Question

这不是直接编程相关的，所以也许它不是主题。

对于我的光伏发电，我已经有了这个postgres表几年了。我做了像

这样的查询

WITH all_sources AS
(
    SELECT DISTINCT origin from solardata
    ORDER by origin
)
SELECT all_sources.origin, count(*) 
FROM solardata, all_sources
WHERE all_sources.origin = solardata.origin
GROUP BY all_sources.origin
ORDER BY all_sources.origin

我得到了这个结果

                origin                | count  
--------------------------------------+--------
 kostal-log-parser: 10.1.log          |   5905
 kostal-log-parser: 5.5.log           |   6059
 kostal-log-parser: LogDaten_10_1.dat |   3474
 kostal-log-parser: LogDaten_5_5.dat  |   3369
 kostal-web-parser                    | 480869
 time-gridder                         |  18432
(6 rows)

但另一方面，如果我跑

select date_time, origin 
from solardata 
order by date_time limit 2;

我得到了

      date_time      | origin 
---------------------+--------
 2009-08-17 18:34:00 | 
 2009-08-17 18:34:00 |

怎么可能？

我的postgres版本是9.4.5

这是解决方案。原因是内连接，但我需要左连接。

WITH all_sources AS
(
    SELECT DISTINCT origin from solardata
    ORDER by origin
)
SELECT all_sources.origin, count(*) 
FROM solardata
LEFT JOIN all_sources on (all_sources.origin = solardata.origin)
GROUP BY all_sources.origin
ORDER BY all_sources.origin

Answer 1

公用表表达式是多余的：

SELECT origin, count(*) 
from solardata 
GROUP BY origin 
ORDER BY origin;

您的查询存在问题：

WHERE   all_sources.origin = solardata.origin

NULL = NULL => NULL(UKNOWN)因此会跳过该行。

的 SqlFiddleDemo_GROUP_BY 的 SqlFiddleDemo_Original

输出：

╔═════════╦═══════╗ ╔═════════╦═══════╗ ║ origin ║ count ║ ║ origin ║ count ║ ╠═════════╬═══════╣ ╠═════════╬═══════╣ ║ (null) ║ 2 ║ ║ ║ 2 ║ ║ ║ 2 ║ vs ║ a ║ 1 ║ ║ a ║ 1 ║ ║ b ║ 1 ║ ║ b ║ 1 ║ ╚═════════╩═══════╝ ╚═════════╩═══════╝

请注意，您的版本中不存在(null)组。

您不应该使用它（冗余cte），但如果您将=更改为IS NOT DISTINCT FROM，您的查询也会有效：

WITH all_sources AS ( SELECT DISTINCT origin from solardata ) SELECT all_sources.origin, count(*) from solardata JOIN all_sources ON all_sources.origin IS NOT DISTINCT FROM solardata.origin GROUP BY all_sources.origin ORDER BY all_sources.origin

Varchar字段大小为0，但未在DISTINCT SQL查询中列出

1 个答案: