如何最小化子查询的使用

如何解决如何最小化子查询的使用

我有下表,我要计算销售额。我用一些子查询来做到这一点。我可以不使用子查询并且不使用表变量来做到这一点吗?

TableA,4 columns - popuplation
pkeyP | population | city | country 
some data
100,1234,london,UK
101,2345,oxford,UK
102,3345,bristol,UK
103,1256,new york,USA
104,5424,LA,USA
105,100,beijing,China

Table B,4 columns - sales_amount
pkeyA | amount | city | country
some data
200,105,UK
201,210,UK
203,23,USA
204,54,USA

the city in first table may not exist in second table
webAuth.authorize(options?: AuthorizeOptions)
subquery 1 will return 
UK,6924
USA,6680
China,100

subquery 2 will return
UK,315
USA,77

final result
UK,6924,6680,77,China,null  

这是一个例子。我的真实查询中有许多这样的小表,我需要先进行分组,然后再加入以进行最终计算。

我不想使用子查询,因为我的真实表具有更多的列并且真实查询非常长。它非常不可读,很难维护。

解决方法

您可以执行以下操作以将单个查询结果合并为一个

select aa.country,aa.popu,bb.amou,cc.smou from 
    (select sum(population) as popu,country from population group by country) aa
inner join 
    (select sum(amount) as amou,country from sales_amount group by countr) bb
on aa.country = bb.country inner join
    (select sum(sales) as smou,country from sales_amount group by countr) cc
on bb.country = cc.country
-- inner join ..  so on as you wrote....
,

编辑,好吧,那为什么不创建聚集已经完成的视图,特别是如果这是您经常使用的信息。如果您主要关注可读性,那么这会有所帮助。例如:

CREATE VIEW vw_aggregated_population AS
SELECT sum(population) AS popu,country 
FROM population GROUP BY country

CREATE VIEW vw_aggregated_sales_amount AS
SELECT sum(amount) AS amou,country 
FROM sales_amount GROUP BY country

之后

SELECT
vw_aggregated_population.country,popu,amou,FROM vw_aggregated_population
LEFT JOIN vw_aggregated_sales_amount
ON vw_aggregated_population.country=vw_aggregated_sales_amount.country
GROUP BY vw_aggregated_population.country
,

您可以将full joingroup byunion all一起使用,以获取所有表中的 all 行。这是使用第二种方法的示例:

with p as (
      select sum(population) as population,country
      from population
      group by country
     ),sa as (
      select sum(amount) as amount,country
      from sales_amount
      group by country
     ) 
select country,max(population),max(amount)
from ((select country,population,null as amount
       from p
      ) union all
      (select country,null,amount
       from sa
      )
     ) ps
group by country;

应该清楚如何将其扩展为更多表。

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