Help for a query

[Himanshu Nagar]

HI Everyone,

I am novice in terms of SQL knowledge , Can some one please explain how is the query executed:

SELECT DISTINCT store_type FROM stores s1
WHERE NOT EXISTS (
SELECT * FROM cities WHERE NOT EXISTS (
SELECT * FROM cities_stores
WHERE cities_stores.city = cities.city
AND cities_stores.store_type = stores.store_type));

I am very confused on this .