将SQL Server varchar转换为日期时间以进行算术运算

Question

I want to combine two separate columns into date time and subtract 20 hours in SQL Server.

This works in Oracle:

create table t ( x varchar2(8),y varchar2(6));

insert into t values ('20151106','090000');

select to_char(to_date(x||' '||y, 'yyyymmdd hh24miss')
             - (20/24),'dd-mm-yyyy hh24:mi:ss') as "date minus 20 hrs" from t;

date minus 20 hrs
- -----------------
05-11-2015 13:00:00

I can't crack it in SQL Server, I can get parts to work but can't combine the two dates (too many functions).

This works:

CREATE TABLE t (x VARCHAR(8), y VARCHAR(6))

INSERT INTO t
VALUES('20151105', '150800')

SELECT  
    DATEADD(HOUR, -20, CAST(CONCAT(SUBSTRING(y, 1, 2), ':', SUBSTRING(y, 3, 2), ':', SUBSTRING(y, 5, 2)) AS TIME))
FROM t

This works:

SELECT DATEADD(HOUR, -20, x)
FROM t

But I can't combine them:

SELECT 
    DATEADD(HOUR, -20,
            CONCAT(SUBSTRING(x, 1, 4), '-', SUBSTRING(x, 5, 2), '-',
                   SUBSTRING(x, 7, 2), ' ',
                   CAST(CONCAT(SUBSTRING(y, 1, 2), ':', SUBSTRING(y, 3, 2),
                               ':', SUBSTRING(y, 5, 2)) AS TIME)))
FROM t

Answer 1

The string you're passing to datepart evaluates to:

select dateadd(hour, -20, '2015-11-05T15:08:00.0000000')

But you'll get a Conversion failed error for more than 3 zeroes. An easy way to fix that is to remove the second column's conversion to time . While you're at it, you could use + instead of concat , which is easier to read:

select dateadd(hour, -20, 
            substring(x,1,4) + '-' +
            substring(x,5,2) + '-' + 
            substring(x,7,2) + 'T' + 
            substring(y,1,2) + ':' + 
            substring(y,3,2) + ':' +
            substring(y,5,2))
from t

See it working at SE Data.

Answer 2

If you CONVERT the values properly to a datetime , there should be no issue with what you're trying to do. You want to give SQL Server a date in the following format, or one of the other acceptable formats:

YYYYMMDD HH:MM:SS    

So applying that to your sample, the date portion is fine, but you need to get the time format correct before you can convert it to a valid date:

CREATE TABLE #t ( x VARCHAR(8), y VARCHAR(6) )
INSERT  INTO #t
VALUES  ( '20151105', '150800' )
-- we want 20151105 15:08:00
SELECT  DATEADD(HOUR, -20,
                ( CONVERT(DATETIME, x + ' ' + 
                            LEFT(y, 2) + ':' + 
                            SUBSTRING(y, 3, 2) + ':' + 
                            RIGHT(y, 2)) )) AS Result
FROM    #t

DROP TABLE #t

Produces

2015-11-04 19:08:00.000