我想把我自定义的指标拼接到固定的查询列后，我使用 join 拼接但是报错了，
t = table(take(`A`B, 5) as sym, 2022.01.02+1..5 as date, rand(10.0, 5) as val)
fields_list = [sqlColAlias(makeCall(sum, sqlCol(`val)), `sum_val)]
sql(select=[sqlCol(`sym), sqlCol(`date), sqlColAlias(makeCall(sum, sqlCol(`val)), `sum_val)], from=t).eval() // OK
sql(select=[sqlCol(`sym), sqlCol(`date)] <-fields_list, from=t).eval() // ERROR
请问元编程里 join 应该怎么书写？
