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); |
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 [id=" + id + ", name=" + name + "]"; } } |
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 [id=" + id + ", name=" + name + ", teacher=" + teacher + "]"; } |
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 id="getTeacher" ...>中的id的值
这种方式，同样可以得到正确的结果