MybatisPlus Lambda表达式 聚合查询 分组查询 COUNT SUM AVG MIN MAX GroupBy

一、序言

众所周知，mybatisPlus在处理单表DAO操作时非常的方便。在处理多表连接连接查询也有优雅的解决方案。今天分享MybatisPlus基于Lambda表达式优雅实现聚合分组查询。

由于视频的交互性更强，保留更多的细节，看视频的朋友，传送门在这里。

下面的内容是博客文字版。

二、代码实现

1、用户实体类

@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、用户聚合类

本类属于核心代码

@TableName(value = "tb_user")public class UserAggr {        private Long deptId;        @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、用户聚合类VO

本类属于核心代码

public class UserAggrVo extends UserAggr {    private String deptName;        public UserAggrVo(UserAggr user) {        super(user);    }}

4、服务层调用

服务层完成调用

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);    // 给deptName完成属性赋值    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、总结

本方案很好的实现了MybatisPlus基于Lambda表达式实现聚合操作，代码在形式上与单表保持统一，代码优雅。各位看官朋友点赞支持一波。

来源地址：https://blog.csdn.net/m0_63836794/article/details/127735973