SQL Server：如何使用计数

Question

我正在使用此SQL Server database。我正在为各国定义以下评论

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 

现在我想要统计每条评论中有多少个国家/地区。我怎样才能做到这一点？我使用下面的查询，但它通过子句不

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

在组，你不能使用列别名，使用的情况下表达，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

结束语与计算列上查询到查询也可以帮助您使用该列：

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为您的列指定别名。

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; 

这将创建一个基于你的密度，可在GROUP BY在以后使用的列Remarks。

Tip: APPLY and Reuse of Column Aliases文章解释了如何使用CROSS APPLY来详细创建可重用的列别名。