Trouble converting from time to decimal time - sql

Question

Part of the task I have been given involves performing calculations on a few columns, 2 of which are in the format of hh.mi.ss and they're varchar. In order for the calculations to work, I need to get them into a time decimal format, whereby 1:30 would be 1.5 . Since I'm currently using SQL Server 2005, I don't have the time or data types built-in and I'm unable to get an upgraded version (not my choice). Working with what I have, I've searched around online and tried to convert it but the result isn't accurate. For example, 13.28 becomes (roughly) 13.5, which is great, however, the seconds go to 100 instead of ending at 60 (since I'm converting it to a float).
For example, using 12.57.46,
CAST(DATEPART(HH, CAST(REPLACE([OASTIM], '.', ':') AS DATETIME)) AS FLOAT) +
(CAST(DATEPART(MI, CAST(REPLACE([OASTIM], '.', ':') AS DATETIME)) AS FLOAT)/60) +
(CAST(DATEPART(SS, CAST(REPLACE([OASTIM], '.', ':') AS DATETIME)) AS FLOAT)/3600)
gave me 12.962...
whereas
CAST(SUBSTRING([OASTIM], 1, 2) AS FLOAT) +
((CAST(SUBSTRING([OASTIM], 4, 5) AS FLOAT) +
CAST(SUBSTRING([OASTIM], 7, 8) AS FLOAT)/60)/60)
gave me 12.970....
and when I tried something simpler,
DATEPART(HOUR, CAST(REPLACE([OASTIM], '.', ':') AS DATETIME))+
(DATEPART(MINUTE, CAST(REPLACE([OASTIM], '.', ':') AS DATETIME))/60)
flopped out and gave me only 12
It's my first exposure to Windows SQL and T-SQL, I've been struggling with this for a few hours. As horrible as it sounds, I'm at the point where I'd be happy with it working even it it means sacrificing performance.

Answer 1

You don't explain what "time decimal" format is. From your example, I'll guess that you mean decimal hours.
A key function in SQL Server for date differences is datediff(). You can convert the time to seconds using a trick. Add the time to a date, then use datediff() to get the number of seconds after midnight. After that, the conversion to decimal hours is just arithmetic.
Here is an example:
select datediff(second,
cast('2000-01-01' as datetime),
cast('2000-01-01 ' + '00:00:59' as datetime)
)/3600.0 as DecimalHours
Note the use of the constant 3600.0. The decimal point is quite important, because SQL Server does integer division on integer inputs. So, 1/2 is 0, rather than 0.5.

Answer 2

You said,
CAST(SUBSTRING([OASTIM], 1, 2) AS FLOAT) +
((CAST(SUBSTRING([OASTIM], 4, 5) AS FLOAT) +
CAST(SUBSTRING([OASTIM], 7, 8) AS FLOAT)/60)/60)
gave me 12.970....
12.970 is wrong for an input of '12.57.46'. The problem is that you are using the SUBSTRING function incorrectly. The 3rd argument represents the number of characters, not the ending character position.
Take a look at this code:
Declare #Sample varchar(20)
Set #Sample = '12.57.46'
select CAST(SUBSTRING(#Sample, 1, 2) AS FLOAT) +
CAST(SUBSTRING(#Sample, 4, 5) AS FLOAT) / 60 +
CAST(SUBSTRING(#Sample, 7, 8) AS FLOAT) / 60 / 60,
SUBSTRING(#Sample, 1, 2),
SUBSTRING(#Sample, 4, 5),
SUBSTRING(#Sample, 7, 8),
CAST(SUBSTRING(#Sample, 1, 2) AS FLOAT) +
CAST(SUBSTRING(#Sample, 4, 2) AS FLOAT) / 60 +
CAST(SUBSTRING(#Sample, 7, 2) AS FLOAT) / 60 / 60
Notice that the minutes is coming out as 57.46 because you are asking for 5 characters. The seconds are coming out correctly because eventhough you are asking for 8 characters, there are only 2 characters left in the string so only those 2 characters are returned.
BTW, I would solve this problem the same way that Gordon did, except I would remove the date stuff so it would look like this:
Select DateDiff(Second,
0,
Convert(DateTime, Replace([OASTIM], '.',':'))) / 3600.0

Answer 3

Here is a simple way to convert time to DecimalHours.
SELECT cast(cast('12.57:46' as datetime) as float) * 24
Result:
~12.963