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Question

我有两列专栏：名字和等级。看起来像这样：

NAME  | GRADE
Adam  | 1
Adam  | 2
Adam  | 2
Adam  | 3
Frank | 2
Frank | 1

现在我想创建一个如下所示的视图：

NAME  | GRADE 1 | GRADE 2 | GRADE 3
Adam  |    1    |    2    |     1
Frank |    1    |    1    |     0

我写了这个：

SELECT     Name, 
    (SELECT COUNT(Grade) 
    FROM dbo.Rodzaj 
    WHERE Grade = '1') as Grade_1,  
    (SELECT COUNT(Grade) 
    FROM dbo.Rodzaj 
    WHERE Grade = '2) as Grade_2,  
    (SELECT COUNT(Grade) 
    FROM dbo.Rodzaj 
    WHERE Grade = '3') as Grade_3
FROM         dbo.Rodzaj
GROUP BY Name

但它不起作用...... 我将不胜感激任何帮助

Answer 1

您正在寻找的是一个“数据透视表”，它由一系列CASE语句完成，每个条件应用1或0，然后SUM()向上和0检索计数。

SELECT
  NAME,
  SUM(CASE WHEN GRADE = 1 THEN 1 ELSE 0 END) AS GRADE1,
  SUM(CASE WHEN GRADE = 2 THEN 1 ELSE 0 END) AS GRADE2,
  SUM(CASE WHEN GRADE = 3 THEN 1 ELSE 0 END) AS GRADE3
FROM Rodzaj
GROUP BY NAME

请注意，如果您需要具有动态数量的列，则必须使用脚本语言和循环来构造查询。 （或存储过程中的循环）