执行 KDB select 时获取默认值而不是空值

Question

我想在特定列值为 null 或空白时获取默认值。

e.g select customer,date,product,orderId,version,size from tableA where date=2020.04.08,product in (`Derivative)

returns :

+----------+----------+------------+---------+---------+------+
| customer |   date   |  product   | orderId | version | size |
+----------+----------+------------+---------+---------+------+
| XYZ fund | 4/8/2020 | Derivative |       1 |       6 |      |
| XYZ fund | 4/8/2020 | Derivative |       2 |       6 | 1000 |
| XYZ fund | 4/8/2020 | Derivative |       3 |       4 |      |
+----------+----------+------------+---------+---------+------+

所以我想要 NA 而不是下面的空白或 null :

+----------+----------+------------+---------+---------+------+
| customer |   date   |  product   | orderId | version | size |
+----------+----------+------------+---------+---------+------+
| XYZ fund | 4/8/2020 | Derivative |       1 |       6 |  NA  |
| XYZ fund | 4/8/2020 | Derivative |       2 |       6 | 1000 |
| XYZ fund | 4/8/2020 | Derivative |       3 |       4 |  NA  |
+----------+----------+------------+---------+---------+------+

Answer 1

这很难用像 NA 这样的值来做，因为 q 更喜欢向量是统一类型。假设您的 size 列是数字类型而不是符号，您将运行遇到问题。

我建议使用 ^ fill 运算符填写一些数值。例如要填零作为默认值：

q)res:select customer,date,product,orderId,version,size from tableA where date=2020.04.08,product in (`Derivative)
q)update 0^size from res
customer date     product    orderId version size
-------------------------------------------------
XYZ fund 4/8/2020 Derivative 1       6       0  
XYZ fund 4/8/2020 Derivative 2       6       1000
XYZ fund 4/8/2020 Derivative 3       4       0

如果使用 NA 值很重要，那么您可以将该列转换为符号类型并填充 `NA:

q)res:select customer,date,product,orderId,version,size from tableA where date=2020.04.08,product in (`Derivative)
q)update `NA^`$string size from res
customer date     product    orderId version size
-------------------------------------------------
XYZ fund 4/8/2020 Derivative 1       6       NA  
XYZ fund 4/8/2020 Derivative 2       6       1000
XYZ fund 4/8/2020 Derivative 3       4       NA

这样做的缺点是，如果不先转换回数字类型，您将无法再对列进行数字运算：

q)select sum size from res
'type
  [0]  select sum size from res
q)select sum "J"$string size from res
size
----
1000

执行 KDB select 时获取默认值而不是空值

Get Default value instead of null while doing KDB select

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