Varchar to Datetime in TSQL - sql

Question

How can I convert date and time value stored like
20200406151341
to DATETIME value 2020/04/06 15:31:41 (YYYY/MM/DD HH:MM:SS.000)? I am unable to find suitable CONVERT() format and the only way so far is to parse the VARCHAR like below.
select dateadd(second, cast(substring('20200406151341',13,2) as int),dateadd(minute, cast(substring('20200406151341',11,2) as int), dateadd(hour,cast(substring('20200406151341',9,2) as int),convert(datetime, left('20200406151341',8), 112)))).
It works yet it's hard to read and understand especially when I have to use it within SELECT statement multiple times.
Also I am surprised query with above conversions is as fast the one with dates stored directly in DATETIME format. Does MSSQL server uses some kind of cache so it does have to do the conversion only once per row?
I use MSSQL Server 2016.

Answer 1

I don't think there is a built in, simple way to do this.
You don't have to go to seconds to do this. You can easily convert the first 8 characters to a date. With some string manipulation, you can convert the last six to a time -- and then add time (as datetime values):
select convert(datetime, left(dt, 8)) + convert(datetime, stuff(stuff(right(dt, 6), 5, 0, ':'), 3, 0, ':'))
from (values ('20200406151341')) v(dt);
You can also use arithmetic rather than 3 dateadd()s:
select dateadd(second,
right(dt, 2) + 60*substring(dt, 11, 2) + 60*60*substring(dt, 9, 2),
convert(datetime, left(dt, 8)))
from (values ('20200406151341')) v(dt)
Note: This uses implicit conversion from a string to an integer (as does your version).

Answer 2

You can use stuff() :
select convert(datetime,
stuff(stuff(stuff(stuff(col, 9, 0, ' '), 10, 0, ''), 12, 0, ':'), 15, 0, ':'
)

Answer 3

Let's throw in a couple more options to the mix:
DECLARE #StrDate varchar(14) = '20200406151341'
SELECT DATETIMEFROMPARTS(
LEFT(#StrDate, 4), -- year
SUBSTRING(#StrDate, 5, 2), -- month
SUBSTRING(#StrDate, 7, 2), -- day
SUBSTRING(#StrDate, 9, 2), -- hour
SUBSTRING(#StrDate, 11, 2), -- minute
SUBSTRING(#StrDate, 13, 2), -- second
0 -- millisecond
) As [Using DateTimeFromParst],
CONVERT(DateTime, LEFT(#StrDate, 8), 112) + -- Date
CONVERT(DateTime, STUFF(STUFF(RIGHT(#StrDate, 6), 5, 0, ':'), 3, 0, ':'), 114) -- Time
As [Using convert and stuff]
Results:
Using DateTimeFromParst Using convert and stuff
2020-04-06 15:13:41 2020-04-06 15:13:41

Answer 4

I suggest using try_convert() or try_cast() in case your strings are invalid, and if invalid they will return NULL instead of raising an error. Refer to the links for further detail on these functions.
declare #val varchar(20)
set #val = '20200416151341'
select try_convert(datetime,
stuff(stuff(stuff(stuff(#val, 9, 0, ' '), 10, 0, ''), 12, 0, ':'), 15, 0, ':')
);
select try_cast(
stuff(stuff(stuff(stuff(#val, 9, 0, ' '), 10, 0, ''), 12, 0, ':'), 15, 0, ':')
as datetime);