How many store are located in district 15?
SELECT count ( * )  FROM store AS t1 JOIN store_district AS t2 ON t1.store_id   =   t2.store_id JOIN district AS t3 ON t2.district_id   =   t3.district_id WHERE t3.district_id  =  15
counting the number of stores in a specific district refers to COUNT(*); district 15 refers to WHERE district_id = 15; joining tables to match stores with their districts.