Mybatis探究-----一对一、一对多关联查询

   1.一对一关联

声明两个实体类

1）部门类

package com.entity;

import java.util.UUID;

/**
 * 部门类
 * 
 * @author yyx 2019年9月17日
 */
public class Department {
    /**
     * 部门ID
     */
    private String deptId;
    /**
     * 部门名称
     */
    private String deptName;

    public Department() {
        super();
    }

    public Department(String deptId, String deptName) {
        super();
        this.deptId = deptId;
        this.deptName = deptName;
    }

    public String getDeptId() {
        return deptId;
    }

    public void setDeptId(String deptId) {
        this.deptId = deptId;
    }

    public String getDeptName() {
        return deptName;
    }

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

    @Override
    public String toString() {
        return "Department [deptId=" + deptId + ", deptName=" + deptName + "]";
    }

    public static void main(String[] args) {
        String str = UUID.randomUUID().toString();
        System.out.println(str);
    }
}

雇员类

package com.entity;

import java.util.UUID;

/**
 * 雇员类
 * 
 * @author yyx 2019年9月17日
 */
public class Employee {
    /**
     * 雇员ID
     */
    private String empId;
    /**
     * 雇员名称
     */
    private String empName;
    /**
     * 部门
     */
    private Department empDept;

    public Employee() {
        super();
    }

    public Employee(String empId, String empName, Department empDept) {
        super();
        this.empId = empId;
        this.empName = empName;
        this.empDept = empDept;
    }

    public String getEmpId() {
        return empId;
    }

    public void setEmpId(String empId) {
        this.empId = empId;
    }

    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }

    public Department getEmpDept() {
        return empDept;
    }

    public void setEmpDept(Department empDept) {
        this.empDept = empDept;
    }

    @Override
    public String toString() {
        return "Employee [empId=" + empId + ", empName=" + empName + ", empDept=" + empDept + "]";
    }

    public static void main(String[] args) {
        String str = UUID.randomUUID().toString();
        System.out.println(str);
    }
}

1.1 级联属性查询

EmployeeMapper.xml配置

<resultMap type="com.entity.Employee" id="EmployeeResult">
    <id column="empId" property="empId" />
    <result column="empName" property="empName" />
    <result column="deptId" property="empDept.deptId" />
    <result column="deptName" property="empDept.deptName" />
</resultMap>

<select id="getEmpAndDept" resultMap="EmployeeResult">
     select e.empId,e.empName,d.deptId,d.deptName from t_emp e,t_dept 
     d where e.deptId=d.deptId
</select>

1.2 单步查询

EmployeeMapper.xml配置

<resultMap type="com.entity.Employee" id="EmployeeResult">
    <id column="empId" property="empId" />
    <result column="empName" property="empName" />
    <!-- association可以指定联合的javaBean对象 
          property="dept"：指定哪个属性是联合的对象 
          javaType:指定这个属性对象的类型[不能省略] 
     -->
     <association property="empDept"
        javaType="com.entity.Department">
        <id column="deptId" property="deptId" />
        <result column="deptName" property="deptName" />
     </association>
</resultMap>

<select id="getEmpAndDept" resultMap="EmployeeResult">
     select e.empId,e.empName,d.deptId,d.deptName from t_emp e,t_dept d 
     where e.deptId=d.deptId
</select>

1.3 分步查询

EmployeeMapper.xml配置

<resultMap type="com.entity.Employee" id="EmployeeResult">
        <id column="empId" property="empId" />
        <result column="empName" property="empName" />
        <!-- association定义关联对象的封装规则 
                 select:表明当前属性是调用select指定的方法查出的结果 
                 column:指定将哪一列的值传给这个方法 
            流程：使用select指定的方法（传入column指定的这列参数的值）查出对象，并封装给property指定的属性 -->
        <association property="empDept"
            select="com.entity.DepartmentMapper.getDeptById" column="deptId">
        </association>
    </resultMap>
    <select id="getEmpAndDept" resultMap="EmployeeResult">
        select * from t_emp
    </select>

DepartmentMapper.xml配置

    <select id="getDeptById" resultType="com.entity.Department">
        select deptId,deptName from t_dept where deptId=#{deptId}
    </select>

   2.一对多关联

声明两个实体类

部门类

package com.entity;

import java.util.List;
import java.util.UUID;

