具有多個條件的情況下即使在某些條件不符合時也會生成輸出

Question

我已經編寫了一個查詢，該查詢使用多條件CASE語句根據聚合函數執行表數據分析，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) 

事情是非常直接的到現在爲止，但我需要的是，當有表中的0行滿足的情況下條件之一，把它顯示在輸出，就像一個空或其他一些預定義值，如：

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

等等on ...

是否有可能實現，考慮到真正的查詢類似於500行，由於when子句中應用了許多條件，並且該表有近60M行？同樣在這個環境中，我無法創建額外的數據庫對象。

Answer 1

你可以嘗試這樣的事情：

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 

輸出：

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