SQL Server：如何使用計數

Question

我正在使用此SQL Server database。我正在爲各國定義以下評論

CASE 
    WHEN density_per_sq_km > 1000 THEN 'Overpopulated' 
    WHEN density_per_sq_km > 500 THEN 'above average' 
    WHEN density_per_sq_km > 250 THEN 'average' 
    WHEN density_per_sq_km > 50 THEN 'below average' 
    ELSE 'Underpopulated' 
END as remarks 

現在我想要統計每條評論中有多少個國家/地區。我怎樣才能做到這一點？我使用下面的查詢，但它通過子句不

SELECT 
    COUNT(country) as no_of_countries, 
    CASE 
     WHEN density_per_sq_km > 1000 THEN 'Overpopulated' 
     WHEN density_per_sq_km > 500 THEN 'above average' 
     WHEN density_per_sq_km > 250 THEN 'average' 
     WHEN density_per_sq_km > 50 THEN 'below average' 
     ELSE 'Underpopulated' 
    END as remarks 
FROM 
    countries_by_population 
GROUP BY 
    remarks; 

Answer 1

在組，你不能使用列別名，使用的情況下表達，而不是

SELECT 
     COUNT(country) AS no_of_countries 
    , CASE 
      WHEN density_per_sq_km > 1000 THEN 'Overpopulated' 
      WHEN density_per_sq_km > 500 THEN 'above average' 
      WHEN density_per_sq_km > 250 THEN 'average' 
      WHEN density_per_sq_km > 50 THEN 'below average' 
      ELSE 'Underpopulated' 
     END AS remarks 
FROM countries_by_population 
GROUP BY 
     CASE 
      WHEN density_per_sq_km > 1000 THEN 'Overpopulated' 
      WHEN density_per_sq_km > 500 THEN 'above average' 
      WHEN density_per_sq_km > 250 THEN 'average' 
      WHEN density_per_sq_km > 50 THEN 'below average' 
      ELSE 'Underpopulated' 
     END 
; 

Answer 2

結束語與計算列上查詢到查詢也可以幫助您使用該列：

SELECT remarks, COUNT(country) as no_of_countries 
FROM (
    SELECT 
    CASE 
     WHEN density_per_sq_km > 1000 THEN 'Overpopulated' 
     WHEN density_per_sq_km > 500 THEN 'above average' 
     WHEN density_per_sq_km > 250 THEN 'average' 
     WHEN density_per_sq_km > 50 THEN 'below average' 
     ELSE 'Underpopulated' 
    END as remarks, 
    country 
    FROM countries_by_population 
) DT 
GROUP BY remarks; 

Answer 3

您可以使用CROSS APPLY爲您的列指定別名。

SELECT T.Remarks, COUNT(*) AS no_of_countries 
FROM countries_by_population AS CBP 
CROSS APPLY (
    SELECT CASE 
      WHEN density_per_sq_km > 1000 THEN 'Overpopulated' 
      WHEN density_per_sq_km > 500 THEN 'Above average' 
      WHEN density_per_sq_km > 250 THEN 'Average' 
      WHEN density_per_sq_km > 50 THEN 'Below average' 
      ELSE 'Underpopulated' 
     END 
    ) AS T(Remarks) 
GROUP BY T.Remarks; 

這將創建一個基於你的密度，可在GROUP BY在以後使用的列Remarks。

Tip: APPLY and Reuse of Column Aliases文章解釋瞭如何使用CROSS APPLY來詳細創建可重用的列別名。