このクエリの内部実行、特にサブクエリからの値がどのように使用されているかを知りたいだけです
SELECT bu, location FROM
(
SELECT DISTINCT bu, hqname, location FROM DOCTOR
UNION
SELECT DISTINCT bu, hqname, location FROM CHEMIST
UNION
SELECT DISTINCT bu, hqname, location FROM STOCKIST
)
GROUP BY bu, location
HAVING COUNT (DISTINCT hqname) > 1;