1

我无法定义别名或选择特定列，只能使用通配符：

在职的：

SELECT DISTINCT ON (r.InsertDate) *  
FROM public.server AS s
LEFT JOIN public.Report AS r ON s.Id = r.ServerId
ORDER BY r.InsertDate DESC;

不工作：

SELECT DISTINCT ON (r.InsertDate) LastReport, s.Id, s.Servername 
FROM public.server AS s
LEFT JOIN public.Report AS r ON s.Id = r.ServerId
ORDER BY r.InsertDate DESC;

错误

ERROR:  column "lastreport" does not exist
LINE 1:  SELECT DISTINCT ON (r.InsertDate) LastReport, s.Id, s.Serve...
                                           ^
SQL state: 42703
Character: 36

也不工作：

SELECT DISTINCT ON (r.InsertDate) , s.Id, s.Servername 
FROM public.server AS s
LEFT JOIN public.Report AS r ON s.Id = r.ServerId
ORDER BY r.InsertDate DESC;

ERROR:  column "lastreport" does not exist
LINE 1:  SELECT DISTINCT ON (r.InsertDate) LastReport, s.Id, s.Serve...
                                           ^
SQL state: 42703
Character: 36

ERROR:  syntax error at or near ","
LINE 1:  SELECT DISTINCT ON (r.InsertDate) , s.Id, s.Servername 
                                           ^
SQL state: 42601
Character: 36

任何想法？

4

1 回答 1

0

你似乎对DISTINCT ON. 它不会“返回一列”。它是一个句法结构。这些是 的变体SELECT：

SELECT ALL
SELECT DISTINCT
SELECT DISTINCT ON ()

（第一个是默认值，在实践中从未使用过。）

列列表遵循这些结构。所以，显然你想要：

SELECT DISTINCT ON (r.InsertDate) r.InsertDate as LastReport, s.Id, s.Servername 
FROM public.server s LEFT JOIN
     public.Report r
     ON s.Id = r.ServerId
ORDER BY r.InsertDate DESC;

事实证明，DISTINCT ON它实际上是在SELECT子句的其余部分之后解析的，因此您可以使用列别名：

SELECT DISTINCT ON (LastReport) r.InsertDate as LastReport, s.Id, s.Servername 
FROM public.server s LEFT JOIN
     public.Report r
     ON s.Id = r.ServerId
ORDER BY LastReport DESC;

但是，别名必须在 . 之后的列列表中定义distinct on。

请注意，您使用的是LEFT JOIN，所以r.InsertDate 可能是NULLL。

于 2020-06-11T11:24:11.757 回答