SQL: conditional statement in GROUP BY clause

问题

I would like the below case statement to change the resultset when the variable @SubjectName is changed between 'English', 'Mathematics' and any other subject eg science.

When the variable is 'English' the column in students called Ks2en should be included in the results set, but when 'Mathematics' is selected the column changes to Ks2ma and when it is anything else Ks2av should be selected.

Where there are any blank values ('') in the results set they are replaced with 'No KS2'.

Here is what I have so far which doesn't work:

@SubjectName varchar(100) ='English'

SELECT CASE WHEN (student.Ks2en = '' and @SubjectName = 'English') OR 
    (student.Ks2ma = '' and @SubjectName = 'Mathematics') OR 
    (student.Ks2ma = '' AND @SubjectName <> 'Mathematics' AND
    @SubjectName <> 'English') 
            THEN 'No KS2' ELSE student.ks2en 
                OR student.Ks2ma OR student.Ks2av END AS 'KS2'
  FROM student JOIN subject 
    ON subject.upn=student.upn 
  WHERE
    [subject.Name] = @SubjectName

Here is an example of the student table:

Ks2en    Ks2ma    Ks2av
4b       3c       3a
4a       4a       4a
3c                3c
         2c       2c
4c       3a       4c
4b       4b       4b

5a       3a       4a

So when @SubjectName = 'English' the result set looks like:

Ks2    
4b       
4a       
3c
No KS2     
4c       
4b
No KS2        
5a

And when @SubjectName = 'Mathematics' the result set looks like:

KS2
3c
4a
No KS2
2c
3a
4b
No KS2 
3a

And when @SubjectName = 'Science' the result set looks like:

KS2
3a
4a
3c
2c
4c
4b
No KS2
4a

回答1:

This code will dynamically return one of three columns depending upon subject.

Is that what you want? You can tack you where clause at the end of the statement if needed.

John

-- dynamically create column
SELECT 
   CASE 
      WHEN lower(coalesce(SubjectName, 'unknown')) = 'english' THEN
        [Ks2en]
      WHEN lower(coalesce(SubjectName, 'unknown')) = 'mathematics' THEN
        [Ks2ma]
      ELSE
        [Ks2av]
   END as KS2
FROM [student] INNER JOIN [subject] ON subject.upn=student.upn 
WHERE [subject].Subjectname LIKE @SubjectName;

I am assuming your table schema is like the following. I am using tuple notation.

student (upn, student name, ...)
subject (upn, subject name, ks2en, ks2ma, ks2av, ...)

If it is not, the code will not work.

来源：https://stackoverflow.com/questions/19098398/sql-conditional-statement-in-group-by-clause

标签

sql-server

variables

operators

case

conditional-statements