mybatis的一对一和一对多查询

一对一

需求：查询出员工的部门+员工的信息（一个员工对应一个部门）

1、建表

　　　　　　　　　部门表（department）

 

 

 　　　　　　　　　　员工表（employee）

 

 

 2、新建两个类（Department和Employee）和接口

 

 

 

public class Department {
    private Integer deptId;
    private String deptName;
    private LocalDate createDate = LocalDate.now();
    private String deptLoc;

　　

public class Employee {
    private Integer empId;
    private String username;
    private String password;
    private String realName;
    private LocalDate bornDate;
    private LocalDate hireDate=LocalDate.now();
    private float salary;
　　 private Department department;

　　

public interface EmployeeMapper {
    /**
     * create by: 
     * description:根据id查询员工信息
     * create time: 2020/8/11
     *
      * @Param: null
     * @return
     */
    Employee efindById(int id);
}

 

3、配置映射表、

对于结果的处理有两种方式：嵌套结果、嵌套查询

嵌套结果

    <resultMap id="employee" type="Employee">
        <id property="empId" column="emp_id">id>
        <result property="hireDate" column="entry_date">result>
        <result property="username" column="username">result>
        <result property="password" column="password">result>
        <result property="bornDate" column="born_date">result>
        <result property="salary" column="salary">result>
        
        <association property="department" column="dept_id" javaType="Department">
            <id property="deptId" column="dept_id">id>
            <result property="deptName" column="dept_name">result>
            <result property="createDate" column="create_Date">result>
            <result property="deptLoc" column="dept_loc">result>
        association>
    resultMap>
    <select id="efindById" parameterType="int" resultMap="employee">
       select d.* ,e.* from department d,employee e where d.dept_id=e.dept_id and e.emp_id=#{empId};
    select>

嵌套查询

   
    <select id="efindById" resultMap="employee2">
        select emp_id,entry_date,username,password,realname,dept_id from employee where emp_id = #{id}
    select>
    <resultMap id="employee2" type="Employee">
        <id property="empId" column="emp_id">id>
        <result property="hireDate" column="entry_date">result>
        <association property="department" select="edu.cduestc.book.Dao.DepartmentDao.findById" column="dept_id"> //这个column表示传递的参数。因为findById查询要使用dept_id这个值
        association>
    resultMap>
    <select id="findById" resultMap="employee2">
        select * from department where dept_id = #{dept_id}
    select>

 4、测试

 

一对多

需求：查询一个部门下有多少员工

1、表和上面一样不变

2、建立类和接口方法

　

1 public class Department {
2     private Integer deptId;
3     private String deptName;
4     private LocalDate createDate = LocalDate.now();
5     private String deptLoc;
6     private List employees;

1 public class Employee {
2     private Integer empId;
3     private String username;
4     private String password;
5     private String realName;
6     private LocalDate bornDate;
7     private LocalDate hireDate=LocalDate.now();
8     private float salary;

1 public interface DepartmentDao {
2          Department findAllEmployee(int dept_id);
3          Department findAllEmployee2(int dept_id);
4 }    

3、配置xml文件

　　嵌套结果

 1 
 2     <select id="findAllEmployee" resultMap="getEmployeelist">
 3         select e.*,d.dept_name 
 4             from employee e,department d 
 5                 where e.dept_id=d.dept_id 
 6                     and e.dept_id=#{dept_id}
 7     select>
 8     <resultMap id="getEmployeelist" type="Department">
 9         <id property="deptId" column="dept_id">id>
10         <result property="deptName" column="dept_name">result>
11         <collection property="employees" ofType="Employee">
12             <id property="empId" column="emp_id">id>
13             <result property="username" column="username">result>
14             <result property="password" column="password">result>
15             <result property="realName" column="realname">result>
16             <result property="hireDate" column="entry_date">result>
17             <result property="bornDate" column="born_date">result>
18             <result property="salary" column="salary">result>
19         collection>
20     resultMap>

嵌套查询

 1 
 2     <select id="findAllEmployee2" resultMap="getEmployeelist2" parameterType="int">
 3         select dept_id,dept_name
 4             from department
 5                 where dept_id=#{dept_id} 
 6     select>
 7     <select id="getEmployees" resultType="Employee" parameterType="int">
 8         select *
 9             from employee
10                 where dept_id=#{dept_id}
11     select>
12     <resultMap id="getEmployeelist2" type="Department">
13         <id property="deptId" column="dept_id">id>
14         <result property="deptName" column="dept_name">result>
15         <collection property="employees" ofType="Employee" column="dept_id" select="getEmployees">
16 
17         collection>
18     resultMap>

 

如果不太记得resultMap的具体使用可以参考以下的介绍

　　

<resultMap id="唯一的标识" type="映射的pojo对象">
  <id column="表的主键字段，或者可以为查询语句中的别名字段" jdbcType="字段类型" property="映射pojo对象的主键属性" />
  <result column="表的一个字段（可以为任意表的一个字段）" jdbcType="字段类型" property="映射到pojo对象的一个属性（须为type定义的pojo对象中的一个属性）"/>
  <association property="pojo的一个对象属性" javaType="pojo关联的pojo对象">
    <id column="关联pojo对象对应表的主键字段" jdbcType="字段类型" property="关联pojo对象的主席属性"/>
    <result  column="任意表的字段" jdbcType="字段类型" property="关联pojo对象的属性"/>
  association>
  
  <collection property="pojo的集合属性" ofType="集合中的pojo对象">
    <id column="集合中pojo对象对应的表的主键字段" jdbcType="字段类型" property="集合中pojo对象的主键属性" />
    <result column="可以为任意表的字段" jdbcType="字段类型" property="集合中的pojo对象的属性" />  
  collection>
resultMap>

collection标签使用的是嵌套查询
 <collection column="传递给嵌套查询语句的字段参数" property="pojo对象中集合属性" ofType="集合属性中的pojo对象" select="嵌套的查询语句" > 
 collection>

 

参考：

https://blog.51cto.com/wuqinglong/1726152

https://blog.51cto.com/wuqinglong/1726099

https://www.cnblogs.com/kenhome/p/7764398.html