在varchar中处理小数时，SQL Server中的ROUND（）

Question

I know the usage of Round() and it works fine for

select round(147.26719,5,1)

which returns 147.26719.

Currently I am working on a table imported from a txt file so all the column are varchar , when I using round() to deal with a decimal in varchar like this

select round('147.26719',5,1)

very weird it returns 147.26718.

I current make it run correctly as

select round(cast('147.26719', decimal(11,6)),5,1)

but can anybody explains to me why this happens? I believe it is something when implicitly convert the varchar to decimal, but just don't know why.

Thanks in advance

Answer 1

By default, the implicit conversion of the string for the round() is into a float , not a decimal. Floating point numbers are imprecise and might be off just a wee, wee bit from what you see -- but enough to matter.

Your solution is the correct one -- cast to a decimal.

You can readily see this by using code such as:

select round('123.3', 1) as col
into x;

select *
from information_schema.columns
where table_name = 'x';