SQL Custom Order By | 易学教程

问题

I can't understand why this doesn't work:

select distinct a.QuestionID,a.QuestionName,b.AnswerID,b.AnswerName
    from @TempExportList a
    join tblAnswers b
    on a.QuestionID = b.QuestionID
    where a.PaperID=@PaperID
    order by (case when a.QuestionName='A' then 0
                   when a.QuestionName='B' then 1
                   else a.QuestionID
              end)

I get the following error -

ORDER BY items must appear in the select list if SELECT DISTINCT is specified.

But this works fine:

select distinct a.QuestionID,a.QuestionName,b.AnswerID,b.AnswerName
    from @TempExportList a
    join tblAnswers b
    on a.QuestionID = b.QuestionID
    where a.PaperID=@PaperID
    order by a.QuestionID

回答1:

The error message explains the problem perfectly.

In the first example the ORDER BY item -- CASE WHEN ... END -- doesn't appear in the SELECT list.

In the second example the ORDER BY item -- a.QuestionID -- does appear in the SELECT list.

To fix the first example you'll need to do something like this:

SELECT DISTINCT a.QuestionID, a.QuestionName, b.AnswerID, b.AnswerName,
                CASE WHEN a.QuestionName = 'A' THEN 0
                     WHEN a.QuestionName = 'B' THEN 1
                     ELSE a.QuestionID
                END
FROM @TempExportList AS a
    JOIN tblAnswers AS b
        ON a.QuestionID = b.QuestionID
WHERE a.PaperID = @PaperID
ORDER BY CASE WHEN a.QuestionName = 'A' THEN 0
              WHEN a.QuestionName = 'B' THEN 1
              ELSE a.QuestionID
         END

回答2:

You can get around this with a CTE

;WITH T AS
(
SELECT DISTINCT a.QuestionID,a.QuestionName,b.AnswerID,b.AnswerName
    FROM @TempExportList a
    JOIN tblAnswers b
    ON a.QuestionID = b.QuestionID
    WHERE a.PaperID=@PaperID
)
SELECT   *
FROM     T
ORDER BY
         CASE
                  WHEN QuestionName='A'
                  THEN 0
                  WHEN QuestionName='B'
                  THEN 1
                  ELSE QuestionID
         END

回答3:

I would have thought the message self explanitory.

You have selected a distinct on a.QuestionID,a.QuestionName,b.AnswerID and b.AnswerName. Therefore, there could be rows of data with the same respective values for each of these fields, but a different one for your case statement.

Consider this

a.QuestionID  a.QuestionName  b.AnswerID   b.AnswerName  [case statement]

1             'One'           2            'Two'         0
1             'One'           2            'Two'         1

How does the query know which value in the last column to use in the order? Is it 0? It is 1? Quite simply, it can't determine, so it can't use it, hence the error.

The second example is fine, because a.QuestionID does appear in the SELECT list, and the query can happily apply the ordering.

来源：https://stackoverflow.com/questions/4335374/sql-custom-order-by

标签

sql-server-2008

sql-order-by