Mybatis-Plus查询返回Map类型数据

待我称王封你为后i 2021-07-16 15:17 1097阅读 0赞

我们前面的案例都是返回的集合List；

集合List的弊端是会把所有的列属性都封装返回，但是我们有时候，只需要返回几个字段，然后再返回到用户端；

所以mp框架给我们提供了List<Map<String, Object>>返回类型，String是列名，Object是值，只返回select的字段；

举例：

/** * 查询每个部门的平均薪资 * sql： SELECT departmentId,AVG(salary) AS avg_salary FROM t_employee GROUP BY department_id; */
    @Test
    public void selectByQueryWrapper9(){ 
        QueryWrapper<Employee> queryWrapper=new QueryWrapper();
        // QueryWrapper<Employee> queryWrapper2=Wrappers.<Employee>query();
        queryWrapper
                 .select("department_id","AVG(salary) AS avg_salary")
                 .groupBy("department_id");
        List<Employee> employeeList = employeeMapper.selectList(queryWrapper);
        System.out.println(employeeList);
    }

返回值：

\[Employee(id=null, name=null, birthday=null, gender=null, email=null, phoneNumber=null, salary=null, department\_id=1, avg\_salary=3000.0000), Employee(id=null, name=null, birthday=null, gender=null, email=null, phoneNumber=null, salary=null, department\_id=2, avg\_salary=3765.0000), Employee(id=null, name=null, birthday=null, gender=null, email=null, phoneNumber=null, salary=null, department\_id=3, avg\_salary=4000.0000), Employee(id=null, name=null, birthday=null, gender=null, email=null, phoneNumber=null, salary=null, department\_id=4, avg\_salary=5000.0000)\]

没用的字段也返回了；

我们改用Map\`

/** * 查询每个部门的平均薪资（返回Map） * sql： SELECT departmentId,AVG(salary) AS avg_salary FROM t_employee GROUP BY department_id; */
    @Test
    public void selectByQueryWrapper10ReturnMap(){ 
        QueryWrapper<Employee> queryWrapper=new QueryWrapper();
        // QueryWrapper<Employee> queryWrapper2=Wrappers.<Employee>query();
        queryWrapper
                 .select("department_id","AVG(salary) AS avg_salary")
                 .groupBy("department_id");
        List<Map<String, Object>> maps = employeeMapper.selectMaps(queryWrapper);
        System.out.println(maps);
    }

返回结果：

\[\{department\_id=1, avg\_salary=3000.0000\}, \{department\_id=2, avg\_salary=3765.0000\}, \{department\_id=3, avg\_salary=4000.0000\}, \{department\_id=4, avg\_salary=5000.0000\}\]

这样的结果才比较友好；