@TableName(value = "tb\_user")

public class User {

    private static final long serialVersionUID = 1L;

    private Integer age;

    private Long deptId;

    @TableId(type = IdType.AUTO)

    private Long userId;

    private String userName;

    

    public User(User user) {

        if (Objects.nonNull(user)) {

            this.age = user.age;

            this.deptId = user.deptId;

            this.userId = user.userId;

            this.userName = user.userName;

        }

    }

}

@TableName(value = "tb\_user")

public class UserAggr {

    

    private Long deptId;

    /\*\*

     \* Новый столбец данных Используется для хранения поля счетчика.

     \* Аннотации очень важны

     \*/

    @TableField(value = "count(\*)", insertStrategy = FieldStrategy.NEVER, updateStrategy = FieldStrategy.NEVER)

    private Long count;

    

    public UserAggr(UserAggr user) {

        if (Objects.nonNull(user)) {

            this.deptId = user.deptId;

            this.count = user.count;

        }

    }

}

public class UserAggrVo extends UserAggr {

    private String deptName;

    /\*\*

     \* Очень важный конструктор

     \*

     \* @param user

     \*/

    public UserAggrVo(UserAggr user) {

        super(user);

    }

}

public List<UserAggrVo> selectList3() {

    LambdaQueryWrapper<UserAggr> wrapper = Wrappers.lambdaQuery(UserAggr.class)

            .select(UserAggr::getDeptId, UserAggr::getCount)

            .groupBy(UserAggr::getDeptId);

    List<UserAggr> userAggrList = userAggrMapper.selectList(wrapper);

    List<UserAggrVo> userAggrVoList = EntityUtils.toList(userAggrList, UserAggrVo::new);

    // Завершить присвоение атрибута deptName

    Set<Long> deptIds = EntityUtils.toSet(userAggrVoList, UserAggrVo::getDeptId);

    if (deptIds.size() > 0) {

        List<Dept> deptList = deptMapper.selectList(Wrappers.lambdaQuery(Dept.class)

                                                    .in(Dept::getDeptId, deptIds));

        Map<Long, String> map = EntityUtils.toMap(deptList, Dept::getDeptId, Dept::getDeptName);

        for (UserAggrVo userAggrVo : userAggrVoList) {

            userAggrVo.setDeptName(map.get(userAggrVo.getDeptId()));

        }

    }

    return userAggrVoList;

}

{

  "code": 200,

  "msg": «Операция прошла успешно»,

  "data": [

    {

      "deptId": "10",

      "count": "1",

      "deptName": "Java"

    },

    {

      "deptId": "11",

      "count": "2",

      "deptName": "Mysql"

    },

    {

      "deptId": "12",

      "count": "3",

      "deptName": "Tomcat"

    }

  ]

}