tSQL - Conversion from varchar to numeric works for all but integer

Question

I have a table with numbers in a varchar(255) field. They're all greater than one and have multiple decimal places. I'd like to convert them to integers. According to every web site I've consulted, including this one on StackOverflow, either of these should work:

SELECT CAST(VarcharCol AS INT) FROM MyTable

SELECT CONVERT(INT, VarcharCol) FROM MyTable

These both work for me for every kind of numeric value but integer - I can convert to float, decimal, etc. just fine, but trying to convert to integer gives me the following error:

Conversion failed when converting the varchar value '7082.7758172' 
to data type int.

I've worked around the problem by converting to data type Decimal(6,0), which works fine. But just for my education, can anyone tell me why converting to data type int (or integer) gives me an error? Thanks.

Answer 1

Converting a varchar value into an int fails when the value includes a decimal point to prevent loss of data.

If you convert to a decimal or float value first, then convert to int, the conversion works.

Either example below will return 7082:

SELECT CONVERT(int, CONVERT(decimal(12,7), '7082.7758172'));
SELECT CAST(CAST('7082.7758172' as float) as int);

Be aware that converting to a float value may result, in rare circumstances, in a loss of precision. I would tend towards using a decimal value, however you'll need to specify precision and scale values that make sense for the varchar data you're converting.

Answer 2

Actually whether there are digits or not is irrelevant. The . (dot) is forbidden if you want to cast to int. Dot can't - logically - be part of Integer definition, so even:

select cast ('7.0' as int)
select cast ('7.' as int)

will fail but both are fine for floats.