Average a time value in SQL Sever 2005

Question

I've got a varchar field in SQL Sever 2005 that's storing a time value in the format "hh:mm"ss.mmmm".

What I really want to do is take the average using the built in aggregate function of those time values. However, this:

SELECT AVG(TimeField) FROM TableWithTimeValues

doesn't work, since (of course) SQL won't average varchars. However, this

SELECT AVG(CAST(TimeField as datetime)) FROM TableWithTimeValues

also doesn't work. As near as I can tell, SQL doesn't know how to convert a value with only time and no date into a datetime field. I've tried a wide variety of things to get SQL to turn that field into a datetime, but so far, no luck.

Can anyone suggest a better way?

Solution 1

SQL Server can convert a time-only portion of a datetime value from string to datetime, however in your example, you have a precision of 4 decimal places. SQL Server 2005 only recognizes 3 places. Therefore, you will need to truncate the right-most character:

create table #TableWithTimeValues
(
    TimeField varchar(13) not null
)

insert into #TableWithTimeValues
select '04:00:00.0000'
union all
select '05:00:00.0000'
union all
select '06:00:00.0000'

SELECT CAST(TimeField as datetime) FROM #TableWithTimeValues
--Msg 241, Level 16, State 1, Line 1
--Conversion failed when converting datetime from character string.

SELECT CAST(LEFT(TimeField, 12) as datetime) FROM #TableWithTimeValues
--Success!

This will convert valid values into a DATETIME starting on 1900-01-01. SQL Server calculates dates based on 1 day = 1 (integer). Portions of days are then portions of the value 1 (i.e. noon is 0.5). Because a date was not specified in the conversion, SQL Server assigned the value of 0 days (1900-01-01), which accommodates our need to average the time portion.

To perform an AVG operation on a DATETIME, you must first convert the DATETIME to a decimal value, perform the aggregation, then cast back. For example

SELECT CAST(AVG(CAST(CAST(LEFT(TimeField, 12) as datetime) AS FLOAT)) AS DATETIME) FROM #TableWithTimeValues
--1900-01-01 05:00:00.000

If you need to store this with an extra decimal place, you can convert the DATETIME to a VARCHAR with time portion only and pad the string back to 13 characters:

SELECT CONVERT(VARCHAR, CAST(AVG(CAST(CAST(LEFT(TimeField, 12) as datetime) AS FLOAT)) AS DATETIME), 114) + '0' FROM #TableWithTimeValues

Solution 2

Try this

AVG(CAST(CAST('1900-01-01 ' + TimeField AS DateTime) AS Float))

You really should store those in a datetime column anyway. Just use a consistent date for that part (1/1/1900 is very common). Then you can just call AVG() and not worry about it.

Solution 3

I used Cadaeic's response to get an answer I was looking for, so I thought I should share the code....

I was looking for a query that would average ALL my times together and give me an overall Turn Around Time for all approvals. Below is a nested statement that gives you the AVG TAT for individual id's and and when nested an overall TAT

SELECT 
-- calculates overall TAT for ALL Approvals for specified period of time
-- depending on parameters of query
CONVERT(VARCHAR, CAST(AVG(CAST(CAST(LEFT(Tat_mins, 12) as datetime) AS FLOAT)) AS DATETIME), 108) + '0'

from 
(
-- tat is for individual approvals 
SELECT 
dbo.credit_application.decision_status,
dbo.credit_application.application_id,
cast(dbo.credit_application.data_entry_complete as date) as'Data Entry Date',
cast(dbo.credit_application.decision_date as DATE) as 'Decision Date',
avg(datediff(minute, dbo.credit_application.data_entry_complete, dbo.credit_application.decision_date)) as 'TAT Minutes',
convert (char(5), DateAdd(minute, Datediff(minute,dbo.credit_application.data_entry_complete, dbo.credit_application.decision_date),'00:00:00'),108) as 'TAT_Mins'
FROM  dbo.credit_application
where Decision_status not in ('P','N')

group by dbo.credit_application.decision_status,
dbo.credit_application.data_entry_complete,
dbo.credit_application.decision_date
--dbo.credit_application.application_id
)bb