Sql 为postgres DB中不存在的行数据显示空值

我有下面的数据集，看起来像这样

t               mean        max     min     std     data_id
4/14/2010 0:00  12.6941 12.6941 12.6941 12.6941          1
4/14/2010 0:00  12.3851 12.3851 12.3851 12.3851          2
4/14/2010 0:20  12.389  12.389  12.389  12.389           1
4/14/2010 0:20  12.1836 12.1836 12.1836 12.1836          2
4/14/2010 0:20  11.3887 11.3887 11.3887 11.3887          6

这里唯一的数据id是（1,2,6），但我有另一个数据id集（1,2,4,5,6），我想用它来获取数据

现在，对于时间t内不存在的所有数据，我想将null（mean，max.std，min）值添加到它们中，因此在这种情况下，我需要以下结果集：-

'2010-04-14 00:00:00','12.6941,12.6941,12.6941,12.6941,12.3851,12.3851,12.3851,12.3851,,,,,,,,,,,,,'
'2010-04-14 00:20:00','12.389,12.389,12.389,12.389,12.1836,12.1836,12.1836,12.1836,,,,,,,,,11.3887,11.3887,11.3887,11.3887'

我使用了以下查询：-

with dataset as (
      select *
      from (values ('2010-04-14T00:00'::TIMESTAMP, 12.6941, 12.6941, 12.6941, 12.6941, 1),
                   ('2010-04-14T00:00'::TIMESTAMP, 12.3851, 12.3851, 12.3851, 12.3851, 2),
                   ('2010-04-14T00:20'::TIMESTAMP, 12.389, 12.389, 12.389, 12.389, 1),
                   ('2010-04-14T00:20'::TIMESTAMP, 12.1836, 12.1836, 12.1836, 12.1836, 2),
                   ('2010-04-14T00:20'::TIMESTAMP, 11.3887, 11.3887, 11.3887, 11.3887, 6)
           ) AS data(t, mean, max, min, std, data_id)
      ),
     dataset_full as (
       select t.t, d.data_id,
              ds.mean, ds.max, ds.min, ds.std
       from (select distinct t from dataset) t cross join
            (select distinct data_id from dataset) d left join
            dataset ds
            on ds.t = t.t and ds.data_id = d.data_id
     )
select t,string_agg(concat(mean, ',', max, ',', min, ',', std), ',' order by data_id)
from dataset_full
group by t
order by t;

我得到以下结果：-

'2010-04-14 00:00:00','12.6941,12.6941,12.6941,12.6941,12.3851,12.3851,12.3851,12.3851,,,,'
    '2010-04-14 00:20:00','12.389,12.389,12.389,12.389,12.1836,12.1836,12.1836,12.1836,11.3887,11.3887,11.3887,11.3887'

---

我没有在=4/14/2010 0:00时获取data_id（4,5,6）的空值，也没有在t=4/14/2010 0:20时获取data_id（4,5）的空值。

只需在定义

数据集满时包含所需的id即可：

dataset_full as (
       select t.t, d.data_id,
              ds.mean, ds.max, ds.min, ds.std
       from (select distinct t from dataset) t cross join
            (values (1), (2), (4), (5), (6)) d(data_id) left join
            dataset ds
            on ds.t = t.t and ds.data_id = d.data_id
     )

交叉联接的目的是获取结果集中所需的所有记录。因此，包括您想要的ID和时间戳。然后，
左连接
会引入适当的数据（如果有）