Give the station pressure status recorded by the weather station which contained no.12 store on 2012/5/15.
no.12 store refers to store_nbr = 12; on 2012/5/15 refers to date = '2012-05-15'; station pressure status refers to stnpressure
SELECT T1.stnpressure FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr WHERE T1.`date` = '2012-05-15' AND T2.store_nbr = 12