Is SQL GROUP BY a design flaw? [closed]

You don’t have to group by the exactly the same thing you’re selecting, e.g. :

SQL:select priority,count(*) from rule_class
group by priority

PRIORITY   COUNT(*)
      70          1
      50          4
      30          1
      90          2
      10          4

SQL:select decode(priority,50,'Norm','Odd'),count(*) from rule_class
group by priority

DECO   COUNT(*)
Odd           1
Norm          4
Odd           1
Odd           2
Odd           4

SQL:select decode(priority,50,'Norm','Odd'),count(*) from rule_class
group by decode(priority,50,'Norm','Odd')

DECO   COUNT(*)
Norm          4
Odd           8