麻烦GROUP BY CASE [英] Trouble with GROUP BY CASE

问题描述

由于（Department_Code，Course_Code）行，以下查询给出错误＃1241 - 操作数应包含1列。当我用（Course_Code）代替它时，它可以工作。然而，这不是我想要的

  SELECT * FROM类
 GROUP BY CASE 
 WHEN（1） THEN 
 Department_Code 
 ELSE CASE WHEN（2）THEN 
（Department_Code，Course_Code）
 ELSE Class_ID 
 END 
 END 
  
 
当条件（2）满足时，我可以通过Department_Code，Course_Code进行分组吗？
 case 表达式只能返回一个值，所以你需要两个 case  $ c>表达式。此外，对每个表达式都使用一个 case 表达式，而不是将两个表达式嵌套在一起： 
 
 
  SELECT * FROM类
 GROUP BY 
 CASE 
当（1）THEN 
 Department_Code 
当（2）THEN 
 Department_Code 
 ELSE 
 Class_ID 
 END，
 CASE 
当（2）THEN 
 Course_Code 
 ELSE 
 1 
 END 
  
 
The following query gives the error "#1241 - Operand should contain 1 column(s)" because of the (Department_Code, Course_Code) line.  When I replace that with just (Course_Code) it works.  However, that's not what I want
SELECT * FROM Classes
GROUP BY CASE 
WHEN (1) THEN
 Department_Code
 ELSE CASE WHEN (2) THEN 
  (Department_Code, Course_Code)
 ELSE Class_ID
 END
END
How can I group by Department_Code, Course_Code when condition (2) is satisfied?
解决方案
A case expression can only return a single value, so you need two case expressions. Also, use a single case expression for each instead of nesting two inside each other:
SELECT * FROM Classes
GROUP BY
  CASE 
  WHEN (1) THEN
    Department_Code
  WHEN (2) THEN 
    Department_Code
  ELSE
    Class_ID
  END,
  CASE 
  WHEN (2) THEN 
    Course_Code
  ELSE
    1
  END


                        
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