Implement contingency table query

First, one association

1. Create a table and the class teacher in a database table

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);

2. Create the appropriate entity classes Classes and Teacher

3. Configure mapping file ClassesMapper.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">

<-! Cid inquiry information corresponding to the class information and teachers ->

<!--

id tag database table corresponding to the primary key attribute entity class

In addition result tag database table corresponding to the primary key attribute of the entity class

column corresponding column attribute database table, property attributes corresponding attribute entity class

-->

<-! Association tag is associated with the return of one configuration label

property attribute object named entity class, javaType full class name of the object is

-->

</association>

</resultMap>

select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=#{cid}

</select>

</mapper>

4.创建dao接口ClassesMapper.java

public interface ClassesMapper{

/**

* 一对一关联

* @param cid

* @return

public Classes select1(int cid);

}

5.创建单元测试

@Test

void testSelect1() throws IOException {

//1.获取配置文件config.xml加入到io流中

InputStream inputStream = Resources.getResourceAsStream("config.xml");

//2.通过SqlSessionFatoryBuilder的builder创建sqlsessionfatory

SqlSessionFactory sqlSessionFatory = new SqlSessionFactoryBuilder().build(inputStream);

//3.通过sqlSessionFactory的openSession方法创建SqlSession

SqlSession sqlSession=sqlSessionFatory.openSession();

//4.通过sqlSession的getMapper传入dao全类名的方法创建UserMapper对象

ClassesMapper um=sqlSession.getMapper(com.zhiyou100.kfs.dao.ClassesMapper.class);

//5.调用UserMapper的selectById来使用通过id查询一个用户

Classes classes=um.select1(1);

System.out.println(classes);

//最后一个页是很重要的一个：关闭资源

sqlSession.close();

}

二、一对多关联

1.在数据库里创建学生表

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);

2.创建对应Student实体类

3.配置映射文件ClassesMapper.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">

<!--

id标签数据库表主键对应实体类属性

result标签数据库表除了主键对应的实体类属性

column属性是对应数据库表的栏，property属性是对应实体类的属性

-->

<!-- collection标签是一对多关联的配置

property是实体类对应的集合名，ofType是集合类的全类名

-->

</collection>