SQLAlchemy:如何编译类似sql的函数。职能。标准

我想对sql进行一些小的更改。职能。标准 当std结果为nan时,我想返回一个不同的值

使用此代码:

from sqlalchemy.sql.expression import FunctionElement

class std(FunctionElement):
    name = 'std'


@compiles(std)
def compile(element, compiler, **kw):
    elem = compiler.process(element.clauses)
    return "if(isnan(std(%s)), 0, std(%s))" % (elem, elem)


t1 = sql.select([sql.literal("1").label('a')]).alias('t1')

q = sql.select([std(t1.c.a)]).select_from(t1)

结果如预期

SELECT if(isnan(std(t1.a)), 0, std(t1.a)) FROM (SELECT '1' AS a) AS t1

使用此查询时

q = sql.select([sql.func.std(t1.c.a)]).select_from(t1)

结果是错误的

SELECT std(t1.a) AS std_1 FROM (SELECT '1' AS a) AS t1