表中的 NULL 值

timestamp = [14:23:35,14:15:03,13:33:30,10:07:59,10:21:34,09:53:41,13:34:52,11:03:24,10:52:13,12:39:42];
qty = 1300 1900 6800 2200 8300 4600 1400 8800 5800 700;
price = 50.76 29.46 174.97 49.6 51.2 50.1 29.1 172.93 173.5 175.1;
sym = `C`MS`IBM`C`C`C`MS`IBM`IBM`IBM$SYMBOL;
qty[qty>4000]=NULL;
price[price<30||price>175]=NULL;
t1 = table(timestamp, sym, qty, price);
t1;

select timestamp, qty, price from t1 where sym=`IBM order by price;

// select records with prices less than the average price for all trades.
select * from t1 where price<avg(price) order by price;

// 选出price大于组平均值的记录
select * from t1 where price>contextby(avg, price, sym);

// 选出price小于组平均值的记录
select * from t1 where price<contextby(avg, price, sym);