MyBatis关于多表联查 关联关系之一--------一对多（单条sql语句查询）

Posted 2020-10-25 明渃筱曦

在MyBatis中，进行多表联查时关联关系主要有这几种：一对多，多对一，多对多，还有一种自关联

1.一对多：有两种方式

（1）用一条sql语句进行查询    （以查询部门和员工为案例）

　首先创建实体类

package entity;

import java.util.List;

/**
 * Created by mycom on 2018/2/26.
 */
public class Dept {//部门
    private Integer deptNo;

    private String deptName;

    private List<Emp> emps;

    public List<Emp> getEmps() {
        return emps;
    }

    public void setEmps(List<Emp> emps) {
        this.emps = emps;
    }

    public Integer getDeptNo() {
        return deptNo;
    }

    public void setDeptNo(Integer deptNo) {
        this.deptNo = deptNo;
    }

    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }



}

然后创建部门接口以及对应的xml文件

package dao;

import entity.Dept;
import entity.Emp;

import java.util.List;

/**
 * Created by mycom on 2018/2/26.
 */
public interface IDeptDao {
    //根据部门编号获得部门名称  员工名称  单条sql
    public Dept getEmpByDeptNo(int deptNo);
}

<?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="dao.IDeptDao">
    <resultMap id="deptMaper" type="Dept">
        <id column="d" property="deptNo"></id>
        <result column="deptName" property="deptName"></result>
        <collection property="emps" ofType="Emp">
            <id column="empNo" property="empNo"></id>
            <result column="empName" property="empName"></result>
        </collection>
    </resultMap>

    <!--查询部门和员工  单条sql语句方案-->
    <select id="getEmpByDeptNo" resultMap="deptMaper">
        SELECT dept.deptNo as d ,deptName,empName FROM dept,emp
        WHERE dept.`deptNo`=emp.`deptNo`
        AND dept.`deptNo`=#{deptNo}
    </select>

</mapper>

编写测试类

//查询部门和员工  单条sql
@Test
public void oneToMany(){
    SqlSession sqlSession = MyBatisUtil.getSession();
    IDeptDao mapper = sqlSession.getMapper(IDeptDao.class);
    Dept dept = mapper.getEmpByDeptNo(1);
    System.out.println("部门名称"+dept.getDeptName());
    for (Emp emp:dept.getEmps()) {
        System.out.println(emp.getEmpName());
    }
    sqlSession.close();
}