Why does SELECT results differ between mysql and sqlite?

Question

I\'m re-asking this question in a simplified and expanded manner.

Consider these sql statements:

create table foo (id INT, score INT);

insert into foo v         


        

Answer 1

Lets look at this two ways, i'll use postgres 9.0 as my reference database

(1)

-- select rows from foo 

select T1.id, avg(T1.score) avg1
from foo T1
group by T1.id
-- where we don't have any rows from T2
having  not exists (
-- select rows from foo
select T2.id, avg(T2.score) avg2
from foo T2
group by T2.id
-- where the average score for any row is greater than the average for 
-- any row in T1
having avg2 > avg1);

 id  |        avg1        
-----+--------------------
 106 | 4.5000000000000000
(1 row)

then let's move some of the logic inside the subquery, getting rid of the 'not' : (2)

-- select rows from foo 
select T1.id, avg(T1.score) avg1
from foo T1
group by T1.id
-- where we do have rows from T2
having  exists (
-- select rows from foo
select T2.id, avg(T2.score) avg2
from foo T2
group by T2.id
-- where the average score is less than or equal than the average for any row in T1
having avg2 <= avg1);
-- I think this expression will be true for all rows as we are in effect doing a
--cartesian join 
-- with the 'having' only we don't display the cartesian row set

 id  |        avg1        
-----+--------------------
 106 | 4.5000000000000000
 107 | 4.0000000000000000
(2 rows)

so you have got to ask yourself -- what do you actually mean when you do this correlated subquery inside a having clause, if it evaluates every row against every row from the primary query we are making a cartesian join and I don't think we should be pointing fingers at the SQL engine.

if you want every row that is less than the maximum average What you should be saying is:

select T1.id, avg(T1.score) avg1 
from foo T1 group by T1.id
having avg1 not in 
(select max(avg1) from (select id,avg(score) avg1 from foo group by id))