有两张表，老师表teacher和班级表class，一个class班级对应一个teacher，一个teacher对应一个class
需求是根据班级id查询班级信息(带老师的信息)
创建teacher和class表：
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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);
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首先定义实体类
老师Teacher类
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package mybatis.bean;
public class Teacher {
private int id;
private String name;
public Teacher() {
}
public Teacher(int id, String name) {
super();
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Teacher [>;
}
}
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班级Class类
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package mybatis.bean;
public class Class {
private int id;
private String name;
private Teacher teacher;
public Class() {
}
public Class(int id, String name, Teacher teacher) {
super();
this.id = id;
this.name = name;
this.teacher = teacher;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Class [>;
}
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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
可以理解为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
classMapper.xml文件如下：
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<?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="mybatis.test5.classMapper">
<!--根据id查询到一个班级信息（带老师信息） -->
<!-- 嵌套结果 -->
<select id="getClass" parameterType="int" resultMap="ClassResultMap">
select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
</select>
<resultMap type="mybatis.bean.Class" id="ClassResultMap">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<association property="teacher" column="teacher_id" javaType="mybatis.bean.Teacher">
<id property="id" column="t_id" />
<result property="name" column="t_name" />
</association>
</resultMap>
</mapper> |
这里使用association标签，association用于一对一的关联查询
-
property- 对象属性的名称 -
javaType- 对象属性的类型 -
column- 所对应的外键字段名称 -
select- 使用另一个查询封装的结果
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
在classMapper.xml中的配置如下：
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<!--嵌套查询 --><select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
select * from class where c_id=#{id}
</select><resultMap type="mybatis.bean.Class" id="ClassResultMap2">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<association property="teacher" column="teacher_id" javaType="mybatis.bean.Teacher" select="getTeacher">
</association>
</resultMap><!-- 使用了sql别名 --><select id="getTeacher" parameterType="int" resultType="mybatis.bean.Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select> |
这里使用了association标签的select属性，其值对应为<select ...>中的id的值
这种方式，同样可以得到正确的结果