8、自定义映射resultMap

新建modules ----->resultMap
t_emp表：

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for t_emp
-- ----------------------------
DROp TABLE IF EXISTS `t_emp`;
CREATE TABLE `t_emp`  (
  `emp_id` int(0) NOT NULL AUTO_INCREMENT,
  `emp_name` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
  `age` int(0) NULL DEFAULT NULL,
  `gender` char(1) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
  `dept_id` int(0) NULL DEFAULT NULL,
  PRIMARY KEY (`emp_id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_emp
-- ----------------------------
INSERT INTO `t_emp` VALUES (1, '张三', 18, '男', 1);
INSERT INTO `t_emp` VALUES (2, '李四', 19, '男', 2);
INSERT INTO `t_emp` VALUES (3, '王五', 20, '女', 1);
INSERT INTO `t_emp` VALUES (4, '赵六', 21, '男', 3);

SET FOREIGN_KEY_CHECKS = 1;

t_dept表：

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for t_dept
-- ----------------------------
DROP TABLE IF EXISTS `t_dept`;
CREATE TABLE `t_dept`  (
  `dept_id` int(0) NOT NULL AUTO_INCREMENT,
  `dept_name` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
  PRIMARY KEY (`dept_id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_dept
-- ----------------------------
INSERT INTO `t_dept` VALUES (1, 'A');
INSERT INTO `t_dept` VALUES (2, 'B');
INSERT INTO `t_dept` VALUES (3, 'C');
INSERT INTO `t_dept` VALUES (4, 'D');

SET FOREIGN_KEY_CHECKS = 1;

    private Integer empId;

    private String empName;

    private Integer age;

    private  String gender;

 
    
    
        empId}-->
        select * from t_emp where emp_id = #{empId}
    
 
 
使用resultMap处理字段名和属性名不一致的问题 
 
    
        
        
        
        
    
    
        select * from t_emp where emp_id = #{empId}
    

Emp中添加dept

private Dept dept;

    
       
        
        
        
        
        
    
    
    
        select t_emp.*,t_dept.*
        from t_emp left join t_dept on
            t_emp.dept_id = t_dept.dept_id
        where t_emp.emp_id = #{empId}
    

 
    @Test
    public void  testGetEmpAndDeptByEmpId(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp empById = mapper.getempAndDeptByEmpId(1);
        System.out.println(empById);
    }
 
 
8.2.2、使用association处理映射关系 
    
    
        
        
        
        
        
            
            
        
    
 
 
8.2.3、分步查询 
EmpMapper接口
 
    //分步查询(1)①查询员工信息
    Emp getempAndDeptByStepOne (@Param("empId") Integer empId);
 
DeptMapper接口
 
    //分步查询（2）②根据员工所对应的部门id查询部门信息
    Dept getEmpAndDeptByStepTwo(@Param("deptId") Integer deptId);
 
EmpMapper.xml
 

    
        
        
        
        
        

        
    
    
    
        select * from t_emp where emp_id = #{empId}
    

DeptMapper.xml

    
    
        select * from t_dept where dept_id = #{deptId}
    
 
测试
 
    @Test
    public void  testGetEmpAndDeptByStep(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp emp = mapper.getempAndDeptByStepOne(2);
        System.out.println(emp);
    }
 
延迟加载 
 
 
分步查询的优点：可以实现延迟加载
 但是必须在核心配置文件中设置全局配置信息：
 lazyLoadingEnabled：延迟加载的全局开关。当开启时，所有关联对象都会延迟加载
 aggressiveLazyLoading：当开启时，任何方法的调用都会加载该对象的所有属性。否则，每个属性会按需加载
 此时就可以实现按需加载，获取的数据是什么，就只会执行相应的sql。此时可通过association和collection中的fetchType属性设置当前的分步查询是否使用延迟加载， fetchType=“lazy(延迟加载)|eager(立即加载)”
 
 
    
        
        
        
        
        
        
    
 
测试：
 
    @Test
    public void  testGetEmpAndDeptByStep(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp emp = mapper.getempAndDeptByStepOne(2);
        System.out.println(emp.getEmpName());
    }
 
 
        
        
        
        
        

        
    
 
fetchType：在开启了延迟加载的环境中，通过该属性设置当前的分步查询是否使用延迟加载
        fetchType="lazy(延迟) | eager(立即加载)"
 
 
8.3、一对多映射处理 
8.3.1、collection 
Dept类增加属性：
 
private List emps;
 
 //查询部门以及部门中员工的信息
    Dept getDeptAndEmpByDeptId(@Param("deptId") Integer deptId);

 


    
        
        
        
            
            
            
            
        
    
    
    
        select * from t_dept
            left join t_emp on
            t_dept.dept_id = t_emp.dept_id
            where t_dept.dept_id = #{deptId}
    

测试

    @Test
    public void  testGetDeptAndEmpByDeptId(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept deptAndEmpByDeptId = mapper.getDeptAndEmpByDeptId(1);
        System.out.println(deptAndEmpByDeptId);
    }

Dept

private List emps;

DeptMapper

    //分步查询（1）查询部门以及部门中的员工
    Dept getDeptAndEmpByStepOne(@Param("deptId") Integer deptId);

    
        
        
        
        
    
    
    
        select * from t_dept where dept_id = #{deptId}
    
 
EmpManner
 
 //分步查询（2）查询部门以及部门中的员工
    List getDeptAndEmpByStepTwo(@Param("deptId") Integer deptId);
 
!--List getDeptAndEmpByStepTwo(@Param("deptId") Integer deptId);-->
    
        select * from t_emp where dept_id = #{deptId};
    

测试

    @Test
    public void  testGetDeptAndEmpByStep(){
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept deptAndEmpByStepOne = mapper.getDeptAndEmpByStepOne(1);
        System.out.println(deptAndEmpByStepOne);
    }