Comments - CASE OPERATOR

Keep on mind that in complex CASE constructions the first matching WHEN is processed only.

So the CASE operator works as if-elseif-else construction, see following examples where each condition of WHEN compare_value is TRUE but only first is matched and its result is returned.

-- First example
SELECT 
  CASE true 
     WHEN (1=1) THEN '1=1' -- result is returned 
     WHEN (1=1 OR 2=2) THEN '1=1 OR 2=2' -- condition not processed
     ELSE 'else'
  END 
;
+-------------------------------------------------------------------------------------+
| CASE true WHEN (1=1) THEN '1=1' WHEN (1=1 OR 2=2) THEN '1=1 OR 2=2' ELSE 'else' END |
+-------------------------------------------------------------------------------------+
+ 1=1                                                                                 +
+-------------------------------------------------------------------------------------+

-- Second example
SELECT 
  CASE true 
     WHEN (1=1 OR 2=2) THEN '1=1 OR 2=2' -- result is returned 
     WHEN (2=2) THEN '2=2' -- condition not processed
     ELSE 'else'
  END 
;
+-------------------------------------------------------------------------------------+
| CASE true WHEN (1=1 OR 2=2) THEN '1=1 OR 2=2' WHEN (2=2) THEN '2=2' ELSE 'else' END |
+-------------------------------------------------------------------------------------+
+ 1=1 OR 2=2                                                                          +
+-------------------------------------------------------------------------------------+

Thanks, clarified in the text.