从postgres的联合结果中选择

Question

我正在尝试查询以下内容：

select currency, sum(volume)
from
    (
        select "baseCurrency" as currency,
               COALESCE("baseVolume", 0) as volume
        from "Pairs"
    )  as a
    union all
    (
        select "quoteCurrency" as currency,
               COALESCE("quoteVolume", 0) as volume
        from "Pairs"
    ) as b
group by currency

但是无论我如何更改查询，都会不断出错。 例如：

ERROR:  syntax error at or near "as"
LINE 11:  ) as b

此查询有什么问题？

Answer 1

不要在union all为子查询使用别名：

select currency, sum(volume)
from ((select "baseCurrency" as currency, COALESCE("baseVolume", 0) as volume
       from "Pairs"
       ) union all
       (select "quoteCurrency" as currency, COALESCE("quoteVolume", 0) as volume
        from "Pairs"
       ) 
      ) c
group by currency;

在最新版本的Postgres中，可以改用横向join ：

select v.currency, sum(v.volume)
from "Pairs" p cross join lateral
     (values ("baseCurrency", "baseVolume"),
             ("quoteCurrency", "quoteVolume")
     ) v(currency, volume)
group by v.currency;

coalesce()可能是不必要的。 如果得到的结果为NULL ，则可以在sum()之后进行处理。 。 。 coalesce(sum(volume), 0) 。

Answer 2

它符合ISO标准。 PostgreSQL被设计为在使用子查询时具有别名。 请转到此处以获取更多信息 。

对于您而言，以下更改将有所帮助：

select b.currency, b.sum(volume)
from
  (  (
        select "baseCurrency" as currency, COALESCE("baseVolume", 0) as volume
        from "Pairs"
    )  
    union all
    (
        select "quoteCurrency" as currency, COALESCE("quoteVolume", 0) as volume
        from "Pairs"
    ) )as b
group by currency