Using SQL to get counts of counts
Expert Rudy Limeback explains how to use SQL to get counts of counts in an Orders table.
I have an Orders table with persons and toys. Sample values are:
person toy 1 puzzle 1 puzzle 1 doll 1 game 2 puzzle 2 doll 3 doll 3 game 3 game 4 doll 4 game 5 doll 5 doll
Persons may have several toys with the same or different names. I'm only interested in persons with games or puzzles. The query results I want from the above table are:
toycount personcount
3 1
2 1
1 2
In English I'd say: Show me a descending count of the number of toys ordered (puzzles and games only) and the number of people who ordered that many. It seems like this should be easy, but I'm just not getting it! Thanks for any help.
The trick here is to do two levels of aggregation. The first level is to count how many of the specified toys (puzzles and games only) were ordered by each person. This gives the "toycount" number for each person. The second level of aggregation is to count how many persons ordered each toycount. Fortunately, it is easy to nest these two levels of aggregation.
select toycount
, count(*) as personcount
from (
select person
, count(*) as toycount
from orders
where toy in ('puzzle','game')
group
by person
) as person_toycounts
group
by toycount
order
by toycount desc
Simple, yes?
