On 24/11/2014 11:26, Erland Sommarskog wrote:   
   > twenty-six@b-mint.net (twenty-six@b-mint.net) writes:   
   >> When I run this query I get lots of 'Unknown'  values, but I'm not sure   
   >> why.   
   >   
   > This happens because   
   >   
   >      CASE expr WHEN val1 THEN ....   
   >                WHEN val2 THEN ....   
   >   
   > is a shortcut for   
   >   
   >     CASE WHEN expr = val1 THEN   
   >          WHEN expr = val2 THEN   
   >   
   > and thus the expression gets evaluated multiple times and then it goes   
   > downhill from there.   
   >   
   > Set up a mapping table instead.   
   >   
   >   
   Ah I see, many thanks.    That would explain it nicely,  I really   
   thought I'd found a bug.   :-)   
      
   By mapping table you mean create a table,  insert 12 rows  with  1, Jan   
      2, Feb  etc.  then join with that?   
      
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