可以用 pivot by 将 a, b 拆成两列，然后对表的每一行做计算：
id = 1 1 2 2 3 4
part = `a`b`a`b`a`b
val = 3 4 2 6 4 8
t = table(id, part, val)
select a -b from select last(val) from t pivot by id, part
可以用 pivot by 将 a, b 拆成两列，然后对表的每一行做计算：
id = 1 1 2 2 3 4
part = `a`b`a`b`a`b
val = 3 4 2 6 4 8
t = table(id, part, val)
select a -b from select last(val) from t pivot by id, part