Question

我编写了一个查询，该查询利用多条件CASE语句基于聚合函数执行表数据分析，例如：

select country,
(case when country in ('DE','FR','IT') then 'Europe'
when country in ('CH','JP','SK') then 'Asia'
when country in ('US','CN') then 'North America'
else 'Other' end) as 'Continent', 
count(*) as NR 
from citizenship
group by country,
(case when country in ('DE','FR','IT') then 'Europe'
when country in ('CH','JP','SK') then 'Asia'
when country in ('US','CN') then 'North America'
else 'Other' end)

直到现在，事情都非常直接，但我需要在表格中有0行以满足其中一个案例条件时，将其显示在输出中，例如使用NULL或其他预定义值，例如为：

 | COUNTRY | Continent | NR |
 |   IT    |   Europe  | 5  |
 |         |   Asia    | 0  |

依旧......

是否有可能实现，考虑到真正的查询类似于500行，由于条款中应用了很多条件，并且表有近60M行？此外，在此环境中，我无法创建其他数据库对象。

Answer 1

您可以尝试这样的事情：

CREATE TABLE continents (NAME VARCHAR(30))
INSERT INTO continents VALUES ('Europe');
INSERT INTO continents VALUES ('Asia');
INSERT INTO continents VALUES ('North America');
INSERT INTO continents VALUES ('Other');
 SELECT * FROM continents

 CREATE TABLE citizenship (country VARCHAR(2));
 INSERT INTO citizenship VALUES ('DE')
  INSERT INTO citizenship VALUES ('IT')
  INSERT INTO citizenship VALUES ('IT')


SELECT A.NAME AS CONTINENT, B.COUNTRY, COALESCE(NR,0) AS NR 
FROM continents A 
LEFT JOIN (
select country
    ,case when country in ('DE','FR','IT') then 'Europe'
    when country in ('CH','JP','SK') then 'Asia'
    when country in ('US','CN') then 'North America'
    else 'Other' end as Continent
    ,COALESCE(count(*),0) as NR 
from citizenship
group by country,
(case when country in ('DE','FR','IT') then 'Europe'
when country in ('CH','JP','SK') then 'Asia'
when country in ('US','CN') then 'North America'
else 'Other' end)
) B ON A.name = B.CONTINENT

输出：

    CONTINENT                      COUNTRY NR
------------------------------ ------- -----------
Europe                         DE      1
Europe                         IT      2
Asia                           NULL    0
North America                  NULL    0
Other                          NULL    0

即使遇到某些条件，也有多种条件产生输出的情况

1 个答案: