如果是 DOUBLE / INT 这种数值类型的列，可以用 toArray 函数把每个组的数据组成一个 array vector 进行存储，比如
tmp = table(rand(10, 10) as stock_id, 1..10 as amount)
select toArray(amount) as amount from tmp group by stock_id
如果是 STRING 这种字符串类型的列，可以指定分隔符把数据拼接成一个 string，使用时，用 split 再拆分一下，比如
tmp = table(rand(10, 10) as stock_id, string(1..10) as amount)
// group by
t = select concat(amount, ";") as amount from tmp group by stock_id
// 取组内第二个字符串
tt = select amount.split(";")[1] as amount from t