我的表有两列：名称和等级.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

但这行不通…

我将不胜感激任何帮助

解决方法:

您要查找的内容称为“数据透视表”,它由一系列CASE语句完成,这些语句对每个条件应用1或0,然后对1和0进行SUM()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的列数,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)

标签：sql,mysql

来源： https://codeday.me/bug/20191201/2078623.html