来自另一个CASE的MySQL CASE导致SQL查询

Question

I have a fairly large query one part of which is:

... 
ROUND(AVG( 
  CASE RA12.answer
    WHEN 'L' THEN 3
    WHEN 'P' THEN 2
    WHEN 'E' THEN 1
    ELSE 0
  END
),0) as 'avg_presentation_format_raw'
...

...and it all works fine including the column above. What I now need to do is another CASE (or something) on the results of the column above. However since it's calculated at run time I cannot reference it. I am guessing I need to use temp variables or something and I found a few things that come close, but nothing that quite does what I am looking for.

What I need is something like what is below based on and in addition to the calculated column above:

...  
  CASE avg_presentation_format_raw <--THIS WON'T WORK BECAUSE IT'S THE CALCULATED COLUMN
    WHEN 3 THEN 'Lecture'
    WHEN 2 THEN 'Poster'
    WHEN 1 THEN 'Either'
    ELSE 'Not Specified'
  END as 'presentation_format'
...

Any thoughts or ideas appreciated!

TIA

Answer 1

Use subquery like:

SELECT 
    CASE t.avg_presentation_format_raw 
       WHEN 3 THEN 'Lecture'
       WHEN 2 THEN 'Poster'
       WHEN 1 THEN 'Either'
       ELSE 'Not Specified'
    END AS 'presentation_format'
    (...)
FROM 
(
   SELECT ROUND(AVG( 
       CASE RA12.answer
          WHEN 'L' THEN 3
          WHEN 'P' THEN 2
          WHEN 'E' THEN 1
          ELSE 0
       END),0) AS 'avg_presentation_format_raw'
       (...)
    FROM (...)
) AS t

Or just wrap it:

SqlFiddleDemo

SELECT
  CASE ROUND(AVG( 
      CASE answer
         WHEN 'L' THEN 3
         WHEN 'P' THEN 2
         WHEN 'E' THEN 1
         ELSE 0
      END
      ),0)
    WHEN 3 THEN 'Lecture'
    WHEN 2 THEN 'Poster'
    WHEN 1 THEN 'Either'
    ELSE 'Not Specified'
  END as 'presentation_format',
  ROUND(AVG( 
   CASE RA12.answer
      WHEN 'L' THEN 3
      WHEN 'P' THEN 2
      WHEN 'E' THEN 1
      ELSE 0
   END),0) AS 'avg_presentation_format_raw'
FROM tab

But you cannot reference like this:

SELECT 
   1 AS 'var1'
   var1 + 1 AS 'var2'

All the results of a single row from a select are atomic. That is, you can view them all as if they occur in parallel and cannot depend on each other.