MybatisPlus Lambda表達式 聚合查詢 分組查詢 COUNT SUM AVG MIN MAX GroupBy

一、序言

衆所周知，MybatisPlus在處理單表DAO操作時非常的友善。在處理多表連接配接連接配接查詢也有優雅的解決方案。今天分享MybatisPlus基于Lambda表達式優雅實作聚合分組查詢。

由于視訊的互動性更強，保留更多的細節，看視訊的朋友，傳送門在這裡。

下面的内容是部落格文字版。

二、代碼實作

1、使用者實體類

@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;
        }
    }
}
           

2、使用者聚合類

本類屬于核心代碼

@TableName(value = "tb_user")
public class UserAggr {
    
    private Long deptId;
    /**
     * 新增的一列資料 用于存儲count字段
     * 注解非常關鍵
     */
    @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;
        }
    }
}
           

3、使用者聚合類VO

public class UserAggrVo extends UserAggr {
    private String deptName;
    /**
     * 很重要構造器
     *
     * @param user
     */
    public UserAggrVo(UserAggr user) {
        super(user);
    }
}
           

4、服務層調用

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);
    // 給deptName完成屬性指派
    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;
}
           

5、效果展示

{
  "code": 200,
  "msg": "操作成功",
  "data": [
    {
      "deptId": "10",
      "count": "1",
      "deptName": "Java"
    },
    {
      "deptId": "11",
      "count": "2",
      "deptName": "Mysql"
    },
    {
      "deptId": "12",
      "count": "3",
      "deptName": "Tomcat"
    }
  ]
}
           

三、總結