Trouble with GROUP BY CASE

Question

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

Answer 1

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

Answer 2

Had the same problem but found a simpler way which was to construct a helper field that can then be referenced in the GROUP BY. Or you can do the same conditional trick placing it in the GROUP BY so long as you Concat the fields.

SELECT *,

If( 'whatever field to check' = 2 , CONCAT(Department_Code,Course_Code), Course_Code) AS 'group1'

FROM Classes
GROUP BY group1