Weird Rounding

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Weird Rounding

Alan G-436699

Mr or Mrs. 500

Points: 504
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March 1, 2010 at 12:56 pm

#145533

I have two tables that are the same - except each one is updated by their own ETL process of which one has been heavily revised - so this is a parallel run.
"Table1" and "PARALLEL_Table1" have a column called TotValue of type FLOAT. On both tables the value in the column for the same row is 49.395 (no other decimal places).
When I run the query:
select convert(decimal(28,2),TotValue) as TotValue
from Table1
where id = 123456
union all
select convert(decimal(28,2),TotValue) as TotValue
from PARALLEL_Table1
where id = 123456
The result set is:
49.39
49.40
This is happening all over the place with one penny of difference in each case. If I change decimal(28,2) to decimal(28,3) then the problem goes away. But I need to understand why the rounding is inconsistent to help me narrow down the differences between the two tables quickly. Can anyone explain these differences? Thanks in advance! 🙂
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mister.magoo SSC-Forever Points: 47068 More actions · Answer 1

FLOAT is an imprecise data type...

try this:

declare @val float

select @val=49.395

select @val,convert(decimal(38,28),@val)

and notice that @val does not exactly equal 49.395

then try

select convert(decimal(38,28),TotValue) as TotValue

from Table1

where id = 123456

union all

select convert(decimal(38,28),TotValue) as TotValue

from PARALLEL_Table1

where id = 123456

and you will no doubt see that one of the values is actually slightly less than 49.395 and one is slightly more, this variance will show itself as different rounding.

MM

select geometry::STGeomFromWKB(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

Alan G-436699 Mr or Mrs. 500 Points: 504 More actions · Answer 2

Thanks Mr Magoo - I'll try what you suggest at work tmw.:-)

Alan G-436699 Mr or Mrs. 500 Points: 504 More actions · Answer 3

After much experimentation, it seems that a method that works quite well to round two slightly different FLOAT numbers like:

41.4750000000000000

41.4749999999999999

is to round firstly to one decimal place (1dp) more than you need and then to the dp that you actually need. Therefore the code for the above would be:

round(round(TotPaid,3),2).

Both numbers above yield 41.48 with this rounding approach which means they can be reconciled. Also, the binary_checksum function will return the same number which doesn't happen with the example two numbers in the standard FLOAT format. I use the binary_checksum function to reconcile rows on two similar tables that are parallel running.:-)

mister.magoo SSC-Forever Points: 47068 More actions · Answer 4

Another approach I sometimes use is to compare like this:

SELECT ....

WHERE ABS(float_1 - float_2)>=0.01

Changing the 0.01 to whatever value you consider to be a variance.

MM

select geometry::STGeomFromWKB(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