英文:
How to create a UDF used for a SQL where clause in DolphinDB?
问题
我已经创建了一个UDF并在where子句中使用它，如下图所示。
def isZero(number, precision=4):
unit = 1 \ pow(10, precision)
ngUnit = -unit
if (ngUnit < number && number < unit):
return true
return false
}
res = select * from diff where isZero(openDiff)
但是报告了以下错误：
发生这种情况的原因是什么？在创建这样的子句时，我需要注意什么吗？
英文:
I have created a UDF and used it in a where clause, which is shown in the following figure.
def isZero(number,precision=4){
unit =1\pow(10,precision)
ngUnit =-unit
if(ngUnit < number && number < unit){
return true
}
return false
}
res= select * from diff where isZero(openDiff)
But an error is reported as follows:
Why is that happening? Is there anything I should be aware of when creating a UDF in such a clause?
答案1
得分: 1
DolphinDB支持向量运算。在where子句中，您UDF的参数openDiff是一列（即向量），因此需要确保您的函数返回相同长度的向量。但您为UDF设计的输出是布尔标量。要解决这个问题，请参考以下方法：
（1）将您的UDF更改为向量函数
（2）使用each(isZero, openDiff)
（3）使用iif：where iif((openDiff < 1.0/pow(10,4) && openDiff > -1.0/pow(10,4)), true, false) == true
（4）使用eqFloat(precision=4)
英文:
DolphinDB supports vector operation. In the where clause, the parameter openDiff of your UDF is a column (i.e., a vector), so you need to ensure that your function returns a vector of the same length. But the output you designed for your UDF is a Boolean scalar. To solve your problem, refer to the following methods:
(1) change your UDF to a vector function
(2) use each(isZero, openDiff)
(3) Use iif: where iif((openDiff<1.0/pow(10,4) && openDiff>-1.0/pow(10,4)),true,false) == true
(4) use eqFloat(precision=4)
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