为什么Postgres的exp / log结果与SQL Server不同? [英] Why the result of exp/log of Postgres differs from SQL Server?
本文介绍了为什么Postgres的exp / log结果与SQL Server不同?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我的包装盒上没有SQL Server,但是为什么以下 answer 在Postgres上返回360?
I don't have SQL Server on my box, but why won't the following answer return 360 on Postgres?
select exp(sum(log(val)))
from (values(4),(5),(3),(6)) as tbl(val)
返回12.888075
returns 12.888075
推荐答案
您必须使用自然对数(ln函数),而不是以10为底的对数(log函数):
You have to use natural logarithm (ln function), not base-10 logarithm (log function):
select exp(sum(ln(val)))
from (values(4),(5),(3),(6)) as tbl(val)
exp
-----
360
(1 row)
但这不是乘行的好方法-它很慢,并且由于四舍五入而容易出错。您应该声明一个总计:
But this is not a good way to multiply rows - it is slow and error prone due to rounding. You should declare an aggregate:
create function multiply(int,int) returns int as $$
select $1*$2;
$$ language sql immutable strict;
create aggregate multiply(int) (
sfunc=multiply,
stype=int,
initcond=1
);
select multiply(val)
from (values(4),(5),(3),(6)) as tbl(val)
multiply
----------
360
(1 row)
这篇关于为什么Postgres的exp / log结果与SQL Server不同?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!
查看全文