【1】需求

如何統計分類數量?

有些分類要合併統計, 例如: 統計亞洲和歐洲有多少國家,剩下的國家統計到"火星"

　　要求結果是這樣的：

【2】在sql group by 中使用 case when

【2.1】常規正向寫法

;WITH t1 AS (
SELECT 'Asia' Area,'china' country UNION all
SELECT 'Asia' Area,'russia' country UNION all
SELECT 'europe' Area,'england' country UNION all
SELECT 'europe
 
' Area,'germany' country UNION all
SELECT 'Africa' area,'india' country
)
SELECT CASE WHEN Area='Asia' THEN Area
            WHEN Area='europe' THEN Area
            ELSE 'spark' END AS AreaName,COUNT(1) country_num
            
FROM t1 
GROUP BY CASE WHEN Area='Asia' THEN Area
            
 
WHEN Area='europe' THEN Area
            ELSE 'spark' end

這個寫法固然可以，但如果不只是亞洲、歐洲，而是有十幾個甚至幾十個相關項，難道要一個一個 when Area=value1 when Area=value2......Area=valueN 嗎？

顯示是不合適且冗餘複雜的，這個時候就用到我們下面的【2.2】寫法

【2.2】反向批量寫法

;WITH t1 AS (
SELECT 'Asia' Area,'china' country UNION all
SELECT 'Asia' Area,'russia
 
' country UNION all
SELECT 'europe' Area,'england' country UNION all
SELECT 'europe' Area,'germany' country UNION all
SELECT 'Africa' area,'india' country
)
SELECT CASE WHEN Area IN ('Asia','europe') THEN Area
            ELSE 'spark' END AS AreaName,COUNT(1) country_num
            
FROM t1 
GROUP BY CASE WHEN Area IN ('Asia','europe') THEN Area
            ELSE 'spark' end

或者也可以反過來用 not in

;WITH t1 AS (
SELECT 'Asia' Area,'china' country UNION all
SELECT 'Asia' Area,'russia' country UNION all
SELECT 'europe' Area,'england' country UNION all
SELECT 'europe' Area,'germany' country UNION all
SELECT 'Africa' area,'india' country
)
SELECT CASE WHEN Area NOT IN ('Asia','europe') THEN 'spark'
            ELSE Area END AS AreaName,COUNT(1) country_num
            
FROM t1 
GROUP BY CASE WHEN Area NOT IN ('Asia','europe') THEN 'spark'
            ELSE Area end