• Mybatis的关联查询。多对一,一对一映射


    我们来看一个实例:

    在数据库中创建两个表

    一、创建表:     

        员工表:
        DROP TABLE IF EXISTS `tbl_employee`;

        CREATE TABLE `tbl_employee` (
        `id` int(11) NOT NULL AUTO_INCREMENT,
        `user_name` varchar(255) DEFAULT NULL,
        `gender` char(1) DEFAULT NULL,
        `email` varchar(255) DEFAULT NULL,
        `d_id` int(11) DEFAULT NULL,
        PRIMARY KEY (`id`),
        KEY `fk_emp_dept` (`d_id`),
        CONSTRAINT `fk_emp_dept` FOREIGN KEY (`d_id`) REFERENCES `tbl_dept` (`id`)
        ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

        部门表:
        CREATE TABLE tbl_dept(
        id INT(11) PRIMARY KEY AUTO_INCREMENT,
        dept_name VARCHAR(255)
        )

    二、mybatis动态代理创建相应的实体类、Mapper接口和Xml文件

      实体类:

    public class tbl_dept {
        private int id;
        private String deptName;
      
    //自己添加get和set方法
    }
    public class tbl_employee {
        private  int id;
        private String userName;
        private String gender;
        private String email;
        private int dId;
        private tbl_dept dept;//声明了一个tbl_dept 对象
    //自己添加get和set方法 }

      mapper接口:

    public interface Tbl_employeeMapper {
        public tbl_employee findTblEmployeeById(int id);
    
    }

       1.xml 文件:使用association标签

     <resultMap type="com.neuedu.bean.tbl_employee" id="findTblEmployeeByIdMap">
            <id  column="id" property="id" />
            <result column="email" property="email" />
            <result column="gender" property="gender" />
            <result column="user_name" property="userName" />
            <result column="d_id" property="dId"/>
         <!-- property 属性指定的POJO中关联的其它POJO在本POJO中的属性,JavaType指定的是关联的POJO的全类名-->
          <!--id 用于指定主键,result用于指定普通的字段,column用于指定数据库字段,property用于指定POJO中的属性 --> <association property="dept" javaType="com.neuedu.bean.tbl_dept"> <id column="id" property="id"/> <result column="dept_name" property="deptName"/> </association> </resultMap> <select id="findTblEmployeeById" resultMap="findTblEmployeeByIdMap" parameterType="int"> select e.id,e.email,e.gender,e.user_name,e.d_id,d.id did,d.dept_name from tbl_employee e ,tbl_dept d <!-- 这里使用了别名--> where e.id=#{id} and d.id=e.id </select>

        2.xml文件使用级联查询

    <resultMap type="com.neuedu.bean.tbl_employee" id="findTblEmployeeByIdMap">
            <id  column="id" property="id" />
            <result column="email" property="email" />
            <result column="gender" property="gender" />
            <result column="user_name" property="userName" />
            <result column="d_id" property="dId"/>
            <result column="did" property="dept.id"/>
            <result column="dept_name" property="dept.deptName"/>
        </resultMap>
    <select id="findTblEmployeeById" resultMap="findTblEmployeeByIdMap" parameterType="int">
            select e.id,e.email,e.gender,e.user_name,e.d_id,d.id did,d.dept_name
            from tbl_employee e ,tbl_dept d
            where  e.id=#{id} and d.id=e.id
        </select>

        3.使用两个查询语句:

       <select id="selectDepartment" resultType="com.neuedu.bean.tbl_dept">
            select * from tbl_dept where id=#{id}
        </select>
        <resultMap type="com.neuedu.bean.tbl_employee" id="findTblEmployeeByIdMap">
            <id  column="id" property="id" />
            <result column="email" property="email" />
            <result column="gender" property="gender" />
            <result column="user_name" property="userName" />
            <!-- select指定上面的statement语句的id  column指定的是上面的查询语句的需要的参数   -->
            <association property="dept" select="selectDepartment" column="d_id"></association>
        </resultMap>
        
        
            
        <select id="findTblEmployeeById" resultMap="findTblEmployeeByIdMap" parameterType="int">
            select e.id,e.email,e.gender,e.user_name,e.d_id
            from tbl_employee e
            where  e.id=#{id}
        </select>
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  • 原文地址:https://www.cnblogs.com/xuesheng/p/7481681.html
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