I think this should be easy, but I can't seem to put the SQL together correctly and would appreciate any help. (I'm using Pg 8.4 in CentOS 5.5, if that matters.)
I have a table of Farms and a table of crops in a 1:M relationship of Farms : Crops. There are lots of different crops to choose form but for now I'm only interested in two crops; corn and soybeans. Some farms grow only corn and some grow only soybeans, and some grow both. What I'd like to know is, which Farms and how many are growing only corn, which and how many are growing soybeans and which and how many are growing both? I can easily get all the corn growers with:
SELECT a.* FROM farms a JOIN crops b ON a.farm_id=b.farm_id WHERE crop_cd='0041'
I can do the same with soybeans (crop_cd= '0081') and then I could subtract the sum of these from the total of all farms that grow either corn or soybeans to get the number of farms growing both, but having to do all those queries sounds very time consuming and inefficient. Is there a better way to get the farm counts or data by categories like farms growing only corn, farms growing only soybeans, farms growing both? I'm also interested in possibly expanding to a general case where I could select more than two crops. and get counts of the permutations.
Here's a sketch of the relevant pieces of the data base.
Tables: farms crops ======= ======= farm_id bigint (pkey) crop_id (pkey) type farm_id foreign key to farms size crop_cd 0041 = corn 0081=soybeans ... year ...
