SQL Date-Time Conversion with nested converts experiences random conversion errors - sql-server

Question

Looking for assistance with a strange issue if anyone has ideas:
I have a SQL that statement works most of the time in a T-SQL script but crashes occasionally. I have identified the data that a crash occurs on and cannot identify any difference between data rows that work.
The goal of this code is to add the time to an already existing datetime value that has 00:00:00 as the time from the second time column (as outlined below). My goal is to combine both columns into YYYY-MM-DD HH:MM:SS format, but I had to convert them to char first to trim off the orignal 00:00:00.
Columns
LogDate - contains date only in DateTime format (YYYY-MM-DD HH:MM:SS)
LogTime - contains the time of the action and is in varchar format (HH:MM)
SQL Conversion
SELECT CONVERT(DATETIME, CONVERT(CHAR(8), LogDate, 112) + ' ' + CONVERT(CHAR(8), LogTime, 108))
FROM TestTable
WHERE EventSerial = '100001'
However, if I change the EventSerial in the above statement to a different row, such as '100002', the statement works.
The data for each row is below:
EventSerial 100001's values:
LogDate: 2015-04-02 00:00:00.000
LogTime: 10:04
EventSerial 100002's values:
LogDate: 2015-04-02 00:00:00.000
LogTime: 10:48
Running with data set 1 fails, running with data set 2 produces output. Also, running the code without the final datetime conversion works, or if I run the code with the string manually it works (as outlined below:)
SELECT CONVERT(CHAR(8), LogDate, 112) + ' ' + CONVERT(CHAR(8), LogTime, 108)
FROM TestTable
WHERE EventSerial = '100001'
SELECT CONVERT(DATETIME, '20150402 10:48')
SELECT CONVERT(DATETIME, '20150402 10:04')
Any suggestions, I'm sure its something silly that I'm missing (and I probably took the long way around the issue anyway. The desired output would be 2015-04-02 10:04:00

Answer 1

First, datetime has no format. (why?)
Second, you don't need to convert the datetime value to char to add hours and minutes, just use DateAdd:
SELECT DATEADD(Minute,
CAST(RIGHT(LogTime, 2) as int),
DATEADD(Hour,
CAST(LEFT(LogTime, 2) as int),
LogDate
)
)
FROM TestTable
WHERE EventSerial = '100001'
Also, note that convert does not hold a style for yyyymmdd hh:mm
Note: code was written directly here, there might be some mistakes.

Answer 2

I'm not sure why you're getting the error... possibly there are some unseen characters in your varchar time field... like a tab or something maybe? Try this query:
SELECT ascii(substring(LogTime,1,1)) Char1,
ascii(substring(LogTime,2,1)) Char2,
ascii(substring(LogTime,3,1)) Char3,
ascii(substring(LogTime,4,1)) Char4,
ascii(substring(LogTime,5,1)) Char5
FROM TestTable
WHERE EventSerial = '100001'
It should show these results:
Char1 Char2 Char3 Char4 Char5
----------- ----------- ----------- ----------- -----------
49 48 58 48 52
(1 row(s) affected)
This would be a bit more efficient:
select dateadd(minute, datediff(minute,0, LogTime), LogDate)
FROM TestTable
But this assumes that your date field always has 00:00:00 time information. If you want to be sure that is stripped out as well you could use:
select dateadd(minute, datediff(minute,0, LogTime), dateadd(day, datediff(day, 0, Logdate),0))
FROM TestTable