What was the average temperature differences during May 2012 for store number 6 and 7?
during May 2012 refers to SUBSTR(date, 1, 7) = '2012-05'; store number 6 refers to store_nbr = 6; store number 7 refers to store_nbr = 7; average temperature difference = Subtract (Divide (Sum(tavg), Count (date) where the store_nbr = 6), Divide (Sum(tavg), Count(date) where store_nbr = 7))
SELECT ( SELECT CAST(SUM(tavg) AS REAL) / COUNT(`date`) FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr AND T1.`date` LIKE '%2012-05%' AND T2.store_nbr = 6 ) - ( SELECT CAST(SUM(tavg) AS REAL) / COUNT(`date`) FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr WHERE T1.`date` LIKE '%2012-05%' AND T2.store_nbr = 7 )
CREATE TABLE sales_in_weather
(
    date      DATE,
    store_nbr INTEGER,
    item_nbr  INTEGER,
    units     INTEGER,
    primary key (store_nbr, date, item_nbr)
);
CREATE TABLE weather
(
    station_nbr INTEGER,
    date        DATE,
    tmax        INTEGER,
    tmin        INTEGER,
    tavg        INTEGER,
    depart      INTEGER,
    dewpoint    INTEGER,
    wetbulb     INTEGER,
    heat        INTEGER,
    cool        INTEGER,
    sunrise     TEXT,
    sunset      TEXT,
    codesum     TEXT,
    snowfall    REAL,
    preciptotal REAL,
    stnpressure REAL,
    sealevel    REAL,
    resultspeed REAL,
    resultdir   INTEGER,
    avgspeed    REAL,
    primary key (station_nbr, date)
);
CREATE TABLE relation
(
    store_nbr   INTEGER
            primary key,
    station_nbr INTEGER,
    foreign key (store_nbr) references sales_in_weather(store_nbr),
    foreign key (station_nbr) references weather(station_nbr)
);
