mybatisplus group by 归组

 QueryWrapper<MetaItem> wrapper = new QueryWrapper<MetaItem>()
                .select("set_id,count(0) as field_count")
                .in("set_id",setids)
                .groupBy("set_id");

        List<Map<String,Object>> list = itemService.listMaps(wrapper);

注意要用 listMaps ，返回的是 Map<String,Object>

示例：

public Result<?> queryPageList(MetaSet metaSet,
								   @RequestParam(name="pageNo", defaultValue="1") Integer pageNo,
								   @RequestParam(name="pageSize", defaultValue="10") Integer pageSize,
								   HttpServletRequest req) {
		QueryWrapper<MetaSet> queryWrapper = QueryGenerator.initQueryWrapper(metaSet, req.getParameterMap());
		Page<MetaSet> page = new Page<MetaSet>(pageNo, pageSize);
		IPage<MetaSet> pageList = metaSetService.page(page, queryWrapper);

        List<MetaSet> records = pageList.getRecords();
        List<String> setids = records.stream().map(f -> f.getId()).collect(Collectors.toList());

        QueryWrapper<MetaItem> wrapper = new QueryWrapper<MetaItem>()
                .select("set_id,count(0) as field_count")
                .in("set_id",setids)
                .groupBy("set_id");

        List<Map<String,Object>> list = itemService.listMaps(wrapper);

        records.forEach(f->{
            Map<String, Object> set_id = list.stream().filter(l -> l.get("set_id").toString().equals(f.getId())).findFirst().orElse(null);
            if(set_id!=null){
                f.setFieldCount(Integer.valueOf(set_id.get("field_count").toString()));
            }
        });
        pageList.setRecords(records);

        return Result.OK(pageList);
	}