MySQL-计算基于另一列的一列中的不同值

Question

我有包含所有数据的表资源，我想要得到上一张表中给出的结果。

Dept Grade
A    E3  
B    M2  
D    E3  
C    M1  
A    E3  
D    E4  
A    E3    

我使用下面的代码，但我无法获得实际的解决方案

SELECT DISTINCT Department,COUNT(DISTINCT GRADE) as Grades FROM reslookup GROUP BY Department;

关于查询的执行结果，

Dept Grade
A    3
B    1
C    1
D    2

但是，我想得到如下所示的结果

Dept E3  E4  M1  M2
A    5   0   2   1
B    4   2   3   0
C    8   9   2   1
D    5   0   2   6

Answer 1

您要执行的操作是将计数行旋转到不同的列中，可以按以下步骤完成：

SELECT Dept,
  SUM(CASE WHEN Grade = 'E3' THEN 1 ELSE 0 END) AS E3, 
  SUM(CASE WHEN Grade = 'E4' THEN 1 ELSE 0 END) AS E4,
  SUM(CASE WHEN Grade = 'M1' THEN 1 ELSE 0 END) AS M1,
  SUM(CASE WHEN Grade = 'M2' THEN 1 ELSE 0 END) AS M2
FROM reslookup 
GROUP BY Dept;

演示

Answer 2

您还可以使用以下代码提供相同的解决方案

SELECT Dept,
      SUM(Grade = 'E3') AS E3, 
      SUM(Grade = 'E4') AS E4,
      SUM(Grade = 'M1') AS M1,
      SUM(Grade = 'M2') AS M2
    FROM reslookup 
    GROUP BY Dept;