Conditional counting: Performance differences in using SUM() vs COUNT()?

Question

Just as a very simple example, let\'s say I have table test with sample data like so:

a     |     b      
-------------
1     |    18
1     |            


        

Answer 1

Personally, I would use

select a, count(b)
  from test
 where b < 50
 group by a

Clear, concise and according to this SQL fiddle a tiny bit quicker than the others (needs less data according to the execution plan, though with a table that small you won't notice a difference):