/**
 * 部门类
 * 
 * @author yyx 2019年9月17日
 */
public class Department {
    /**
     * 部门ID
     */
    private String deptId;
    /**
     * 部门名称
     */
    private String deptName;
    
    private List<Employee> deptEmps;

    public Department() {
        super();
    }

    public Department(String deptId, String deptName) {
        super();
        this.deptId = deptId;
        this.deptName = deptName;
    }

    public String getDeptId() {
        return deptId;
    }

    public void setDeptId(String deptId) {
        this.deptId = deptId;
    }

    public String getDeptName() {
        return deptName;
    }

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

    @Override
    public String toString() {
        return "Department [deptId=" + deptId + ", deptName=" + deptName + ", deptEmps=" + deptEmps+ "]";
    }

    public static void main(String[] args) {
        String str = UUID.randomUUID().toString();
        System.out.println(str);
    }
}

雇员类

package com.entity;

import java.util.UUID;

/**
 * 雇员类
 * 
 * @author yyx 2019年9月17日
 */
public class Employee {
    /**
     * 雇员ID
     */
    private String empId;
    /**
     * 雇员名称
     */
    private String empName;

    public Employee() {
        super();
    }

    public Employee(String empId, String empName) {
        super();
        this.empId = empId;
        this.empName = empName;
    }

    public String getEmpId() {
        return empId;
    }

    public void setEmpId(String empId) {
        this.empId = empId;
    }

    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }    

    @Override
    public String toString() {
        return "Employee [empId=" + empId + ", empName=" + empName + "]";
    }

    public static void main(String[] args) {
        String str = UUID.randomUUID().toString();
        System.out.println(str);
    }
}

2.1 单步查询

DepartmentMapper.xml配置

<resultMap type="com.entity.Department" id="DepartmentResult">
        <id column="deptId" property="deptId" />
        <result column="deptName" property="deptName" />
        <collection property="deptEmps" ofType="com.entity.Employee">
            <id column="empId" property="empId" />
            <result column="empName" property="empName" />
        </collection>
    </resultMap>

    <select id="getDeptAndEmps" resultMap="DepartmentResult">
        select
        d.deptId,d.deptName,e.empId,e.empName
        from t_dept d left join t_emp e
        on d.deptId=e.deptId
    </select>

2.2 分步查询

DepartmentMapper.xml配置

<resultMap type="com.entity.Department" id="DepartmentResult">
        <id column="deptId" property="deptId" />
        <id column="deptName" property="deptName" />
        <!-- 扩展：多列的值传递过去： 将多列的值封装map传递； column="{key1=column1,key2=column2}" fetchType="lazy"：表示使用延迟加载； 
            - lazy：延迟 - eager：立即 -->
        <collection property="deptEmps" select="com.dao.EmployeeMapper.getEmps"
            column="deptId" fetchType="lazy"></collection>
    </resultMap>

    <select id="getDeptAndEmps" resultMap="DepartmentResult">
        select * from t_dept
    </select>

EmployeeMapper.xml配置

<resultMap type="com.entity.Employee" id="EmployeeResult">
        <id column="empId" property="empId" />
        <result column="empName" property="empName" />
    </resultMap>

    <select id="getEmps" resultMap="EmployeeResult">
        select * from t_emp where deptId=#{deptId}
    </select>

一对一、多对一数据库语句

/*
SQLyog Ultimate v12.09 (64 bit)
MySQL - 5.7.9-log : Database - db_mybatis_associate
*********************************************************************
*/


/*!40101 SET NAMES utf8 */;

/*!40101 SET SQL_MODE=''*/;

/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;
CREATE DATABASE /*!32312 IF NOT EXISTS*/`db_mybatis_associate` /*!40100 DEFAULT CHARACTER SET utf8 */;

USE `db_mybatis_associate`;

/*Table structure for table `t_dept` */

DROP TABLE IF EXISTS `t_dept`;

CREATE TABLE `t_dept` (
  `deptId` varchar(50) NOT NULL,
  `deptName` varchar(20) NOT NULL,
  PRIMARY KEY (`deptId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

/*Data for the table `t_dept` */

insert  into `t_dept`(`deptId`,`deptName`) values ('6cf35a30-3b09-432b-bae6-14ab8cb6c964','销售部'),('7135aeab-f50e-4db2-b0ea-6cd007223203','生产部');

/*Table structure for table `t_emp` */

DROP TABLE IF EXISTS `t_emp`;

CREATE TABLE `t_emp` (
  `empId` varchar(50) NOT NULL,
  `empName` varchar(20) NOT NULL,
  `deptId` varchar(50) NOT NULL,
  PRIMARY KEY (`empId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

/*Data for the table `t_emp` */

insert  into `t_emp`(`empId`,`empName`,`deptId`) values ('b14af873-ff15-45d3-ab37-8a2718d8fb35','杜甫','6cf35a30-3b09-432b-bae6-14ab8cb6c964'),('cebdaba7-bca3-4812-a52f-e9b0d38a13e4','李斯','6cf35a30-3b09-432b-bae6-14ab8cb6c964');

/*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;
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原文地址：https://www.cnblogs.com/fengfuwanliu/p/10620916.html