PostgreSQL 区别开

Question

我无法定义别名或选择特定列，只能使用通配符：

工作：

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

也不行：

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

有什么想法吗？

Answer 1

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

SELECT ALL
SELECT DISTINCT
SELECT DISTINCT ON ()

（第一个是默认的，实际中从未使用过。）

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 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distinct on之后的列列表中定义。

请注意，您使用的是LEFT JOIN，所以r.InsertDate 可以是NULLL。