROM student WHERE class_id=1 //1是第一个查询得到的c_id字段的值
<select id="getClass2" resultMap="getClass2Map">
select * from class where c_id=#{id}
</select>
<select id="getTeacher" resultType="com.mybatis.test04.Teacher">
select t_id id, t_name name from teacher where t_id=#{id}
</select>
<select id="getStudent" resultType="com.mybatis.test04.Student">
select s_id id, s_name name from student where class_id=#{id}
</select>
<resultMap type="com.mybatis.test04.Classes" id="getClass2Map">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" select="getTeacher"></association>
<collection property="students" column="c_id" select="getStudent"></collection>
</resultMap>
collection 一对多关联
ofType 指定集合中元素对象的类型
附：建表语句
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher` (
`t_id` int(11) NOT NULL auto_increment,
`t_name` varchar(20) default NULL,
PRIMARY KEY (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
INSERT INTO `teacher` VALUES ('1', '孔子');
INSERT INTO `teacher` VALUES ('2', '孟子');
DROP TABLE IF EXISTS `class`;
CREATE TABLE `class` (
`c_id` int(11) NOT NULL auto_increment,
`c_name` varchar(20) default NULL,
`teacher_id` int(11) default NULL,
PRIMARY KEY (`c_id`),
KEY `fk_teacher_id` (`teacher_id`),
CONSTRAINT `fk_teacher_id` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
INSERT INTO `class` VALUES ('1', '1121班', '1');
INSERT INTO `class` VALUES ('2', '1122班', '2');
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`s_id` int(11) NOT NULL auto_increment,
`s_name` varchar(20) default NULL,
`class_id` int(11) default NULL,
PRIMARY KEY (`s_id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
INSERT INTO `student` VALUES ('1', '张三', '1');
INSERT INTO `student` VALUES ('2', '李四', '1');
INSERT INTO `student` VALUES ('3', '王五', '1');
INSERT INTO `student` VALUES ('4', 'Tom', '2');
INSERT INTO `student` VALUES ('5', 'Jack', '2');
INSERT INTO `student` VALUES ('6', 'Robert', '2');