表“g”缺少 FROM 子句条目

Question

select distinct "BillingCountry", count(g."Name") as genre_occurance from 
        (select "BillingCountry", g."Name" as sub2 from "Invoice" 
        join "InvoiceLine" as invl on invl."InvoiceId" = "Invoice"."InvoiceId" 
        join "Track" as t on t."TrackId" = invl."TrackId" 
        join "Genre" as g on g."GenreId" = t."GenreId" 
        where "BillingCountry" in ('USA', 'Canada', 'Brazil', 'France', 'Germany')
        group by "Invoice"."BillingCountry", g."Name") as sub1

我知道有很多针对此类错误的解决方案，但我尝试遵循这些解决方案，但我无法让查询正常工作，我只是不了解解决方案，我需要这方面的帮助。

ERROR:  missing FROM-clause entry for table "g"
LINE 1: select distinct "BillingCountry", count(g."Name") as genre_o...

Answer 1

g.Name 列的g.Name是子查询，所以在外部查询中是看不到的。
此外，在子查询中g.Name别名为sub2 ，因此您应该在外部查询中使用此别名。

但是，在子查询中，您group by Invoice.BillingCountry, g.Name而不进行任何聚合。 为什么？

子查询的结果是BillingCountry和g.Name的所有不同组合，您也可以通过使用DISTINCT而不是GROUP BY获得：

select distinct BillingCountry, g.Name as sub2 
from Invoice 
join InvoiceLine as invl on invl.InvoiceId = Invoice.InvoiceId 
join Track as t on t.TrackId = invl.TrackId 
join Genre as g on g.GenreId = t.GenreId 
where BillingCountry in ('USA', 'Canada', 'Brazil', 'France', 'Germany')

最后在外部查询中，您使用没有GROUP BY子句的聚合。

也许您想在外部查询中进行聚合，如下所示：

select BillingCountry, count(Name) as genre_occurance 
from (
  select i.BillingCountry, g.Name 
  from Invoice i 
  join InvoiceLine as invl on invl.InvoiceId = i.InvoiceId 
  join Track as t on t.TrackId = invl.TrackId 
  join Genre as g on g.GenreId = t.GenreId 
  where i.BillingCountry in ('USA', 'Canada', 'Brazil', 'France', 'Germany')
) as t
group by BillingCountry

可以在没有子查询的情况下编写：

select i.BillingCountry, count(g.Name) as genre_occurance 
from Invoice i 
join InvoiceLine as invl on invl.InvoiceId = i.InvoiceId 
join Track as t on t.TrackId = invl.TrackId 
join Genre as g on g.GenreId = t.GenreId 
where i.BillingCountry in ('USA', 'Canada', 'Brazil', 'France', 'Germany')
group by i.BillingCountry

Answer 2

您在内部查询中将 g.name 别名为 sub2，因此外部 select 不知道“名称”列，外部查询也不知道内部查询中的内容，而是您在 select 中返回的内容。 您可以像这样修复您的查询：

select distinct "BillingCountry", count(sub2) as genre_occurance 
from  (
            select "BillingCountry", g."Name" as sub2 
            from "Invoice" 
            join "InvoiceLine" as invl on invl."InvoiceId" = "Invoice"."InvoiceId" 
            join "Track" as t on t."TrackId" = invl."TrackId" 
            join "Genre" as g on g."GenreId" = t."GenreId" 
            where "BillingCountry" in ('USA', 'Canada', 'Brazil', 'France', 'Germany')
            group by "Invoice"."BillingCountry", g."Name"
        ) as sub1